IE doesn't do CSS correctly, so often if you do the workarounds to get style sheets working well with IE it can break the rendering in Firefox and other standards-compliant browsers. I've found that developing with Visual Studio.NET you get two main browser choices to develop to: IE 5 or Netscape 4 (which is totally broken for CSS, making IE look awesome in comparison). So if you want to use CSS, it writes it to be compatible with IE5. It would be nice if they put a "Mozilla minus what IE can't understand" as an option.
Fortunately the users for my current app all use IE, but I'm about to develop some customer-facing sites so I definately need to make sure they are compatable.