When I put ubuntu on my machine at home firefox 3 seemed substantially slower than it did on windows. I'm not sure if FF3 is crappy on iinux or if there was something about my install that was screwed up.
For example, I've seen several websites that crash Linux in Firefox -- not good. The rendering in general is also not as good as win/mac, esp. with respect to transparency.
Haven't had any crashing problems, either.