Firefox 52: Introducing WebAssembly, CSS Grid and the Grid Inspector


It is definitely an exciting time in the evolution of the web with the adoption of new standards, performance gains, better features for designers, and new tooling. Firefox 52 represents the fruition of a number of features that have been in progress for several years. While many of these will continue to evolve and improve, there’s plenty to celebrate in today’s release of Firefox.

In this article, the Developer Relations team covers some of the most innovative features to land, including WebAssembly, CSS Grid, the CSS Grid Inspector Tool, an improved Responsive Design Mode, and Async and Await support for JavaScript.

WebAssembly breaks barriers between web and native

Firefox 52 supports WebAssembly, a new format for safe, portable, and efficient binary programs on the Web. As an emerging open standard developed by Mozilla, Google, Microsoft, and Apple, WebAssembly will eventually run everywhere that JavaScript does: in every major browser, and in browser-derived runtimes like Node.js and Electron. WebAssembly is designed to be ubiquitous.

Compilers like Mozilla’s Emscripten can target the WebAssembly virtual architecture, making it possible to run portable C/C++ on the web at near-native speeds. In addition to C/C++, the Rust programming language has preliminary support for WebAssembly, and LLVM itself includes an experimental WebAssembly backend. We expect many other languages to add support for WebAssembly in the coming years.

Via Emscripten, WebAssembly makes it straightforward to port entire games and native applications to the Web, but it can also do much more. Thanks to its speed and easy interoperability with JavaScript, tasks which were previously too demanding or impractical for the web are now within reach.

(You can click here to play the full Zen Garden demo. Firefox 52 required, desktop only at this time. )

JavaScript functions can call WebAssembly functions, and vice versa. This makes it possible to mix-and-match between high-level JavaScript and low-level C/C++/Rust within a single web application. Developers can reuse WebAssembly modules without needing to understand their internals, much as they do today with minified JavaScript libraries.

In areas where consistent performance is most important—games, audio/video manipulation, data analysis, raw computation, codecs, etc.—WebAssembly offers clear advantages in size and speed. For this reason, we expect that many popular libraries and frameworks will eventually come to rely on WebAssembly, either directly or indirectly.

In terms of code reuse and software architecture, the wall between “native” and the web is falling, and this is just the beginning. Tooling and debugging will continue to improve, as will interoperability, performance, and raw capabilities. For example, multi-threading and SIMD are already on the WebAssembly Roadmap.

Get started with WebAssembly on MDN, and find the latest information direct from the creators of WebAssembly at

CSS Grid and the Grid Inspector

Firefox 52 includes support for CSS Grid Layout Module Level 1, a CSS specification that defines 18 new CSS properties. CSS Grid is a two-dimensional layout system for the web, making it much easier to code many of the layout patterns we’ve been solving for years using grid frameworks, natively in the browser. And it opens up a world of new possibilities for graphic design. Whether you are focusing on user interfaces for app experiences or the editorial design of content, there’s a lot of power for you in this new toolset.

CSS Grid works by defining rows and columns, and placing items into areas on the grid. The rows and columns can be given a specific size (fixed, fluid, or a mix), or they can be defined to resize themselves depending on the size of content. Items on the grid can be explicitly placed in CSS, or they might be placed by the browser according to the Grid auto-placement algorithm. These sizing and placement options give CSS Grid more power and flexibility than any of the existing layout frameworks. In addition, the ability to define and place things in rows is completely new.

"variations on a grid" screenshot

We are also proud to announce our new Grid Inspector Tool, which allows you to see the grid lines directly on the page, making it easier to see what’s happening.

See the examples from this video at And find a playground of more examples at

Interested in learning Grid? We have in-depth guides on MDN:

Here are answers to the two most frequently asked questions about CSS Grid:

Should I use Grid or Flexbox? Which is better?

You’ll use both, mixing CSS Grid with Flexbox and the other CSS Properties that affect layout (floats, margins, multicolumn, writing modes, inline block, etc.). It’s not a choose-only-one situation. Grid is the right tool when you want to control sizing and alignment in two dimensions. Flexbox is the right tool when you are only concerned with controlling one dimension. Most projects will use both, each on a different little piece of the page. Once you understand the differences between the two, it’ll be clear how they work together brilliantly.

Why should I get excited about CSS Grid now? Won’t it take years before Grid is supported in enough browsers to be able to use it?

