Isn't it always good to know how to fix something before possible breaking it? Well, if you want to go back to Firefox 3.6 after trying Firefox 4.0, here is how to get your old extensions back. Neat!
Update: I just updated to Firefox 4.0. One extension needed to be updated. The rest worked as is. I didn't "lose" any. So far I've found no downside to the update.