The main reason people use Internet Explorer is because they have Windows. And Windows comes with it. A lot of these people do not have an open mind, which is required to try firefox.
I think a lot of Firefox users after using Firefox for a long period of time find it is a much better browser than Internet Explorer and discontinue using it.
Firefox is superior:
-Looks better, it is smoother and easier to use
-Easier, it has tabbed browsing and built in good stuff
-More stuff, it has plugins and themes
-Faster, not only is it faster but you can make it even more faster with fasterfox extension