On Windows XP I use 3.5, on Linux I use 3.0 both are fine - saw a comparison chart of speeds Firefox vs other browsers somewhere and it came out very well.
I'm on v3.5.2 it works well, been using F./Fox at home for years & now find Explorer hard to get on with, it just seems clunky & not well thought out compared.
Only thing I'd say is if you are using any 'Add-ons' with firefox, check before you upgrade you can get a newer version of the add-on compatible with 3.5. If not then the add-on will get disabled after the upgrade, which can be annoying.
Otherwise it's fine and funky, been using 3.5.2 for about a week now.