Mozilla released the first beta version of Firefox 3.1, you can grab it over here. Here's the changelog:
Firefox 3.1 Beta 1 is based on the Gecko 1.9.1 rendering platform, which has been under development for the past 6 months. Gecko 1.9.1 is an incremental release on the previous version with significant changes to improve web compatibility, performance, and ease of use:
* Web standards improvements in the Gecko layout engine
* Added support for CSS 2.1 and CSS 3 properties
* A new tab-switching shortcut that shows previews of the tab you're switching to
* Improved control over the Smart Location Bar using special characters to restrict your search
The feature that users were all talking about before the release was TraceMonkey, this boosts rendering speeds up to 40 times faster than the previous version of Firefox. This feature has been switched off, you can enable it by typing “about:config” in the address bar.
1. Type about:config in the URL bar
3. Double-click on the entry to set the value to true
4. Close the browser window and restart the browser
I just tried it myself and it definitely feels a bit snappier than Firefox 3.0.3.