I see no reason to change from FF3 to FF4 until sometime around 4.1 ...having been with Firefox since...dunno, must have been v.1 or something, I know when they make a major release its best to wait until several months have passed before its (a) stable (b) reasonably bug free and compatible with all websites (c) the 3rd party addons that make it a great browser actually all work. Its only when the addon developers start dropping the older versions its worth moving on...
but hey, feel free to install it and find all the broken websites and addons for me, I'm bored with beta testing "release" products that don't work as well as their predecesors, which seems to be the norm with all software nowadays...