Tell me more about this intriguing future

Firefox 1.0 was released on the 9th of November of 2004, and I still remember the buzz. We were all excitedly downloading it because our browser had finally reached v1.0.

Using Firefox at that time, with all the developer extensions, gave you such an advantage over other web developers. Adding in the tabs, and the way it was predictable (CSS standards wise), and that it wouldn’t get infected with stuff as often as Internet Explorer, made it into such a joyous experience.

Now, if you had told me back then that I’d be contributing code to Firefox, I’d be laughing in your face. But then I’d stop and ask: Wait… what? Tell me more about this intriguing future!

Fast forward thirteen years. I am working at Mozilla, and tomorrow we release Firefox Quantum to the general public. It’s, as the name says, a “quantum leap” between this and previous Firefox versions.

I’m personally excited that I’ve contributed code to this release. I worked on removing dependencies on the (now defunct) Add-on SDK from the code base of Developer Tools. This means that the SDK code could be finally removed from Firefox, as the new WebExtensions format that Firefox uses now does not make use of that SDK. Results? Safer and leaner Firefox (the old SDK exposed way too many internals). Oh, and that warm and fuzzy feeling after deleting code…

So I didn’t contribute to a big initiative such as a new rendering engine or whatnot, but it’s often the little non-glamourous things that need to be done. I’m proud of this work (which was also done on time). My team were great!

Another aspect I’m very thrilled about is how this work has set us up for more successes already, as we’ve developed new tools and systems to find out ‘bad stuff’ in our code, and now we’re using these outside of the Firefox “core” team to identify more things we’ll want to improve in the upcoming months. There’s a momentum here!

Who knows what else will the future bring? Maybe in 10 years time I’ll be telling you I shipped code for the new rendering engine in Firefox indeed! One has to be open to the possibilities…

Update: my colleague Lin has explained how Firefox Quantum is a browser for the future, using modern technology.

How to solve the “aborting due to worker thread panic” error message while compiling Firefox on a virtual machine

Short answer: allocate more memory to your virtual machine.

The error is produced because the build process ran out of memory when compiling Servo’s style crate.

I tried with 4096Mb and it seems to be happily chugging along now. I guess your mileage may vary, yadda yadda… 💁🏻

This is nothing short of miraculous as it’s a Virtual Machine running Linux in a Macbook retina (with 8 Gb of RAM), which is a moderately underpowered-for-compiling-things laptop. But hey, lightweight, right?

Talking about Web Audio in WeCodeSign Podcast

I recorded an episode for the WeCodeSign podcast. It’s in Spanish!

You can download / listen from their website.

We actually talked about more than Web Audio; there’s a list of links to things we mentioned during the episode. From progressive enhancement to Firefox’s Web Audio editor, to the old PCMania tracking stories, to Firefox for iOS… lots of things!

I was really pleased with the experience. The guys were really good at planning, and did a great job editing the podcast as well (and they use Audacity!).

Totally recommended—in fact I suggested that both my fantastic colleague Belén and the very cool Buriticá are interviewed at some point in the future.

I’d love to hear what they have to say!

Throwback to the last time I recorded a podcast in Spanish–at least this time I wasn’t under a massive cold! 🙃

Web Animations: why and when to use them, and some demos we wrote

There’s been much talk of Web Animations for a long time but it wasn’t quite clear what that meant exactly, or even when could we get that new thing working in our browsers, or whether we should actually care about it.

A few weeks ago, Belén, Sam and me met to brainstorm and build some demos to accompany the Firefox 48 release with early support for Web Animations. We had been asked with very little time, but we reckoned we could build at least a handful of them.

We tried to look at this from a different, pragmatic perspective.

Most graphic API demos tend to be very technical and abstract, which tends to excite graphics people but mean nothing to web developers. The problem web developers face is not making circles bounce randomly on the screen, it is to display and format text on screen, and they have to write CSS and work with designers who want to play a bit with the CSS code to give it as much ‘swooosh’ as they envision. Any help we can provide to these developers so they can evaluate new APIs quickly and decide if it’s going to make their work easier is going to be highly appreciated.

So our demos focused less on moving DIVs around and more on delivering value, either to developers by offering side by side comparisons (JS vs CSS code and JS vs CSS performance) or to end users with an example of an style guide where designers can experiment with the animation values in situ, and another example where the user can adjust the amount of animation to their liking—we see this as a way of using the API for improving accessibility.

You can also download all the demos from the repo:

git clone

And should you use CSS or should you use JS? The answer is: it depends.

Let’s look at the same animation defined with CSS and JS, taken straight from the CSS vs JS demo:


.rainbow {
    animation: rainbow 2s alternate infinite;

@keyframes rainbow {
    0% { background: #ff004d; }
    20% { background: #ff77ab; }
    50% { background: #00e756; }
    80% { background: #29adff; }
    100% { background: #ff77ab;}


let el = document.querySelector('.rainbow');
    { background: '#ff004d', offset: 0 },
    { background: '#ff77ab', offset: 0.20 },
    { background: '#00e756', offset: 0.5 },
    { background: '#29adff', offset: 0.80 },
    { background: '#ff77ab', offset: 1 }
], {
    duration: 2000,
    direction: 'alternate',
    iterations: Infinity

As you can see, this is fairly similar.

CSS’s syntax for animations has been with us for a while. We know how it works, there is good support across browsers and there is tooling to generate keyframes. But once the keyframes are defined, there’s no going back. Modifying them on the fly is difficult unless you start messing with strings to build styles.

