Only difference I noticed was that some add-ons failed to work. While I noticed a nice difference between Firefox 3.6 and 4.0/5.0, it was still lagging a bit. Therefore, my primary browser is Chrome. Even IE9 to me I would rather use than Firefox, since it is much more fluid and less laggy.
I've just dumped firefox a few days ago in favor of chrome. youtube and flash were giving me issues where coming out of full screen will cause a plugin crash and jumping from one part of the video to another causes a plugin crash. I don't know whoe's at fault Firefox or flash player but it seems to be working fine in chrome. I will however check back and see what updates come around and see if the issue gets fixed