A lot of javascript routines that work fine in IE 6,7,8,9, Opera, Chrome earlier versions of FireFox do not work in FireFox 4. Have yet to figure out why. As a web developer, this has cost a ton of time and money.
How come the fast push in releases? Are they looking to catch up their numbers to the other competitors to make it look more advanced (Because it really is the most advanced browser, so it should have high numbers!)