Firefox 4:
- is much, MUCH faster than before
- is multithreaded as much as 3.6.5+ for now, and the same, uses multiple processes: one process for the browser, one process to host the plugins (in case of plugins crashes (not like other browsers, that start a process per tab); while it doesn't help in case of browser bugs (a crash leads to the whole browser closing, Fx has the best crash recovery process of all I've tested
- its UI is still as open to configuration as ever, and going back to its 'legacy' UI is but a few clicks away - and once you've restored the menus, you can restore the status bar
- if Firefox 3.6 crashes a lot for you, I'd recommend backing up your shortcuts and security keys, make a list of your extensions, and then delete the user profile: more often than not, crashes are experienced due to accumulated cruft from version to version, a bad setting left behind by a plugin, a misapplied update etc. So a backup and restore should solve it. You could also open a Sync account, enable Firefox to backup all your personal settings, then delete the profile, and reconnect to Sync.
I find Chrome's interface offensive, personally, and Opera too "fluffy" - but then it's a matter of personal taste.