Latest version of Mozilla (well, not just the latest either) is more compliant to the HTML standards than IE ... just most people either don't know about NS, or can't be bothered when IE comes with Windows.
Of course, those of us who have to use non-MS operating systems don't get the choice of using IE, so I'm glad there's at least one viable alternative :wink:
(NB: In one of the modules last year, we had to write a web page that displayed fine in *Lynx*. I mean, that is fairly extreme compatibility ... but our lecturer liked Lynx, so...)