IE for mac is very bad, but IE 6 on XP isnt that bad, it just has a few differences from FF. not extremely hard to make compliant with IE6.

but its not cuz MS doesnt want to be css compliant or wants ppl to have to be with them or w/e its probably because IE6 is like 3 years old or something. FF is new thats y it has the best support for things, IE7 will most likely have support for the more newer standards and might be better than FF.