Firebox 4.0 beta 4 is now available for download

Firefox 4 Beta 4 is available in 39 languages.
Firefox Sync is now included by default.
A new feature gives users a visual overview of all open tabs, allowing them to be sorted and grouped.
An experimental API is included to provide more efficient Javascript animations.
Firefox now supports the HTML5 video “buffered” property.
See the complete changelist from the previous beta.

As well as these features from previous Firefox 4 Betas:

Tabs are now on top by default on Windows and OSX – Linux will be changing when the theme has been modified to support the change.
You can turn any tab into an “App Tab” by right-clicking on it and selecting “Make into App Tab” from the context menu.
Web developers can animate content using CSS Transitions.
Responsiveness and scrolling improvements from the new retained layers layout system.
JavaScript speed improvements due to engine optimizations.
Changes to how XPCOM components are registered in order to help startup time and process separation.
You can search for and switch to already open tabs in the Smart Location Bar.
New Addons Manager and extension management API (UI will be changed before final release).
Significant API improvements are available for JS-ctypes, a foreign function interface for extensions.
The stop and reload buttons have been merged into a single button on Windows, Mac and Linux.
The Bookmarks Toolbar has been replaced with a Bookmarks Button by default (you can switch it back if you’d like).
Crash protection for Windows, Linux, and Mac when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins.
CSS Transitions are partially supported.
Full WebGL support is included but disabled by default at this time.
Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently.
Native support for the HD HTML5 WebM video format.
An experimental Direct2D rendering backend is available on Windows, turned off by default.
Web developers can use Websockets for a low complexity, low latency, bidirectional communications API.
Web developers can update the URL field without reloading the page using HTML History APIs.
More responsive page rendering using lazy frame construction.
Link history lookup is done asynchronously to provide better responsiveness during pageload.
CSS :visited selectors have been changed to block websites from being able to check a user’s browsing history.
New HTML5 parser.
Support for more HTML5 form controls.
Web authors can now get touch events from Firefox users on Windows 7 machines.
A new way of representing values in JavaScript that allows Firefox to execute heavy, numeric code (used for things like graphics and animations) more efficiently.

Developers can find out about all the changes and new features at the Mozilla Developer Center.