Firefox 4 Beta 9 – a huge pile of awesome

Hello, and welcome to the post for Firefox 4 beta 9. If you’re reading this then you’re interested in what we’ve got coming down the pipe for the latest beta for the wonderful browser known as Firefox 4. We’re starting to reach the end of our development cycle for the next release of Firefox and I thought it might be worth it to give a full overview of everything that we’ve got in store for web developers vs. Firefox 3.6. So if you’ve read previous releases, some of this will look familiar. But this is a new beta and we’ve been busy, so there’s something new in here for everyone.


One of Firefox 4’s main themes is Performance. As such, we’ve done a huge amount of work to improve browser performance across the board, from startup time, to JavaScript performance, to interactive performance and making the user interface easier and more efficient to drive. That being said, here are some of the things we’ve done to improve performance up to Beta 9.

The JaegerMonkey has landed.

And you might have noticed that it’s really fast. This is the world’s first third-generation JavaScript engine, using Baseline JIT technology similar to engines found in other browsers and kicked up a level with the Tracing engine found in Firefox 3.6. As such, we’re competitive on benchmarks such as Sunspider and V8, but we’re also fast at a whole mess of things that we expect to see in the set of next-generation web applications, hence Kraken.

Hardware Acceleration

Firefox 4 supports full hardware acceleration on Windows 7 and Windows Vista via a combination of D2D, DX9 and DX10. This allows us to accelerate everything from Canvas drawing to video rendering. Windows XP users will also enjoy hardware acceleration for many operations because we’re using our new Layers infrastructure along with DX9. And, of course, OSX users have excellent OpenGL support, so we’ve got that covered as well.


Compartments, added in Beta 7, is infrastructure that we’ve added to Firefox to improve performance, give us a better base for security and, over the long term, give us better responsiveness because of its positive effects on garbage collection. (Note that there are some parts of Compartments that still have not landed.) If you want to learn about why Compartments are so important, have a look at Andreas Gal’s great post on the topic. He does a great job of explaining what they are and how they work.

Improved DOM performance and style resolution

We’ve made a huge number of improvements to our overall DOM and style resolution performance as well. Although JavaScript gets much of the focus these days, it turns out that in page load and interactive performance, the DOM and style resolution (usually as part of reflowing a document) often affect page performance more than JavaScript. And as such we’ve seen improvement across the board in this area with Firefox 4. In some tests we’re a solid 2x faster than Firefox 3.6.

JavaScript typed arrays

If you’re doing processing with JavaScript and you want to process low-level data, Firefox now supports native typed arrays in JavaScript. This can make a gigantic difference in how fast items are processed if you’re doing WebGL or image manipulation.

In addition, if you’re getting data from an XHR request, you can get that data as a JavaScript native typed array for fast processing with mozResponseArrayBuffer.

JavaScript animation scheduling APIs

We now support events that let you schedule animations from JavaScript. Doing this means you aren’t wasting CPU and battery by running your animations as fast as possible when the underlying rendering engine isn’t going to draw them anyway. (Gecko now has an internal timer for all drawing and will render at a maximum of 60fps, the edge of human perception. It will also turn this timer down to 1fps for tabs that aren’t in the foreground. Use it.) This is most useful for people building libraries, but you should be aware that there’s a better way to animate in Gecko.

Retained layers

As mentioned above, Firefox now has an internal Layers system for improved drawing performance for many types of web pages. Images, fixed backgrounds, inline video, etc. are now rendered to an internal layer and then composited with other parts of a web page in a way that can take advantage of hardware acceleration. This improves the page load time and interactive performance of many web pages.

Asynchronous plug-in painting

On Windows and Linux we now paint plug-ins asynchronously. In previous releases when we wanted to paint a web page that included a plug-in, we would have to ask the plug-in for the data to paint. If a plug-in was slow or was hung, it would make the browser feel slow. Now we’re able to get that data asynchronously, which means that the overall responsiveness of the browser should be improved.

Vastly improved cache

Firefox 4 has a vastly improved disk cache. This disk cache should result in faster start-up performance and runtime performance. But why mention this here? Our expectation is that site owners should see much higher cache hit rates with Firefox 4 as we dynamically set the cache size based on how much space is free on an end-user’s hard drive.


Firefox 4 now has WebGL enabled by default. Based on the original 3-D Canvas work by Vladimir Vukićević, this is being widely implemented in browsers. The WebGL spec is on the short path to a 1.0 release and we’re very excited to see this be used in the wild.

