I have used both and there's a reason that IE7 is designed to emulate Firefox (the same way Vista was designed to compete with OS
. Microsoft is taking notice that the features and add ons of these other softwares are eating away at their market share. Ask yourself why IE now has tabbed browsing... Thank you Firefox and other open source web browsers.
In a nutshell, Firefox is faster (install fasterfox and follow this
link for much faster web browsing (for high speed connections only). It has a lot of very useful add-ons that are easy to find and always evolving, and it's a piece of software that improves with every edition, not just throwing out a new edition with a few bug-fixes.
Find a problem with Firefox? Bugzilla allows you to report it and rest assured, if it's a wide spread problem, Firefox will fix it. See if you can get Microsoft to deal with IE bugs as quickly...