A more stable Firefox. Always a good thing.
If you noticed that your Firefox asked if it could update your browser to 3.6.4, it has good reason to – it's for crash protection.
UPDATE: The version jumped to 3.6.6 over the weekend. The main new feature is still crash protection, but going from 3.6.4 to 3.6.6 resulted in a single extra bug fix (increase hang detector timeout). If you're really curious to see what makes 3.6.6 the new hotness over 3.6.4, check out the change log here.
This latest build of Firefox isolates third party plug-ins in a separate process, so when they crash, they won't bring the entire browser down with it. This is something that's already done by other browsers, like Chrome. (Isn't competition of free products great?)
Mozilla wrote in its blog post:
Results from our beta testing show Firefox 3.6.4 will significantly reduce the number of Firefox crashes experienced by users who are watching online videos or playing games. When a plugin crashes or freezes while using Firefox, users can enjoy uninterrupted browsing by simply refreshing the page.
The new build and feature is available now for Windows and Linux.