Anatomy of a Firefox Update

Alessio (:Dexter) recently landed a new ping for Firefox 56: the “update” ping with reason “ready”. It lets us know when a client’s Firefox has downloaded and installed an update and is only waiting for the user to restart the browser for the update to take effect.

In Firefox 57 he added a second reason for the “update” ping: reason “success”. This lets us know when the user’s started their newly-updated Firefox.

I thought I might as well see what sort of information we could glean from this new data, using the recent shipping of the new Firefox Quantum Beta as a case study.

This is exploratory work and you know what that means[citation needed]: Lots of pretty graphs!

First: the data we knew before the “update” ping: Nothing.

Well, nothing specific. We would know when a given client would use a newly-released build because their Telemetry pings would suddenly have the new version number in them. Whenever the user got around to sending them to us.

We do have data about installs, though. Our stub installer lets us know how and when installs are downloaded and applied. We compile those notifications into a dataset called download_stats. (for anyone who’s interested: this particular data collection isn’t Telemetry. These data points are packaged and sent in different ways.) Its data looks like this:Screenshot-2017-9-29 Recent Beta Downloads.png

Whoops. Well that ain’t good.

On the left we have the tailing edge of users continuing to download installs for Firefox Beta 56 at the rate of 50-150 per hour… and then only a trace level of Firefox Beta 57 after the build was pushed.

It turns out that the stub installer notifications were being rejected as malformed. Luckily we kept the malformed reports around so that after we fixed the problem we could backfill the dataset:Screenshot-2017-10-4 Recent Beta Downloads

Now that’s better. We can see up to 4000 installs per hour of users migrating to Beta 57, with distinct time-of-day effects. Perfectly cromulent, though the volume seems a little low.

But that’s installs, not updates.

What do we get with “update” pings? Well, for one, we can run queries rather quickly. Querying “main” pings to find the one where a user switched versions requires sifting through terabytes of data. The query below took two minutes to run:

Screenshot-2017-10-3 Users Updating to Firefox Quantum Beta 57(1)

The red line is update/ready: the number of pings we received in that hour telling us that the user had downloaded an update to Beta 57 and it was ready to go. The blue line is update/success: the number of pings we received that hour telling us the user had started their new Firefox Quantum Beta instance.

And here it is per-minute, just because we can:Screenshot-2017-10-3 Users Updating to Firefox Quantum Beta 57(2).png

September 30 and October 1 were the weekend. As such, we’d expect their volumes to be lower than the weekdays surrounding them. However, looking at the per-minute graph for update/ready (red), why is Friday the 29th the same height as Saturday the 30th? Fridays are usually noticeably busier than Saturdays.

Friday was Navarati in India (one of our largest market for Beta) but that’s a multi-day festival that started on the Wednesday (and other sources for client data show only a 15% or so dip in user activity on that date in India), so it’s unlikely to have caused a single day’s dip. Friday wasn’t a holiday at all in any of our other larger markets. There weren’t any problems with the updater or “update” ping ingestion. There haven’t been any dataset failures that would explain it. So what gives?

It turns out that Friday’s numbers weren’t low: Saturday’s were high. In order to improve the stability of what was going to become the Firefox 56 release we began on the 26th to offer updates to the new Firefox Quantum Beta to only half of updating Firefox Beta users. To the other half we offered an update to the Firefox 56 Release Candidate.

What is a Release Candidate? Well, for Firefox it is the stabilized, optimized, rebuilt, rebranded version of Firefox that is just about ready to ship to our release population. It is the last chance we have to catch things before it reaches hundreds of millions of users.

It wasn’t until late on the 29th that we opened the floodgates and let the rest of the Beta users update to Beta 57. This contributed to a higher than expected update volume on the 30th, allowing the Saturday numbers to be nearly as voluminous as the Friday ones. You can actually see exactly when we made the change: there’s a sharp jump in the red line late on September 29 that you can see clearly on both “update”-ping-derived plots.

That’s something we wouldn’t see in “main” pings: they only report what version the user is running, not what version they downloaded and when. And that’s not all we get.

The “update”-ping-fueled graphs have two lines. This rather abruptly piques my curiosity about how they might relate to each other. Visually, the update/ready line (red) is almost always higher than the update/success line (blue). This means that we have more clients downloading and installing updates than we have clients restarting into the updated browser in those intervals. We can count these clients by subtracting the blue line from the red and summing over time:Screenshot-2017-10-3 Outstanding Updates for Users Updating to Firefox Quantum Beta 57

There are, as of the time I was drafting this post, about one half of one million Beta clients who have the new Firefox Quantum Beta… but haven’t run it yet.

Given the delicious quantity of improvements in the new Firefox Quantum Beta, they’re in for a pleasant surprise when they do.

And you can join in, if you’d like.


(NOTE: earlier revisions of this post erroneously said download_stats counted updater notifications. It counts stub installer notifications. I have reworded the post to correct for this error. Many thanks to :ddurst for catching that)


Data Science is Hard: Dangerous Data

I sit next to a developer at my coworking location (I’m one of the many Mozilla staff who work remotely) who recently installed the new Firefox Quantum Beta on his home and work machines. I showed him what I was working on at the time (that graph below showing how nicely our Nightly population has increased in the past six months), and we talked about how we count users.

Screenshot-2017-9-28 Desktop Nightly DAU MAU for the Last Six Months by Version

=> “But of course we’ll be counting you twice, since you started a fresh profile on each Beta you installed. Actually four times, since you used Nightly to download and install those builds.” This, among other reasons, is why counting users is hard.

<= “Well, you just have to link it to my Firefox Account and then I’ll only count as one.” He figured it’d be a quick join and then we’d have better numbers for some users.

=> “Are you nuts?! We don’t link your Firefox Account to Telemetry! Imagine what an attacker could do with that!”

In a world with adversarial trackers, advertising trackers, and ever more additional trackers, it was novel to this pseudo-coworker of mine that Mozilla would specifically not integrate its systems.

Wouldn’t it be helpful to ourselves and our partners to know more about our users? About their Firefox Accounts? About their browsing history…

Mozilla doesn’t play that game. And our mission, our policies, and our practices help keep us from accidentally providing “value” of this kind for anyone else.

We know the size of users’ history databases, but not what’s in them.

We know you’re the same user when you close and reopen Firefox, but not who you are.

We know whether users have a Firefox Account, but not which ones they are.

We know how many bookmarks users have, but not what they’re for.

We know how many tabs users have open, but not why. (And for those users reporting over 1000 tabs: WHY?!)

And even this much we only know when you let us:


Why? Why do we hamstring our revenue stream like this? Why do we compromise on the certainty that having complete information would provide? Why do we allow ourselves to wonder and move cautiously into the unknown when we could measure and react with surety?

Why do we make Data Science even harder by doing this?

Because we care about our users. We think about what a Bad Actor could do if they had access to the data we collect. Before we okay a new data collection we think of all the ways it could be abused: Can it identify the user? Does it link to another dataset? Might it reveal something sensitive?

Yes, we have confidence in our security, our defenses in depth, our privacy policies, and our motivations to work for users and their interests.

But we are also confident that others have motivations and processes and policies that don’t align with ours… and might be given either the authority or the opportunity to gain access in the future.

This is why Firefox Send doesn’t know your encryption key for the files you share with your friends. This is why Firefox Accounts only knows six things (two of them optional) about you, and why Firefox Sync cannot read the data it’s storing for you.

And this is why Telemetry doesn’t know your Firefox Account id.


So I’ve Finished Final Fantasy XV

(Spoilers may lie within for a game that’s a year old.)

For a game that obviously had a lot of time (10 years!), effort, and money dumped into it… it feels unfortunately uneven.

The story, though standard Final Fantasy fare, is told poorly enough that characters react emotionally to situations that haven’t been earned, the player is forced to have her character make decisions without knowledge that her character has, and injects a person from the tie-in movie in a prominent part of the endgame without having ever, in the plot or the story, meeting the player character.

I like a good mystery. I like plots that surface only about a tenth of a world’s lore. I’m happy to think about questions posed by the narrative, and intrigued by the choices made by writers and directors about what pieces to include and which to omit.

