How many people think its a huge mistake to suggest to users to view the site in FF before entering (if they have IE)?
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.
hopefully they will be non existent within the next year or so
I'd suggest throwing a banner across the top that says something like, "Your browser is outdated and some parts of this site may not function correctly. Upgrade here"
The more sites that encourage users off ie6 the better off we'll all be.
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
Most importantly, look at how many of your users are using IE6, or anything other than FF. Will the better experience for your FF (or simply IE6+) users outweigh losing almost all of your IE6 users? Keep in mind that many people who use IE6 now simply can't upgrade because of hardware or administrative constraints.