New features include migration of Chrome history, bookmarks and cookies, synching of addons between computers, and support for CSS text-size-adjust property. Here's the full changelog:
What’s New
NEW: Firefox can now migrate your bookmarks, history, and cookies from Google Chrome
NEW: With Sync enabled, add-ons can now be synchronized across your computers
NEW: The CSS text-size-adjust property is now supported
CHANGED: Redesigned media controls for HTML5 video
HTML5: The outerHTML property is now supported on HTML elements
HTML5: View source syntax highlighting now uses the HTML5 parser (see bug 482921)
DEVELOPER: The Style Editor for CSS editing is now available to web developers
DEVELOPER: Web developers can now visualize a web page in 3D using the Page Inspector 3D View
DEVELOPER: SPDY protocol support for faster page loads is now testable
DEVELOPER: XMLHttpRequest now supports HTML parsing
DEVELOPER: Files can now be stored in IndexedDB (see bug 661877)
DEVELOPER: Websockets has now been unprefixed
FIXED: Various security fixes
FIXED: Firefox notifications may not work properly with Growl 1.3 or later (691662)
Known Issues
If you try to start Firefox using a locked profile, it will crash (see 573369)
For some users, scrolling in the main GMail window will be slower than usual (see 579260)
Some synaptic touch pads are unable to vertical scroll (see 622410)
OS X: WebGL performance may be degraded on some hardware (see 713305) - Resolved in v12
Windows: The use of Microsoft's System Restore functionality shortly after updating Firefox may prevent future updates (see 730285)