I appologies for not reading more then the first page.
IE lets you be a sloppy coder, where as FF does not, I never worry about Opera or Safari. but if you code for FF, IE should work, but if you code purely for IE, FF wont necessarily work. IE was the first to support XMLHttpRequest, which is the basis for AJAX or Web2.0 my first drag and drop application in a browser only could work with IE, it gives reason to use IE over firefox for intranet development. I love firefox, I have gotten in quite a bit of trouble at work using it. But that said I can see why some people develop only for IE, it allows for sloppier code, and more features are supported in IE than in W3C compliant browsers/