I downloaded Firefox 4.0. When I installed it I was horrified at the interface. It was like a whole new learning process. Practically none of the addons were functional and except for may be 2 or 3 no compatibles ones available. I quickly uninstalled it and went back to 3.6.16
I will now seriously consider Chrome as alternative !
Interestingly, moving to Chrome won't solve the "whole new learning process" or addon compatibility problems you're experiencing. Just a thought.
Give it a chance, update your addons or find new ones that work similarly. You will be rewarded because FF4 is definitely better than FF3 in many ways.