Want a demo? Try the amazing Flight of the Navigator where we combine WebGL with a number of other technologies in Firefox 4 — HTML5 video, the Audio API, all remixed with live data from the web.


The new JaegerMonkey engine in Firefox 4 isn’t just faster, it’s also got some new tricks as well. Our goal is to bring new tools to developers and that includes evolving the JavaScript language. Here are two examples of where we’ve made other improvements:

ECMAScript 5

Firefox 4 Beta 9 contains much of the new version of the JavaScript language. Our goal with Firefox 4 is to have industry-leading support for ECMAScript 5, including the new strict mode. You can track our progress on the ES5 bug.

Web Console

Firefox 4 will include the Web Console. This is a new tool that will let you inspect a web page as it’s running, see network activity, see messages logged with console.log, see warnings for a page’s CSS, and a number of other things.

Note this that is something that we’ll be including with Firefox 4 directly. It’s not an add-on.

(Also Firebug will be ready for the final Firefox 4 release.)


Firefox has always had excellent support for HTML5, even as far back as Firefox 3.5. Firefox 4 builds on that history with a bunch of new HTML5-related features.


We’ve started to implement much of HTML5 forms. We include support for new input types, datalist support, new input attributes like autofocus and placeholder, decoupled forms, form options, validation mechanisms, constraint validation and new CSS selectors to bind them all together. There’s a great post up on the Hacks site covering our HTML5 forms support in Firefox 4. If you want more information, check it out.


Firefox 4 includes an HTML5-ready parser. This parser brings with it some new capabilities, most notably inline SVG, but also improves performance by running the parsing algorithm on its own processor. It also brings our parser algorithm closer to the standard and lays the foundation for consistent parsing across browsers.

Support for WebM

Firefox 4 includes support for WebM, the new royalty-free format for video on the web. It works on YouTube if you join their HTML5 beta. It also works with embedded YouTube videos if you use their new iframe embedding API. (Join the beta and see an example at the bottom of this post.)

Video Buffer API

We now support the buffered attribute for HTML5 video. This means it’s possible to give users an accurate measure of how much video has been buffered instead of having to guess from the download rate and your current position in the stream.

Video “preload” support

We’ve replaced the “autobuffer” attribute for HTML5 video with the new “preload” attribute. This attribute gives you much better control over how videos are pre-buffered if they are included on a page rather than the old on/off system that was included with Firefox 3.5.

History pushState and replaceState

Firefox 4 now supports the HTML5 pushState and replaceState history modification calls. These allow you to create or modify the browser navigation history. These are extremely useful for applications that might want to generate history entries using the hash after the URL (useful for HTML-based slides, for example.) Other sites have been using these APIs to hide history information so that site-internal navigation aids aren’t revealed as part of HTTP-referrers.

Audio sampling and generation API

Firefox 4 includes the Firefox Audio Data API. This API allows you read, modify and write data from audio and video elements. This is now being widely experimented with and people are building some very beautiful things with it.


Firefox 4 includes a bunch of new DOM functionality, in addition to the performance improvements I mentioned before.

Added support for .click() to the File upload control

You can now call .click() on a hidden File control in order to bring up the platform file upload widget. This means you don’t have to expose the (ugly) file upload control; instead you can build your own. If you combine this with the new File APIs and progress events and you’ve got a recipe for a very nice file upload experience.

Support for .slice in the File API

We now support the Blob API and the .slice APIs that come with it. This is useful for people who want to want to process small parts of an otherwise large File object from JavaScript. You no longer have to load the entire file into memory.

It’s also useful for people who want to reliably upload large files. With some server and JS code you can now split a large file into small sections and upload those chunks, including re-retrying failed sections, or even uploading several sections in parallel.

Support for File API URLs

We now support the .url attribute for the File API. This lets you take objects that you’ve gotten through the File API and use them for images, video, HTML or other URL-powered objects.

Touch and multi-touch events

Firefox 4 supports both touch and multi-touch events, exposed to the DOM. Support for this functionality is available on Windows 7 systems.

Detect click-to-touch

You can now tell if a mouse or finger generated an event with the mozInputSource property. This is useful with the touch and multi-touch events and makes it possible to build apps that treat touch and mouse input differently.


