''re :everything checks, but when it asks for an address, which is from Xfinity, and a password, which I’m not sure they’re asking for so I tried the Xfinity, Yahoo, and Mozilla passwords.'' So in Thundrebird you 'New Account > Email then you entered your name, email address and password (so I'm presuming you know that password) Then you get a browser prompt from yahoo asking for same email address and password. That sounds like the set up has got to the stage where it's creating the Oauth token and needs you to confirm you really are you and to select to allow Thunderbird access to the server. The email address and password is specifically for the account you are setting up. You have already entered that at the start of creating the account - it's just yahoo double checking and asking you to allow permission for Thunderbird to access the server. IF you had a 'comcast' email address, then it's the password you use to access that comcast webmail account/email address regardless of what server it's on. You will only have the one password to access the webmail account for that email address. But you should have already entered that data when you started to create the account in Thunderbird. Have you already been migrated over to the yahoo server ? if yes, have you already logged on to the webmail account to view your emails via https://login.yahoo.com/ As per info here: https://help.yahoo.com/kb/SLN36803.html If no then do it now because that is the password you would use when creating the account in thunderbird. It's got nothing to do with the Mozilla account password. That's just used when accessing eg: this support forum.