i dont disagree with FF being a better browser, I am using. But I know still over 88% of the world uses IE.
not to mention that the headers do not always return IE in them if the user is using IE.
Some return nt4.0 if using older IE versions and so on.
If you are basing it off of headers you could just do a redirect to the FF download page with a frame that says, if you want to view my site install this application.....lol
And if you wanted to get real smart you would download FF for developers, and create your own home page with advertising and google adsense search and have them install that version....