Because of changes to the way browser companies work together to create new CSS, wide support for CSS Grid will arrive at an unprecedented speed. Mozilla is shipping support first, in Firefox 52 on March 7th. Chrome 57 will support Grid a week later, on March 14. Safari 10.1 will ship support for Grid; it’s currently in beta. Internet Explorer 10 and 11 already have support for a much earlier version of the specification, behind an -ms prefix. (You may want to utilize it, or you may not. Learn about the details before deciding.) MS Edge also has current support for the original draft of the spec, with an update to the current spec coming sometime in the future.

You can ship websites that use CSS Grid today, before 100% of your users have a browser with CSS Grid, by thinking through the structure of your code and planning for what happens in all browsers. Feature Queries are a key tool for making sure all users have a good experience on your site.

Async functions and the await keyword

Firefox 52 also includes a brand new JavaScript feature from ES2017: asynchronous functions and their companion, the await operator. Async functions build on top of ES2015 Promises, allowing authors to write asynchronous code in a similar way to how they would write their synchronous equivalents.

Take the following example, which takes the result of one asynchronous request, and uses part of it as the argument to a second asynchronous function. Here’s how it would look with a traditional callback approach:

function loadProfile() {
  getUser(function (err, user) {
    if (err) {
    } else {
      getProfile(, function (err, profile) {
        if (err) {
        } else {      

Relatively straightforward, but if we were to need to do additional processing and asynchronous requests, the levels of nesting or series of callback functions could become difficult to manage. Also, with more complex callback sequences, it can become difficult to determine the flow of the code, making debugging difficult.
Promises, introduced in ES2015, allow for a more compact representation of the same flow:

function loadProfile() {
    .then(function (user) {
      return getProfile(;

Promises excel at simplifying these sequential method sequences. In this example, instead of passing a function to getUser and getProfile, the functions now return a Promise, which will be resolved when the function’s result is available. However, when additional processing or conditional calls are required, the nesting can still become quite deep and control flow can again be hard to follow.
Async functions allow us to re-write the example to resemble the way we would write a synchronous equivalent, without blocking the thread the way the synchronous code would!:

async function loadProfile() {
  try {
    let user = await getUser();
    displayProfile(await getProfile(;    
  } catch (err) {

The async keyword in front of the function tells the JS engine that the following function can be paused by asynchronous requests, and that the result of the function will be a Promise. Each time we need to wait for an asynchronous result, we use the await keyword. This will pause execution of the function without stopping other functions from running. Also, getUser and getProfile don’t need to be changed from how they would be written in the Promise example.
Async functions aren’t a cure-all for complex control flow, but for many cases they can simplify the authoring and maintenance of async code, without importing costly libraries. To learn more see the async and await MDN documentation.

Responsive Design Mode

In addition to the Grid Inspector described above, Firefox now includes an improved Responsive Design Mode (RDM). The improved RDM tool can do network throttling, simulating various connection speeds primarily experienced by mobile users. In addition, various screen size and pixel density simulations are available for common devices. Many of the features were described in an earlier post introducing RDM. Currently this feature is only enabled if e10s is also enabled in the browser. Be sure to read over the complete documentation for RDM on MDN.

More Firefox 52 goodness

These are some highlights of the game-changing features we’ve brought to the browser in Firefox 52. To see the a detailed list of all release changes, including a feature for identifying auto-generated whitespace, and the ability to detect insecure password fields, see the Firefox 52 Release notes.

About Dan Callahan

Engineer with Mozilla Developer Relations, former Mozilla Persona developer.

More articles by Dan Callahan…


  1. Luke

    Is this supposed to show more than a black screen? Firefox 52.0b9

    March 7th, 2017 at 19:06

    1. Jason Weathersby

      Did you update Firefox?

      March 9th, 2017 at 12:50

  2. Ravi Kasireddy

    great article! indeed!!

    March 8th, 2017 at 04:54

  3. Travis Doherty

    Nice release!

    March 8th, 2017 at 07:51

  4. chathuranga

    Great article.. thanks

    March 8th, 2017 at 09:05

  5. Igor Cavour Oliveira

    error on firefox 52 (epic zen garden demo)
    Ubuntu 14.04 (live-cd)

    Initial memory size: 320MB (MIN_MEMORY: 32MB, IDEAL_MEMORY: 320MB, MAX_MEMORY: 2047.9375MB, RESERVE_MAX: true, heuristicIs64Bit: true) EpicZenGarden.html:105:3

    [editor’s note: I copied off the details of your error report, and passed them along for further investigation. Thanks!]

