firefox is nice; i use it currently, but it took 8 extensions and a new skin for me to be satisfied with it..
and it does load alot slower than IE (or any other browser for that matter, including opera).
further still, opera has mozilla/firefox beat when it comes to installing new skins/extensions. the way opera does skin installation is literally fast, simple and perfect: you just click on a theme, and 3 secs later, its changed the whole browser to it, and it then asks you if you want to keep it or not.
no browser restart, no large skins to download.
i took up firefox because of some serious IE exploits a while back, but i question the decision now, given my brothers machines still SP 1, and he uses IE all the time, and hasnt had anything go wrong (i check both our machines for spyware, viruses, etc. and both our machines are fine).
if you want extensions, check out extensionroom.mozdev.org, and check out the mozilla update page for hundreds of skins..