I'm a little undecided over FF3, purely becuase the roundy curvy-ness of it all looks a bit too much like IE, which really winds me up.
But the biggest pain by far is the disappearance of the drop-down menu for going back a page... I like to be able to just hit that a select the previous page from the list that appears.
Yes, I know I can right click it, but why should I have to when the 'previous page' button has a drop-down, so where's the one for going back???