Firefox 3 is focused on improved memory handling, performance, and stability, improved XUL, and new core components such as application data stored in SQLite. Firefox 3 could break a few existing extensions and applications built on top of Firefox, and it will definitely include new optimizations if you like to build on the popular browser. Firefox 3 should have additional JavaScript and SVG features if you're into that sort of thing.
Download: Mozilla Firefox 3.0 Pre Alpha 4 For Windows