Firefox 4 will include a super-early version of IndexedDB. This emerging standard for local storage is still undergoing change and will be private-prefixed until it’s stable. We have some early API docs on IndexedDB, but the spec and implementation we have in Firefox 4 have both changed since the docs were last updated. As we get closer to release time we’ll be updating the docs and talk more about the implementation. Meanwhile, you can read the IndexedDB primer for an overview of using the IndexedDB API.


Firefox 4 includes support for the new FormData object, which makes it easier to interact with HTML forms. It also enables some new capabilities, like making it easy to upload a file as part of a form accessed via the File API.

SVG Animation and SMIL

In Firefox 4 you can now animate SVG with SMIL.

SVG as Images and CSS backgrounds

You can now use SVG as the source of an <img> tag. You can even animate them with SMIL. And you can also use SVGs as backgrounds in CSS.

Get a Canvas as a File object

Many people wanted a Canvas accessible as a file object for uploads and other purposes. You can now use the mozGetAsFile() method on a canvas and it will return an image file.

Resizable text areas

If you’ve been using a beta, you’ve noticed that text areas are now resizable by default. You can disable that with the new resize CSS property.


Firefox 4 includes a huge pile of new CSS properties and promotions from the private CSS namespace into the final namespace due to the maturation of some of the standards in this space.

CSS Transitions

Firefox 4 will include support for CSS Transitions. Since the spec is still early, these are still -moz prefixed extensions.


We now support an early version of calc private-namespaced as -moz-calc. This will let you use simple expressions anywhere you would normally want to use a length. This can make the CSS layout of pages much simpler. (No more divs for spacing!)


We now support a new extremely useful CSS extension: -moz-any() selector grouping. This will be very useful in HTML5 contexts. Please read the post for more information.


The -moz-element is an extension to the background-image property that lets you use any element as the background for another element. This is hugely useful & powerful and we hope that other browsers will adopt it as well.


The :-moz-placeholder allows you to change the attributes of background text that’s a placeholder in an HTML5 form. This allows you to change the color or other attribute of the text. This can be very useful if you’ve changed the style of the actual text box and you want the background text to match.


The border-radius attribute is now supported, replacing the old -moz-border-radius attribute.


box-shadow has replaced -moz-box-shadow.


Firefox 4 will include support for exposing much more of the capabilities of TrueType fonts with the -moz-font-feature-settings property. It’s possible to take advantage of all kinds of font capabilities — kerning, ligatures, alternates, real small caps, and stylistic sets, to name just a few.

Consistent CSS units

We’ve changed our handling of pixel sizes to match Internet Explorer, Safari and Chrome so that 1 inch always equals 96 pixels. See Robert’s post for more information on these changes.

Support for a physical CSS unit

We’ve introduced a new CSS unit, mozmm for the rare instance where you actually want a physical size to be used. Once again, see Robert’s post for more information on this new unit.


Firefox now supports the -moz-device-pixel-ratio media query that gives you the number of device pixels per CSS pixel.


As mentioned above, we now have a resize CSS property that lets you disable resizable input text areas.


We now support the -moz-tab-size property that lets you specify the width in space characters of how to render a tab character (U+0009) when rendering text.


The new CSS pseudo-selector, -moz-focusring lets you specify the appearance of an element when it’s focused and it would have a focus ring drawn around it. Different operating systems have different conventions for when to draw a focus ring or not and this lets you control the look of form controls while keeping to platform conventions.


The new -moz-image-rect lets you use a sub-rectangle of an image as part of a background or background-image.


Last but not least, Firefox 4 supports a huge number of new security tools, useful for both users and web developers alike. Here’s a quick overview of the new technologies we’re delivering:

Content Security Policy

Content Security Policy (CSP) is a set of tools that web developers can use to help prevent a couple different classes of attacks. In particular, it offers tools to mitigate cross-site scripting attacks, click-jacking and packet sniffing attacks.

Another important piece of CSP is that when one of the rules is violated, Firefox can send back information about that violation to the web site, making it a useful canary to improve security for other browsers too.


Firefox 4 now supports the X-Frame-Options header, one defense against clickjacking. This response header is supported by other browsers as well. (This was also delivered as part of a Firefox 3.6 update, but is worth calling out since it’s new since the original 3.6 release.)


Firefox 4 supports the obtusely-named HTTP Strict Transport Security (HSTS) headers. You can use these headers to tell the browser that it should never, ever contact a site over unencrypted HTTP.

