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.

You are actually using adobe's craptactular PDF reader? What the hell is wrong with you. Go get Foxit or one of the mayh other readers that DON'T chew up 200+Mb, install unwanted crap (photoshop album, adobe downloader, etc) and slow your computer and browser to a crawl.
I didn't even notice 3.6.4 was out yet, lmao. Time to update.
3.6.6 is only a minor update I think.
Chrome still has a long way to match FF in add-ons and customization.
That was what I was going to say and I run FF on my Mac.
Now the right question is: how do ngo's make money?