Convoluted answer here... On the largest percentage of web pages that use web standard coding, FF renders the page faster than IE. HOWEVER, since MS has enjoyed a monopoly in many areas of software, there are a significant number of web pages out there that were coded using (for instance) MS Frontpage software and or Active X. These are non-standard pages (unless you are a die-hard MS koolaide drinker) and will give Firefox/Mozilla fits. (It should be pointed out that one of the BIG reasons that IE is so insecure is because it supports axtive-x - which allows web pages to download and execute stuff on your machine without your even knowing its doing it..) For these pages, you will need to keep IE handy (although I have started avoiding them. IF they want me to visit, they need to redo their pages to be more standard-friendly...)
I use Firefox, and have used it since before it was to the 1.0 version. I'm keeping it.
There only one thing for which IE is better than Firefox. It's not just MS-specific pages that render better in IE, but pages with broken HTML in general. IE is much more forgiving for these pages.