Firefox users can also use the STS UI add-on to add and remove sites from the HSTS list, even sites that don’t natively support HSTS.

CORS Improvements

We’ve fixed some bugs in our CORS implementation.

:visited changes

Firefox 4 includes the changes required to help improve your privacy online by closing a decade-old hole in CSS rules that let any website query your browsing history. These changes have also been picked up by WebKit-based browsers and we’ve heard rumors that IE 9 might pick up this important change as well.

That’s a lot of stuff

Yes, it really is! We hope that you like it.


  1. Mike

    After using the Betas for sometime, switching to 3.6 is just… ugh.

    So while there are obviously a lot of things that can be done to push FF further, I for one and really happy to see the UI changes, startup speed, JM… the Lot. Its great!

    I run 4b10 portable so i don’t use the IE7 installed on this company’s computer, and should I remind you guys on this XP 32bit computer, Sunspider is done in something like 250ms… where IE7 is something like 22,000ms. The biggest jump is one test is done 2800 times faster.

    We’re never going to get it perfect, but certainly we are starting to lay down some ground breaking stuff. No all the issues reported are FF, some are users ignorance, others the OS (and im not pointing my finger at the Linux users), and some are just probably poorly configured stuff. who knows, or cares. Fix it or use… whatever else. why come to mozilla and just complain.

    I think we all need to be happy that we have this new browser coming out and its like what FF3 did to FF2… I remember the day FF3 came out and was just… completely wowed. Using the Beta, I feel the same way. And yea, I don’t use the panorama thing, i think it sucks too!… and blank tabs are dumb as well, but who cares… Someone’ll come up with something and if your that upset about it, well… instead of bitching, man up and do it, or find someone to do it for you. Go ahead… use IE.

    January 29th, 2011 at 13:17

  2. R

    I hope 4 matches all the hype. I was an early user of Firefox and used to recommend it to all who would listen but gave up on it last year and switched to Chrome and the regular fallback, IE. Too many crashes and other problems with 3. I’ll wait until the final release of 4 to try it but I’ll be gone quickly after the first sign of trouble. Developers may be OK with a bad browser they can tinker with but I just want something that works. I run Win7 Pro.
    I’m good with followup comments but no spam please. Thanks

    February 9th, 2011 at 16:06


    This really sounds great. I have been using FireFox 4 and it keeps crashing and will not clear the history. I uninstalled it and hope that when I download FireFox 4 9 it will be much better.

    February 14th, 2011 at 21:38

  4. Wissem

    Firefox is all about the Open Web right ? You guys need to do more and more about Linux Desktop users.I bet that Linux users are the most bug reporters you had from the very begining of Firefox beta ;)

    March 3rd, 2011 at 17:31

    1. Paul Rouget

      > I bet that Linux users are the most bug reporters you had from the very begining of Firefox beta ;)

      No, you’re wrong here.

      But yes, we know that Linux users are important :)

      March 3rd, 2011 at 18:34

  5. Siddharth

    Firefox 4 is really cool. Ever since I started using Firefox (Fanboy since FF2.) It has crashed like 10 times? Less than 10 I guess. I love the skin of Chrome but I don’t want to leave Firefox. So I downloaded Firefox 4 and installed a Chrome skin. What was cool was, I was able to see the Panorama button on the far right. I wasn’t able to see it in the default theme. But now I could.

    Firefox rocks! :)

    I have all the tricks and tips of Firefox 4 listed here.

    March 3rd, 2011 at 22:41

  6. Lars

    Letting any website modify (read and write?) my browser history sounds a like a really bad idea (-> privacy). Also, I often review my history to find an article I read the other day. What if the website doesnt want me to and deletes any specific links, so that I always have to start from the front page again (to see more adds along the way)?

    If you let any website popup an Open / Save / Upload dialog without my explicit wish, please at least keep the “Really want to upload these files?” dialog ;-)

    The other features sound great!

    March 4th, 2011 at 04:26

  7. pip010

    jaaa PUT some attention to Linux users. Mozilla you are a shame to OSS!

    April 18th, 2011 at 03:37

    1. louisremi

      Many developers at Mozilla do use Linux as their main OS and report bugs/issues they encounter on their platform. Of course, more attention and time is spent on the most widely used platform: windows.

      April 18th, 2011 at 09:09

  8. arbsb

    I think firefox 4 should have an option to retain its old UI, at least until people gain confidence to try and get used to the new

    December 12th, 2012 at 11:18

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