Startup: Firefox sidebar for chatting with fellow visitors to any website.

Definitely. I have seen at least couple other firefox extensions to do the same thing (here's one: http://www.yakalike.com/). I was involved in a drawing/graffiti type one last year (http://drawhere.com). It was fun to try and get done in javascript, but it was hard to hold people's interest for an extended period. There's a history of this stuff failing: http://wired-vig.wired.com/news/business/0,1367,42803-2,00.html

I'll be curious to see if/how someone gets it to stick.

here's another one: https://addons.mozilla.org/addon.php?id=1825

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.

Couldn't they just put Adsense on the sidebar to monetise it?