As well as these features from previous Firefox 4:

* Firefox 4 Beta is available in 76 languages
* Overhaul of the bookmarks and history code, enabling faster bookmarking and startup performance
* Per-compartment garbage collection is now enabled, reducing work done during complex animations
* The Firefox Sync setup experience has been greatly improved across desktop and mobile devices
* Speed, functionality, and compatibility improvements to WebGL
* Additional polish for the Firefox Add-ons Manager
* Uses JägerMonkey, a new, faster JavaScript engine
* Certain rendering operations are now hardware-accelerated using Direct3D 9 on Windows XP, Direct3D
10 on Windows Vista and 7, and OpenGL on Mac OS X
* Improved web typography using OpenType with support for ligatures, kerning and font variants
* HTML5 Forms API makes web based forms easier to implement and validate
* Support for the new proposed Audio Data API
* Direct2D Hardware Acceleration is now on by default for Windows 7 users
* Firefox button has a new look for Windows Vista and Windows 7 users
* Support for HSTS security protocol allowing sites to insist that they only be loaded over SSL
* 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
* Tabs are now on top by default on Windows, Mac OS X, and Linux
* 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 OS X, 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 OS X 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
* 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
* 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