FireFox 9 was officially released this past Monday. The newest version of FF totes an overall speed increase plus bug and stability fixes. The Mac version now allows for two finger swipe navigation and deeper theme integration. There is also additional HTML5 and CSS support plus updated "Do Not Track" hooks for developers.

The full list of changes from the Mozilla website are:
Added Type Inference, significantly improving JavaScript performance
Improved theme integration for Mac OS X Lion
Added two finger swipe navigation for Mac OS X Lion
Added support for querying Do Not Track status via JavaScript
Added support for font-stretch
Improved support for text-overflow
Improved standards support for HTML5, MathML, and CSS
Fixed several stability issues
Fixed several security issues

As always, its best to upgrade to the latest version asap in order to avoid any of those bugs & stability issues, so head to http://www.mozilla.org/en-US/firefox/new/ to update.