3
well, in one site, as I close Firefox (3.0.1) (don't log out) and open it again, I'm logged out!
in another one, I had a problem only with FF (everything's ok in IE 7) that I've got sth like session time-out or sth, as I don't do any activities for some minutes, and then I try a link, I see I'm logged out and then, even when I log in and receive welcome message, in the next page, I'm still logged out!
in this case, I cleared FF cookies and problem was solved, but I think it can't be a permanent solution!