FireFox is more "standard" compliant. Standards are those recommended by W3C. IE is less complaint. For example, the right column shouldn't be moved at all on this page http://www.w3.org/Style/CSS/ -- a bad CSS implementation in IE.
Most IE only stuff (other than ActiveX controls) on Microsoft website can be done nicely using web standards (CSS, DOM, JavaScript/ECMAScript).
Even former Microsoft program manager prefers Firefox (www.joelonsoftware.com 6/15/2004) -- Great Minds Think Alike. Ahah :-)