    March 8th, 2017 at 20:50

  6. Steve McWilliam

    Just tried the Zen Garden Demo and it says that WebGl isn’t supported please try Firefox 52 – I am currently running on 52.0 (32 bit) – so why isn’t it working ???

    March 9th, 2017 at 08:34

  7. Paweł P.

    “SyntaxError: illegal character” – some error on Zen Garden.
    Nice job for for the future – like I always say – Web Browser is place where “all” meets :)

    Paweł Pałczyński
    Agencja Interaktywna – DTL.PL

    March 9th, 2017 at 13:02

  8. Dan kokot


    March 9th, 2017 at 14:42

  9. Michal Hantl

    Web Assembly!!!!

    March 9th, 2017 at 23:02

  10. Adam

    WebAssembly is really interesting.
    Do I understand well that with WebAssembly it is possible to write more complex applications working off-line but with a web-technology-based UI?
    Or do I have to worry about websites calling Windows API DLLs without notice?

    March 9th, 2017 at 23:48

    1. Dan Callahan

      Yes, if you combine WebAssembly with a technology like Service Workers, you could make complex applications that work completely off-line and use HTML/CSS/JavaScript for UI.

      As for safety, WebAssembly is generally implemented within each browser’s existing JavaScript VM (Spidermonkey for Firefox, V8 for Chrome, etc.), and runs with the same sandboxing and restrictions as JavaScript. Thus, there shouldn’t be an appreciable number of exploits that are unique to WebAssembly.

      March 10th, 2017 at 07:37

  11. giorgos

    trying to run the webassembly demo:

    Your browser does not support WebAssembly. Please try with Firefox 52 or newer.
    Current user agent: Mozilla/5.0 (Windows NT 5.1; rv:52.0) Gecko/20100101 Firefox/52.0

    March 12th, 2017 at 07:43

  12. Joshua Cogliati

    Hm, all I get from the EpicZenGarden is it downloads for a bit, and then I get: “QuotaExceededError TypeError: is null” (This is on Firefox 52.0)

    March 12th, 2017 at 07:49

  13. Akash

    Thank You! hey i have one query about this is css3 syntax right? if i’m correct then why not in my browser showing me the correct grid?

    March 12th, 2017 at 10:39

  14. Gary H

    I really HATE Firefox 52 !!!
    Dropping support for Java is like shooting yourself in the foot.
    Businesses have developed programs for customers to use that require Java runtime support in the browser.
    For Mozilla to drop support of Java means users must find another browser.
    It will take along time for Businesses to replace existing Java based programs with something else. In the mean time, Firefox 51 or Firefox 52-ESR can be
    used , but the clock is ticking on their life time.

    About this WebAssembly stuff, does that mean that every time the user starts an application that it gets recompiled before it gets executed ?
    If so, where is the stability in that ? The end user is faced with an unstable environment every day. ARGH !!!

    March 13th, 2017 at 08:45

    1. Johnny Walker

      No point in getting annoyed with FireFox, this is a continued trend throughout the the industry. Oracle themselves are killing off the plugin.

      March 14th, 2017 at 06:44

  15. Eliah

    Will there be an API to use WebAssembly instead of JS for developing WebExtentions.

    March 13th, 2017 at 09:04

  16. Johnny Walker

    Here’s what the Zen Garden demo shows me:

    March 14th, 2017 at 06:46

  17. Albert

    Kudos to the team! I hope that Edge will get an update from their early practical test version for CSS Grid soon, too. Once Safari and Opera have been updated it’d be pretty much ready to go :)

    The ZenGarden sample fails for me in 52 (and 54) as well. There are many errors in the console, started with “UnknownError”, “Failed to openIndexedDB, proceeding without reading or storing contents to IndexedDB! Error: “, “Failed to read file wasmModule from IndexedDB, error: IndexedDB not available!”, …

    It works fine and smooth in Nightly 55.

    March 17th, 2017 at 05:42

  18. Jhons

    Hi nice feature about web assemly and css grid as i am looking to develop Firefox extension and i read the article at but still unable to understand… Kindly guide me

    March 28th, 2017 at 16:11

    1. Dan Callahan

      The official Firefox Add-on docs at are a great place to start

      March 31st, 2017 at 14:28

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