http://www.mozilla.com/en-US/firefox/4.0/releasenotes/

What’s New in this Firefox 4 RC

General stability, performance, and compatibility improvements
See the complete changelist from the previous beta

As well as these features from previous Firefox 4 Betas:

Firefox 4 RC is available in 79 languages
Support for the proposed Do Not Track ("DNT") header
Connection status messages are now shown in a small overlay
WebGL is enabled on all platforms, but, like for other hardware acceleration features, many buggy drivers or driver versions are blocked
The default homepage design has been refreshed
Firefox no longer switches into offline mode automatically
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
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

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