After using mozilla for well over a year now, I can look back and tell you why IE was the suckiest browser ever.
The worst thing about IE was the pop ups. Basically if someone was malicious enough in their scripting, they could change your home page, continuously pop up more banner ads until the end of time, resize windows, change your desktop, do whatever they wanted, Mozilla has ways to prevent this, IE does not. Then look at IE, it is ugly. At least mozilla is skinnable. I dont care what some stupid mother fucking Windows users think, Mozilla is just as fast as IE and I have never had a problem opening any website, but I can count the times I have visited a Microsoft website in the past year on one hand. If it is absolutly necessary to use IE to visit a website, you can tell mozilla to identify as IE easily. I doubt you could get IE to identify as Mozilla no matter how hard you try. Plus flash and java work just fine in mozilla so dont expect an inferior experience. The worst the about IE is the security. There were more serious security vulnerabilities in IE than any other browser, and there are many that microsoft has yet to fix. In my opinion, the best browsing experience is on any OS, with Mozilla.