Looking at the Firefox 3.5 Release Noteswe see the major new features;
- Support for the HTML5
- Improved tools for controlling your private data, including a Private Browsing Mode.
- The ability to share your location with websites using Location Aware Browsing. (Try it here!)
- Support for native JSON, and web worker threads.
- Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
You can grab your copy of Firefox 3.5 final at Mozilla.com.