Well, there are standards (www.w3.org), and Firefox (and other Gecko-based browsers) is the browser I'm familiar with that the best job of supporting those standards which fall within its bailiwick (I understand KDE's khtml engine and Opera are right up there as well, but hearing that from me is at second hand at best).
Generally speaking, if it looks one way in IE, and another way in Firefox, it's Firefox that is showing it correctly. If it looks wrong, the bug is in the page, not the browser. (That's not 100% true of course - even Firefox contains bugs - but it's much more likely than the converse.)
More generally, if the browser does something that conflicts with The Standards™, then it's the browser that's wrong. (Otherwise the standards are useless.)