I haven't used Firefox regularly in quite a while and I would never use FF over IE if I had a choice.
There were times when I've used FF for it's security while browsing some pretty **** sites, but I'm done with that. I've learned more and I'm able to browse without having to rely on FF's security.
I just looked at the process list, and my current version of FF runs with ~22mb of memory used while IE (it's IE6) runs with only ~16mb. My current computer is pretty outdated, so the lower memory usage is noticable.
I don't like IE7 (assuming that's the version currently being used in my school), though. It felt like using something just as bulky as FF.
Programs are losing the simple look (solid, grey menus and bars), which I don't like. They're getting too much of a "user-friendly" look to them.
All of the add-ons and such aren't really a necessity, or even much of a convenience, to me, either. It's just more clutter.
The security from FF isn't even really needed. I've been using IE6 for a while now (with the updater turned off), and I haven't had any "virus" problems. I think it's just the lack of knowledge of the user that makes them think FF is more secure.
The tab thing, I don't like it. I'd much rather have the page open in a new window than a tab. I'm much more accustomed to clicking down on the taskbar or "alt+tab"ing to change which page I'm viewing.
IE6 > FF > IE7
FF > IE7 only because the Final Fantasy is alright.