You can comply til you're blue in the face with w3's standards but I don't care how you slice it your sites will never look the same in all the varied browsers out there. You HAVE to think IE first or you'll never survive. Yes, FF and others have made inroads but 90% of all users are still using IE and if I have to make a design choice that favors one over the other I'll choose IE every time.
Firefox has never bothered with design esthetics. Never has, never will. Firefox came on with a big advantage in security over IE but for pratical purposes of just viewing it offers a webmaster several disadvantages. It's clunky. I'm yet to see it display a vBulletin site right or even a phpBB, and these are the most common (and compliant, I might add) sites online. It also shreds common portal sites such as Nuke and it makes using eCommerce applications such as OS Commerce and even Amazon difficult. Who needs that?
Firefox is making a living out of being anti-Microsoft. Sooner or later they need to be pro-user and render sites faster and more to design standards.
Jeff <-- there, I feel better having got my FF rant out of the way
ps -- Actually, it's not my Christmas sites I'm irritated about most with FF. It's my baseball sites and the fact that I cannot read online banking sites with FF either. :-x