Gotta say..
Firefox 4 is so far pretty outstanding, I usually make the switch from current to the next version when it hits the RC stage. (to use as my main browser)
There are still little niggles, like a webpage which hits the cpu hard with some bad js slows/freezes up the UI so closing the tab is a pain (a problem Chrome does not suffer from).
Hopefully when they finally get Electrolysis up and running everything will be hunky dory in that regard.