This isn’t that. I mean it is, in places. I don’t need and didn’t receive a cutscene and backing barks about how Ifrit was the one who wrought the Starscourge. That’s a fine piece of information to put in an in-game codex or tie-in novel or whatever.

At the very least you must give the player time with secondary characters before fridging them if you want an emotional response. I didn’t know who Jared was before you killed him. The only reason I knew he was important was because the main characters became mopey-faced when they heard of his off-screen demise. And for the relationship in the game’s own logo I only have the characters’ words to go by to determine how much Noctis and Luna loved each other despite never being in the same place for ten years. But boy howdy was her death rendered beautifully and with excellent scoring.

If it were just the story that was uneven, I’d still be upset. But this unevenness extends throughout the title.

Barks during the fishing minigame are timed to the wrong events; only half of the casual conversations have lip-syncing; you can only have one “Kill <some monster(s)> and get <some reward(s)>” quest active at once; the control schemes for Chocobo riding, car driving, and walking all have different buttons for jump; the map doesn’t zoom in far enough to discriminate icons in town; the fog of war on dungeon maps only shows on the full map not the minimap…

I work in software. I know how bugs creep into release. But the only reason I can think of to explain three different jump buttons and unmarkable maps with different sort orders on quest lists is that Squenix ignores their interaction designers.

Story and mechanics aren’t the whole of it either: Final Fantasy XV’s representation of healthy male relationships is above anything I can remember from any Final Fantasy title. They even cry together, our roadtrip boys… if you wait midway through the credits for it. Yet relationships with women are tropish, boring, and underwritten. Despite the backlash Square Enix received after Episode Duscae (the first of the playable demos) they declined to design Cindy some mechanics coveralls or exclude superfluous car washing scenes. Iris is a schoolgirl stereotype the game cannot decide whether I’m attracted to, embarrassed by, or protective of. Luna is a damsel no matter how much we’re told her actions drive the plot. Aranea is a spinny death machine that battles in heels and bared midriff (though she almost has a character arc)…

You might think in reading this that I didn’t have fun playing FFXV and didn’t enjoy the game. I did, really… It’s beautiful, the four main characters have acceptable chemistry, story actions have story consequences, the battle system is fast and reasonably fun, the minigames are diverting, they finally learned how to communicate enemy scale, and did I mention it’s beautiful?

But when Dragon Age: Inquisition can, two years and one console generation earlier, “Open World” better that a mainline Final Fantasy… I just wonder what went wrong.

Two Days, or How Long Until The Data Is In

Two days.

It doesn’t seem like long, but that is how long you need to wait before looking at a day’s Firefox data and being sure than 95% of it has been received.

There are some caveats, of course. This only applies to current versions of Firefox (55 and later). This will very occasionally be wrong (like, say, immediately after Labour Day when people finally get around to waking up their computers that have been sleeping for quite some time). And if you have a special case (like trying to count nearly everything instead of just 95% of it) you might want to wait a bit longer.

But for most cases: Two Days.

As part of my 2017 Q3 Deliverables I looked into how long it takes clients to send their anonymous usage statistics to us using Telemetry. This was a culmination of earlier ponderings on client delay, previous work in establishing Telemetry client health, and an eighteen-month (or more!) push to actually look at our data from a data perspective (meta-data).

This led to a meeting in San Francisco where :mreid, :kparlante, :frank, :gfritzsche, and I settled upon a list of metrics that we ought to measure to determine how healthy our Telemetry system is.

Number one on that list: latency.

It turns out there’s a delay between a user doing something (opening a tab, for instance) and them sending that information to us. This is client delay and is broken into two smaller pieces: recording delay (how long from when the user does something until when we’ve put it in a ping for transport), and submission delay (how long it takes that ready-for-transport ping to get to Mozilla).

If you want to know how many tabs were opened on Tuesday, September the 5th, 2017, you couldn’t tell on the day itself. All the tabs people open late at night won’t even be in pings, and anyone who puts their computer to sleep won’t send their pings until they wake their computer in the morning of the 6th.

This is where “Two Days” comes in: On Thursday the 7th you can be reasonably sure that we have received 95% of all pings containing data from the 5th. In fact, by the 7th, you should even have that data in some scheduled datasets like main_summary.

