I wish everyone used Mozilla, but the fact is that 80% of Web surfers are using IE, so you pretty much have to write IE compliant Web pages. I've done several designs that looked perfect in Mozilla but looked screwey the first time I checked them in IE.
Yes, the reason is because Microsoft prefers to write their own standards rather than conform to the W3C, but 99% of your IE using site visitors don't care about that. All they see is that your site looks crappy on their screen.