Reasons I use FireFox over IE on a Windows box:
1. FireFox is quicker for me to get into and use compared to IE. Why? Well, IE seems to like checking all my drives and network shares when I type in a URL, locking it up for a decent amount of time. At both work and home, I have a decent amount of mapped drives and virtual CD drives. IE also seems to do this randomly later on accessing a site as well.
2.
Extensions. I like one browser window, so I use an extension to ensure that even links that try to pop up in new windows are instead opened as a new tab.
3. Lack of major security vunerabilities. Sure, it doesn't affect me too much, but honestly, I dislike having popup windows, and I also dislike having popups from IE asking if I want some new spyware.
4. Form saving. IE has a bad habit of loosing what you had in a form. Firefox seems to hold what I typed in, so even if the BBS glitches, I can just hit back and try again.
5. Ability to not only stop popups, but also other annoyning javascript like resizing windows for no reason, playing with my status bar text, and other things.
6. Right click, block images from this server. Comes in handy for stopping ad banners.
7. If the above is not good enough, grab a custom .css to apply to pages to stop almost all ad banners.
8. The new download manager is rather nice. I like keeping multiple downloads in one window as well (can you tell I dislike taskbar clutter?)