How do we know this? We measured it:

Screenshot-2017-9-12 Client "main" Ping Delay for Latest Version(1).png(Remember what I said about Labour Day? That’s the exceptional case on beta 56)

Most data, most days, comes in within a single day. Add a day to get it into your favourite dataset, and there you have it: Two Days.

Why is this such a big deal? Currently the only information circulating in Mozilla about how long you need to wait for data is received wisdom from a pre-Firefox-55 (pre-pingsender) world. Some teams wait up to ten full days (!!) before trusting that the data they see is complete enough to make decisions about.

This slows Mozilla down. If we are making decisions on data, our data needs to be fast and reliably so.

It just so happens that, since Firefox 55, it has been.

Now comes the hard part: communicating that it has changed and changing those long-held rules of thumb and idées fixes to adhere to our new, speedy reality.

Which brings us to this blog post. Consider this your notice that we have looked into the latency of Telemetry Data and is looks pretty darn quick these days. If you want to know about what happened on a particular day, you don’t need to wait for ten days any more.

Just Two Days. Then you can have your answers.


(Much thanks to :gsvelto and :Dexter’s work on pingsender and using it for shutdown pings, :Dexter’s analyses on ping delay that first showed these amazing improvements, and everyone in the data teams for keeping the data flowing while I poked at SQL and rearranged words in documents.)


The Photonization of about:telemetry

This summer I mentored :flyingrub for a Google Summer of Code project to redesign about:telemetry. You can read his Project Submission Document here.


Google Summer of Code is a program funded by Google to pay students worldwide to contribute in meaningful ways to open source projects.

about:telemetry is a piece of Firefox’s UI that allows users to inspect the anonymous usage data we collect to improve Firefox. For instance, we look at the maximum number of tabs our users have open during a session (someone or several someones have more than one thousand tabs open!). If you open up a tab in Firefox and type in about:telemetry (then press Enter), you’ll see the interface we provide for users to examine their own data.

Mozilla is committed to putting users in control of their data. about:telemetry is a part of that.


When :flyingrub started work on about:telemetry, it looked like this (Firefox 55):


It was… functional. Mostly it was intended to be used by developers to ensure that data collection changes to Firefox actually changed the data that was collected. It didn’t look like part of Firefox. It didn’t look like any other about: page (browse to about:about to see a list of about: pages). It didn’t look like much of anything.


After a few months of polishing and tweaking and input from UX, it looks like this (Firefox Nightly 57):


Well that’s different, isn’t it?

It has been redesigned to follow the Photon Design System so that it matches how Firefox 57 looks. It has been reorganized into more functional groups, has a new top-level search, and dozens of small tweaks to usability and visibility so you can see more of your data at once and get to it faster.



Just because Google Summer of Code is done doesn’t mean about:telemetry is done. Work on about:telemetry continues… and if you know some HTML, CSS, and JavaScript you can help out! Just pick a bug from the “Depends on” list here, and post a comment asking if you can help out. We’ll be right with you to help get you started. (Though you may wish to read this first, since it is more comprehensive than this blog post.)

Even if you can’t or don’t want to help out, you can take sneak a peek at the new design by downloading and using Firefox Nightly. It is blazing fast with a slick new design and comes with excellent new features to help be your agent on the Web.

We expect :flyingrub will continue to contribute to Firefox (as his studies allow, of course. He is a student, and his studies should be first priority now that GSoC is done), and we thank him very much for all of his good work this Summer.


Another Advantage of Decreasing Data Latency: Flatter Graphs

I’ve muttered before about how difficult it can be to measure application crashes. The most important lesson is that you can’t just count the number of crashes, you must normalize it by some “usage” value in order to determine whether a crashy day is because the application got crashier or because the application was just being used more.

Thus you have a numerator (number of crashes) and a denominator (some proxy of application usage) to determine the crash rate: crashes-per-use.

The current dominant denominator for Firefox is “thousand hours that Firefox is open,” or “kilo-usage-hours (kuh).”

The biggest problem we’ve been facing lately is how our numerator (number of crashes) comes in at a different rate and time than our denominator (kilo-usage-hours) due to the former being transmitted nearly-immediately via “crash” ping and the former being transmitted occasionally via “main” ping.

