Thunderbird won't accept the email password of a "Roadrunner" account.

I'm trying to add a Roadrunner email account to a new instance of Thunderbird (I bought a new computer). The Roadrunner account already exists in my Thunderbird account and on the previous instance of Thunderbird. When I attempt to add the Roadrunner account, Thunderbird rejects the Roadrunner account's password. Note that I can use the password get into the account on the Roadrunner site, but Thunderbird won't accept it. Of course, Thunderbird's error message is vague to the point of being worthless, so I don't know why the Roadrunner account is being rejected. What am I doing wrong?