Internet Explorer Pros (Just my opinion on all of these)- Easy to use, allows tabbing, IE7 has better security, has built in phishing detection, pop-up blocker, easy to configure, etc.
IE Cons- Active X controls are the problem with IE, because most of the time they're what causes the non-stop error messages making you have to close out IE and perhaps lose important work and sites, I don't like how it "finds" words compared to Firefox, it is very unstable on slower computers, the more tabs you open the longer you have to wait then the more chance it'll just give up and crash, etc.
Firefox Pros- I like Firefox a lot better than IE, the only reason I have IE is for Microsoft Update, or if a website requires it. It is more stable than IE, doesn't rely on Active X controls, automatically tells you that you have updates for you plugins, "finds" words better, doesn't crash as often so more stable, when you open more tabs it doesn't give up on you, I feel it handles pictures, and videos in a quicker more efficient matter, your favorites are optional.. you can chooses where to place them, etc.
Firefox Cons- Not compatible with some website along with Microsoft Update so therefore some plugins may need to be installed so that some webpages will work with it, sometimes when you update it your history will no longer work.. but this is for the older version... other than that I can't really think of any cons.
Hope this helps (: