I can login to ATT with password but Thunderbird cannot gain entry

Far as I can see the server set is wrong. On their page are different settings: Incoming email server - imap.mail.att.net (and you have only att.net) Port - 993 Server requires SSL is selected Outgoing email server - smtp.mail.att.net (also here you have only att.net) Port - 465 or 587 Server requires SSL is selected Outgoing server requires authentication is selected

The inbound.att.net and outbound.att.net servers are correct, but you need to use a secure mail key instead of the regular account password. Delete the old passwords in Saved Passwords, restart TB, and enter the key when prompted.

https://support.mozilla.org/en-US/questions/1288812