The release handles a several stability issues which can be found here.
On our end, it appears that this latest version is no longer locking up under Windows 7, which was a problem on some PCs. (Google's Chrome and Microsoft's Internet Explorer have been running seemlessly under Windows 7 with Firefox exhibiting a more-than-occasional lock-up).
If you are a Firefox user and haven't been automatically updated, you can click 'Help' at the top of the browser and then 'Check for Updates' for the latest version, now 3.5.5
Updated versions in most languages, for Windows, MAC, and Linux, have been posted to the Mozilla website here.
Update (Nov 11, 2009 9 AM ET) Mozilla has now released the second Beta of Firefox 3.6 (which so far looks like a screamer?!). Over 190 changes since Firefox 3.6 Beta 1. You can find out more here.
Updated Sat Nov 7, 2009 8 AM ET