• Uses JägerMonkey, a new, faster JavaScript engine
• WebGL is enabled by default on Windows and Mac OS X. WebGL support requires an OpenGL-capable graphics card. Support for other graphics cards on Windows (specifically Intel GPUs) and Linux will be coming in a future beta
• 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
• See the complete changelist from the previous beta

As well as these features from previous Firefox 4 Betas:

• Firefox 4 Beta is available in 57 languages
• 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 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
• 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