In the corporate world, most people still use IE.
1) It's free and comes with windows
2) it's technically supported by M$, however all of the IE specific problems I have ever run into at work were outside of M$ scope of help. Many little problems with DLL's and registry things... many web-based applications break often (every time IE updates... the release of IE7 was a disaster for several of my clients)
3) people know how to use IE - it is rather straight forward
4) Almost everything can work with/use IE, even if IE isn't the best possible choice
5) many people simply do not know of other web browsers. as far as they are concerned IE is simply "the internet". You say "firefox" and they say "Fire what?"
6) Group Policy can manage IE - but it can only install, uninstall or block other browsers
7) Since IE is standard, there are many corporate apps which do not work well on FF (some not at all) - but this can be corrected by the new add-ons which make FF look like IE
Commercially speaking, there are advantages to using IE, such as group policy. Most users simply do not need the additional functions that FF supports. All they need is "Favorites" and that's it.