Critical mass for something like this is a nasty problem, but I think it could be cracked. Firefox is a bit of a selective user base to begin with (the +/- 10% userbase are probably very heavily weighted towards the tech savvy), and if you could get critical mass for one site that has a large Firefox user base on which the chat would be very helpful (say, Wikipedia) spreading out from there could be just a matter of time if you play your cards right.

Also, I haven't tried it, but I do hope there's an option to easily disable it because, after all, _I_ am only looking at the realnursestakeiteverywhere.com for educational purposes, but I definitely don't want to talk with those _other_ losers there.