I downloaded FF3 beta some weeks ago. It's faster, it has a new downloader and an improved checker for plugins, also a password manager. When I downloaded it, GreaseMonkey, Firebug and TabMix did not work. It seems GM and TabMix work now, I haven't checked. Of my plugins (8 in total) only 2 did not work.
It's faster, but still (it seems to me) a tad slower than Opera. This are my informal times (with some applications opened):
Open
FF 5 seconds, Opera 9.5 3 seconds, IE7 2 seconds
Open NewYorkTimes page
FF 35, Opera 31, IE 40
Reopen NewYorkTimes
FF 4, Opera 8, IE 20
Still lacks an integrated bittorrent manager, IRC, text to voice translator, integrated contact list or mail (you can get them separatedly). I'll download it tomorrow, but I plan to stick with Opera.
NOTE: I don't know why after a bright start, now FF takes half a minute to open (with more apps already opened: Excel, Opera, Word, Adobe and a game). The AccuWeather thingie is not working either. It's still fast for reloading a cached page.