Firefox just works better, if only because of tabbed browsing.
The differences between IE and FF aren't that much, but it's all the little differences that can add up real quick when you've got a lot of code doing a lot of different things.
And I agree swedguy. Simple, clean code works across all browsers. That's kinda the point.
Also, be aware that CSS is still pretty new and advanced applications don't always look the same in all browsers.
