the error message you're getting is just an elaborate way of saying "login failed". judging by the mozilla bug, the problem might be that you're using different capitalization of your login name.
I don't get that from the mozilla bug, they mention capitalization of mechanisms, but not of logins, see for instance this diff:
https://bugzilla.mozilla.org/attachment.cgi?id=8888351&action=diff where they replace "login" and "plain" by "LOGIN" and "PLAIN".
alternatively, the login *is* wrong, because the password expired or something like that. if neither applies, try configuring "AuthMech LOGIN".
AuthMech LOGIN works indeed (so my logins and passwords are correct).
