The first noticeable difference is load times. Every version ever of IE I have ever used takes forever to load initially, let alone how long it takes to render a web page.
Firefox has had issues in the past with memory leaks, I have noticed also that the more add-ons used with FF, the slower it runs.
Opera and chrome are my top two favorites. Chrome for its simplicity, speed and versatility. Opera for its customization, speed and look.
The main gripe with IE is how far behind the curve it always is and how locked down it is as far as ANY customization is concerned. Being an avid linux user, i want to be able to have my own personal browsing experience if I so choose. Oh, and let's not go into the numerous IE security leaks......
"What can be asserted without proof, can be dismissed without proof."-Hitch.