i think its a huge mistake to _require_ users to get FF by either forcibly denying them, or by 100% ignoring cross-browser compatibility in such a way that your site is unusable in IE6. not yet, anyway, as IE6 still has a chunk of the market (15-ish%).

for the most part, building a web application that degrades gracefully is a good idea, and you can forcibly degrade it for older browsers.

and making a suggestion is good. there are quite a few sites that will show a message suggesting browser upgrades.

/$0.02