I've found myself using Chrome more and more lately, been finding Firefox slower, even after upgrading to 4 and then 5.
Given the frequency with which it hangs on me, I'm assuming there's something like a dodgy plugin causing issues, but I've wasted enought time trying to track it down
[edit]
Don't know if it's related, but after my core temp hit 94C yesterday I took the laptop apart and found the main vent completely blocked with dust. Everything's a lot better behaved since I cleared it out, FF included.
Moral: Avoid FF5, it clogs your vents with dust.