Firefox 3.5 Beta 4 features a Private Browsing Mode, faster performance with the new TraceMonkey JavaScript engine, improvements to the Gecko layout engine, native JSON support and various other improvements. Here's the full changelog:
Improved tools for controlling your private data, including a Private Browsing Mode. Better performance and stability with the new TraceMonkey JavaScript engine. The ability to provide Location Aware Browsing using web standards for geolocation. Support for native JSON, and web worker threads. Improvements to the Gecko layout engine, including speculative parsing for faster content rendering. Support for new web technologies such as: HTML5