Firefox 3.1 Beta 2.0 includes the following features:

* This beta is now available in 54 languages - get your local version.
* A new Private Browsing Mode that allows you to browse without Firefox storing any traces of where you’ve been - perfect for online holiday shopping!
* New functions that make it easy to remove the history of your past few hours of browsing, or remove all traces of a website.
* New support for web worker threads.
* The new TraceMonkey JavaScript engine is now on by default for web content.
* Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
* Removed the new tab-switching & preview behavior based on feedback from Beta 1 users
* Support for new web technologies such as the and elements, the W3C Geolocation API, JavaScript query selectors, CSS 2.1 and 3 properties, SVG transforms and offline applications.

Further discussion on this is obtained from the forum webmasterworld.com/firefox_browser/3802992.htm