I disagree, Firefox isn't the only alternative to IE and there're many people who use Opera who'll get this message as it identifies itself as IE6 by default. I don't like this kind of agressive Firefox marketing and it's one of my biggest dislikes about the Firefox community.
Too many options can kill a man.
If Opera identifies itself as IE6 by default, that's very fucking stupid. The Opera developers should've expected their users to fall into these IE traps, afterall, THEY'VE GOT MICROSOFT IN THE AGENT STRING!
Some developers will check that Opera isn't included in the agent string aswell, but not all of them will for different reasons ("not my fault, I think I'm doing this the right way. Fix your fucking agent string.", "there's THREE web browsers?").