I have no idea how this happened, and I couldn't say if it was cause mirc 7.02 or Firefox for sure, or both:
I just visited http://www.betanews.com/auth/login.php
and clicked the username box to select my username from the drop down list - and every username from a forum irc channel I sit in was in the username dropdown.
I have never mass selected the users in that irc channel, I have never pasted their info into a website.
Each name in the dropdown is an individual, with no password, so it's not like a bunch of users were accidentally copied and pasted in when trying to log in one day, as that would have been taken as one user with a really long name.
I did not even have mirc running at the time! (this must have of course have happened at a previous time when I did, I didn't see it, and they've just been stored).
I'm sorry I know it's one of those things that may be 100% impossible
to track down or replicate with such little info, but I had to report it, it totally blew my mind.