- Added support for font-stretch
- Improved support for text-overflow
- Improved standards support for HTML5, MathML, and CSS
You can obtain the new version via the built-in update mechanism (Help / About Firefox / Check for Updates), or you can download a complete installation package, in a variety of (human) languages.
Update Wednesday, 21 December, 20:28 EST
The WebMonkey site has a review article discussing the changes in Firefox 9.0.