I have already replied to you question about the differences between Firefox and IE.
IE7 is a new version of Internet Explorer, not an update or upgrade of IE6. In my opinion, Microsoft should not have sent IE7 out with their regular Windows Updates. Many people who got automatic updates found themselves with IE7 and many bugs, some of which still have to be worked out.
One appeal of IE7 is the ability to use tabbed browsing. In fact, Microsoft launched IE7 ahead of schedule (it was going to come out with Vista) due to the popularity of Firefox, it's use of tabbed browsing, safety and the ability to customize it using extensions.
Many users who found themselves with IE7 uninstalled it and went back to using IE6.
I have been using Firefox since it was released to the public almost three years ago.