For me it depends on which ever icon i happen to click, for the most part IE7 because it works everywhere. the only reason i run firefox is for spellchecking at times. but even then, thats what google toolbar is for.
Internet Explorer 6.0. Mozilla doesn't work for me - too many bugs, and IE 7.0...oh boy, I wouldn't even know where to start - there are just too many problems with it.