Firefox is a bit more standards compliant. Also, because FF is less commonly used than IE, and not tied directly into the operating system, and is patched more frequently, it is more secure. Firefox is also more extensible than IE, with literally thousands of plugins available for almost any need. LiveHTTPHeaders, YSlow,
SEO Toolbar, Total Validator, and Firebug are just some of the diagnostic addins I have in my Firefox, which help me troubleshoot network and web site problems, and none of which have an IE counterpart.