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.
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.

Sad that open source software like FF falls victim to closed source monopolists and their filthy practices... And I'm *not* pointing at Adobe and Apple.
I'm a hardcore Opera user as well and never experienced the problems you describe, execpt with the 10.50 alpha. Opera hasn't crashed a single time since 10.50, not even the 10.60 beta version I'm writing this post in.
Chrome is great, but I find it lacks features. And Google's privacy record is rather scary...