There's like a beta release or something of updates to firefox 3.0 but for some reason with their GNPU (or whatever it's called, general users license) it's not allowed to be called Firefox and it can't have the same icon, but it's called Minefield and has a very close imitation of the icon you probably won't even notice the difference of once it's in your dock.

Reset your shapeshifter theme to the default apple set, launch Firefox 3.0/Minefield, then switch your theme with shapeshifter, problem solved. Then just don't quit your browser or else you'll have to downgrade to FF 2.0.x.