This is where the JS API proves its worth. It can tap into live animations, and it can create new animations. And you can create animations that are very tedious to define in CSS, even using preprocessors, or just plain impossible to predict: imagine that you want custom colour animations starting from an unknown colour A and going to another unknown colour B. Trying to pregenerate all these unknown transitions from the beginning would result in an enormous CSS file which is hard to maintain, and would not even be as accurate as a newly created animation could be.

There’s another advantage of using the Web Animations API versus animating with a library such as tween.js—the API is hardware accelerated and also runs on the compositor thread, which means incredibly smooth performance and pretty much perfect timing. When you animate DOM elements using JavaScript you have to create lots of strings to set the style property of these DOM elements. This will eventually kick a pretty massive Garbage Collector, which will make your animations pause for a bit: the dreaded jank!

Finally another cool feature of these native animations is that you can inspect them using the browser developer tools. And you can scrub back and forth, change their speed to slow them down or make them faster, etc. You cannot tap into JS-library-defined animations (e.g. tween.js) unless there’s support from the library such an inspector, etc.

Essentially: Web Animations cheat by running at browser level. This is the best option if you want to animate DOM elements. For interpolating between plain values (e.g. a three.js object position vector x, y, z properties) you should use tween.js or similar.

If you want to know more here’s an article at Hacks talking about compositors, layers and other internals, and inspecting with DevTools. And with plenty of foxes!

The full API isn’t implemented in any browser yet, but both Firefox and Chrome stable have initial support already. There’s no excuse: start animating things already! I’m also excited that both browsers shipped this early support more or less simultaneously—makes things so much easier for developers.

With mega thanks to Belén for pushing this forward and to Sam for going out of his internship way to help us with this 🙌🏼

Score another one for the web!

Last week I made a quick trip to Spain. It was a pretty early flight and I was quite sleepy and so… I totally forgot my laptop! I indeed thought that my bag felt “a bit weird”, as the laptop makes the back flat (when it’s in the bag), but I was quite zombified, and so I just kept heading to the station.

I realised my laptop wasn’t there by the time I had to take my wallet out to buy a train ticket. You see, TFL have been making a really big noise about the fact that you can now use your Oyster to travel to Gatwick. But they have been very quiet about requiring people to have enough credit in their cards to pay the full amount of the ticket. And since I use “auto top up”, sometimes my card might have £18. Sometimes it won’t, as in this case.

Anyway, I didn’t want to go back for the laptop, as I was going on a short holidays trip, and a break from computers would be good. Except… I did have stuff to do, namely researching for my next trip!

I could use my phone, but I quite dislike using phones for researching trips: the screen is just too small, the keyboard is insufferable, and I want to open many tabs, look at maps, go back and forth, which isn’t easy on a phone, etc. I could also borrow some relative’s laptop… or I could try to resuscitate and old tablet that I hadn’t used since 2013!

It had become faulty at the beginning of 2013, but I thought I had fixed it. But months after, it decided to enter its mad loop of “restart, restart, restart and repeat” during a transatlantic flight. I had to hide it in my bag and let it expire its battery. And then I was very bored during both the rest of the flight, and the flight back, as all my carefully compiled entertainment was on it. Bah! And so I stopped using it and at some point I brought it to Spain, “just in case”.

Who would have guessed I’d end up using it again!?

I first spent about 30 minutes looking for a suitable plug for the charger. This tablet requires 2A and all the USB chargers I could find were 0.35A or 0.5A. The charger only had USA style pins, but that part could be removed, and revealed a “Mickey mouse” connector, or C7/C8 coupler if you want to be absolutely specific. A few years ago you could find plenty of appliances using this connector, but nowadays? I eventually found the charger for an old camera, with one of these cables! So I made a Frankenchargenstein out of old parts. Perfect.

The tablet took a long time to even show the charging screen. After a while I could finally turn it on, and oh wow, Android has changed a lot for the better since 3.1. But even if this tablet could be updated easily, I had no laptop and no will to install developer tools on somebody else’s laptop. So I was stuck in 3.1.

The Play Store behaved weirdly, with random glitches here and there. Many apps would not show any update, as developers have moved on to use newer versions of the SDK in order to use new hardware features and what not, and I don’t blame them, because programming apps that can work with different SDKs and operating system versions in Android is a terribly painful experience. So the easiest way to deal with old hardware or software versions is just not supporting them at all. But this leaves out people using outdated devices.

One of these “discriminatory apps” I wanted to install for my research was a travel app which lets you save stuff you would like to visit, and displays it on a map, which is very convenient for playing it by ear when you’re out and about. Sadly, it did not offer a version compatible with my device.

But I thought: Firefox still works in Android 3.1!

I got it updated to the latest version and opened the website for this app/service, and guess what? I could access the same functionalities I was looking for, via the web.

And being really honest, it was even better than using the app. I could have a tab with the search results, and open the interesting ones in a different tab, then close them when I was done perusing, without losing the scrolling point in the list. You know… like we do with normal websites. And in fact we’re not even doing anything superspecial with the app either. It’s not like it’s a high end game or like it works offline (which it doesn’t). Heck, it doesn’t even work properly when the network is a bit flaky… like most of the apps out there 😛

So sending a huge thanks to all the Firefox for Android team for extending the life of my ancient device, and a sincere message to app makers: make websites, not apps 😉