I think it depends on the site. Most programmers are probably already using FF while they browse - but if your target is the average home user, it might be more

I think it depends on the site. Most programmers are probably already using FF while they browse - but if your target is the average home user, it might be more of a problem. Personally, I don't like it when a website tells me I have to use a certain tool. Now granted, most of the problem is that IE chose to be different than everything else, but still - when I'm working, I would rather have a smaller feature-set that runs on everything than a larger feature-set that requires a certain browser. I would rather be told to use FF than IE (my utility company requires IE, and it bothers me because it's all financial transactions - and Microsoft has, on occasion, not even trusted its own product). But still - I would advise against requiring a certain browser.