The boys have just released yet another beta, which is said to improve the following:
Firefox 3.1 Beta 3 is based on the Gecko 1.9.1 rendering platform, which has been under development for the past 9 months. Firefox 3.1 is an incremental release on the previous version with significant changes to improve web compatibility, performance, and ease of use:
* This beta is now available in 64 languages – get your local version.
* Improved the new Private Browsing Mode.
* Improvements to web worker thread support.
* New native JSON support.
* Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
Further information can be had here: