Username or EmailPassword
It's difficult to quantify the number of users, because one may have downloaded it and never used it again. Others like myself have downloaded the Windows and Linux version and installed it on more than 20 clients computers. Also various Linux distros ship with firefox, so all those people who used the version from their distro probably don't count in the download stats. Also there are people who pass on their copies to friends etc. Personally I think the number of users is probably more than the number of downloads.