i use firefox as primary, but i still have to use IE from time to time. it seems with newer versions of firefox, it can handle most of the pages out there, but i still have to check pages with IE. face it, everything on the net is built for IE (and most of the time exclusively, because a lot of web designers either don't care or are oblivious to checking browser compatibility).
i used opera a bit here and there, and it is nice (don't know if they have it fixed or not but it used to have a horrible time with animated gifs - played them super slow, and was annoying). but i just never got into it really.
probably what i really like about mozilla though is its more standards-oriented html handling, the security, and the plugin support (like the very handy "view page in IE" plugin). IE really isn't that horrible - aside from some security issues it's basically just as good as everything else (aside from its occasional childlike html rendering). if mozilla was as popular as IE, i'd bet there would be as many exploits for firefox as there are for IE
i'll stand by mozilla as my personal preference, but if you work with the web a lot, you can't ignore IE at all. last stats i had it still had an EXTREMELY commanding lead on the net, and probably only loses that on hardcore techy sites like this.
oh yeah, the main thing that pisses me off with mozilla/IE, is that if you make something crappy (written crappy) that works in IE, it's pretty common that it doesn't work in mozilla. that's generally pretty easy to fix (most of the time). BUT, when you build for mozilla (which is about 95% compatibility with IE), and a page breaks that way in IE, it's usually 10 times more difficult to fix. several times i've had perfect code break in IE but work perfectly in mozilla, all html standards etc. sometimes IE just doesn't understand the pages