With pingsender now sending most “main” pings as soon as they’re created, our client submission delay for “main” pings is now roughly in line with the client submission delay of “crash” pings.

What does this mean? Well, look at this graph from

Screenshot-2017-7-25 Crash Rates (Telemetry)

This is the Firefox Beta Main Crash Rate (number of main process crashes on Firefox Beta divided by the number of thousands of hours users had Firefox Beta running) over the past three months or so. The spike in the middle is when we switched from Firefox Beta 54 to Firefox Beta 55. (Most of that spike is a measuring artefact due to a delay between a beta being available and people installing it. Feel free to ignore it for our purposes.)

On the left in the Beta 54 data there is a seven-day cycle where Sundays are the lowest point and Saturday is the highest point.

On the right in the Beta 55 data, there is no seven-day cycle. The rate is flat. (It is a little high, but flat. Feel free to ignore its height for our purposes.)

This is because sending “main” pings with pingsender is behaviour that ships in Firefox 55. Starting with 55, instead of having most of our denominator data (usage hours) coming in one day late due to “main” ping delay, we have that data in-sync with the numerator data (main crashes), resulting in a flat rate.

You can see it in the difference between Firefox ESR 52 (yellow) and Beta 55 (green) in the kusage_hours graph also on

Screenshot-2017-7-27 Crash Rates (Telemetry)

On the left, before Firefox Beta 55’s release, they were both in sync with each other, but one day behind the crash counts. On the right, after Beta 55’s release, notice that Beta 55’s cycle is now one day ahead of ESR 52’s.

This results in still more graphs that are quite satisfying. To me at least.

It also, somewhat more importantly, now makes the crash rate graph less time-variable. This reduces cognitive load on people looking at the graphs for explanations of what Firefox users experience in the wild. Decision-makers looking at these graphs no longer need to mentally subtract from the graph for Saturday numbers, adding that back in somehow for Sundays (and conducting more subtle adjustments through the week).

Now the rate is just the rate. And any change is much more likely to mean a change in crashiness, not some odd day-of-week measurement you can ignore.

I’m not making these graphs to have them ignored.

(many thanks to :philipp for noticing this effect and forcing me to explain it)


Latency Improvements, or, Yet Another Satisfying Graph

This is the third in my ongoing series of posts containing satisfying graphs.

Today’s feature: a plot of the mean and 95th percentile submission delays of “main” pings received by Firefox Telemetry from users running Firefox Beta.

Screenshot-2017-7-12 Beta _Main_ Ping Submission Delay in hours (mean, 95th %ile)

We went from receiving 95% of pings after about, say, 130 hours (or 5.5 days) down to getting them within about 55 hours (2 days and change). And the numbers will continue to fall as more beta users get the modern beta builds with lower latency ping sending thanks to pingsender.

What does this mean? This means that you should no longer have to wait a week to get a decently-rigorous count of data that comes in via “main” pings (which is most of our data). Instead, you only have to wait a couple of days.

Some teams were using the rule-of-thumb of ten (10) days before counting anything that came in from “main” pings. We should be able to reduce that significantly.

How significantly? Time, and data, will tell. This quarter I’m looking into what guarantees we might be able to extend about our data quality, which includes timeliness… so stay tuned.

For a more rigorous take on this, partake in any of dexter’s recent reports on RTMO. He’s been tracking the latency improvements and possible increases in duplicate ping rates as these changes have ridden the trains towards release. He’s blogged about it if you want all the rigour but none of Python.


FINE PRINT: Yes, due to how these graphs work they will always look better towards the end because the really delayed stuff hasn’t reached us yet. However, even by the standards of the pre-pingsender mean and 95th percentiles we are far enough after the massive improvement for it to be exceedingly unlikely to change much as more data is received. By the post-pingsender standards, it is almost impossible. So there.

FINER PRINT: These figures include adjustments for client clocks having skewed relative to server clocks. Time is a really hard problem when even on a single computer and trying to reconcile it between many computers separated by oceans both literal and metaphorical is the subject of several dissertations and, likely, therapy sessions. As I mentioned above, for rigour and detail about this and other aspects, see RTMO.