Seems like Firefox 3.6 got released recently, for anyone who uses Firefox.
The main features that you probably care about is the new Personas theme system and some speed improvements.

For more details, see the release notes.

Or you can download it here: http://www.mozilla.com/en-US/firefox/all.html
For Windows 64-bit version, you can get it from here: http://wiki.mozilla-x86-64.com/Firefox:Download (note that Flash does not natively work on x64 versions, though there is a development x64 build of Flash)