I use Adelphia
When you first set up a profile in Tbird, it doesn't ask you for the password, just the username. Then, the first time you try sending/receiving mail, it prompts you for the password for the mailserver. Do you not remember the password for the profile or something? Have you tried logging in to the Adelphia webmail page and resetting the password for the profile you're trying to set up?
I had a problem like that several weeks ago. All e-mail accounts for two different users were suddenly requesting manual password entry (which did no good). The next morning no problem. Earthlink's mail servers had been down. Nothing to do with the mail client I was using.
Perhaps you are having the same problem with Adelphia?