Number one issue that is driving people to switch over from IE is the security flaws and scripts written directed at IE itself. A good case in point is spyware itself. Most are written to be picked up in IE, while it's a little harder to catch using Mozilla or other browsers. You may still get some, but not as many.
Hence why I stopped using IE altogether. My brother in law is convinced that IE is the best browser out there. I've made mention to him about all the security flaws and scripts, but he's still convinced. But then again, he checks for spyware religiously, Firefox has been great at decreasing the threat of spyware on my PC, it blocks out a good majority of it.
Next up is all the very useful tools that you can add into firefox. I just like the usefulness of the firefox browser (and some of the other not-so-useful, but entertaining extensions like FireSomething). Tabed browsing, once gotten used to, is GREAT for browsing. I don't spend more than 10 minutes in here before I hit a few tabs open.
Another reason I use Firefox. The themes are great as well as the extensions. An extensible browser is so cool, I got used to tabbed browsing within minutes of downloading Firefox.
The small flaws I see so far is that images seem to load just a tad slow for my taste. There may be a cure out there, but I haven't found it yet. I also notice that the web-functions of my Logitech Keyboard don't open links/work with FireFox, but not that big of a deal IMO. Mind you....this is still beta! The real browser isn't even released in v1.0 yet. my guess is that it will be an even greater browser. I have read about many of the big-dogs making a joint venture into Mozilla to really get it perfect.
Considering the fact that Firefox is stil 0.9, it's pretty darn fast. I think that the developers are looking to secure the browser first before making any touch-ups to performance.
IMO, a browser should be more useful to you than to just open webpages, and that's what FireFox offers me. I won't go back to IE. It's just too bad that IE is so deeply rooted into Windows.
And that's why, again, I use Firefox. I can't even stand to use IE anymore. I think that the fact that IE is deeply rooted in Windows is one of the biggest reasons to stay away from IE. A malicious IE script can take down your Windows system (assuming here, not sure if it's possible) because of the deep integration of IE into Windows.