I personally see no reason to use Firefox except for the Firebug extension. I personally use IEx64. x64 (64-bit) browsers are MUCH better than x86 browsers (32-bit); as the x86 browsers tend to have massive memory leaks; FF especially. When it comes down to it, almost every website on the net these days require some sort of scripting; and Firefox has scripting problems; it can't even handle the DOM (Document Object Model) correctly. Yes, IE is not standards compliant; but thats why I like it as a web-publisher. The standards are arcane and retarded. Not to mention, pages generally look different in IE (they look better). I mean, name me a banking website that works as it should in FF; they don't exist.
Yes, you can use an extension that lets you use the IE engine in FF, so you get all the IE features; like beautification, DOM control and general scripting advances... but at that point... JUST YOU IE! Sure, everyone says that FF is more secure than IE; but thats just snake oil. IE is just as secure as FF; the difference is that much of the security is OFF by default because you need them off to browse even some of the more basic of websites. At least in IE, there are switches, in FF you can't disable the security. Technically I HAVE TO use both, because I need to test every site I make in a multitude of browsers...
But if you dont like IE... use Opera, dont use that FF shit...
IE = non-standard, but is light weight and allows scripting...
OP = extremely standards compliant and EXTREMELY light weight...
FF = has extensions, but is very heavy and memory intensive...
Chrome = seriously... you have no reason to ever use this. Its not as standards compliant as OP, doesn't offer the extensions of FF, and doesn't have the scripting or beautification of IE... so why use it?