Are you trying to use the instructions provided by ATT? where you need to use a mailkey instead of your password. Or are you trying to set the account up as a yahoo account using all yahoo settings exept the ATT mail address? and using oauth authentication instead of the mailkey. I this case Cookies are required to be enabled in Thunderbird. They are on by default, but we see a steady trickle of folk that have installed helper applications that have disabled them.

I have tried both the automatic account configuration in Thunderbird and the ATT Yahoo instructions Neither worked. I never got anything about mailkey. I don't know where to turn on cookies in Thunderbird.

Yes cookies are enabled.

mailkey is here https://www.att.com/support/article/email-support/KM1240308/

Yahoo setting are published here. IMAP https://help.yahoo.com/kb/SLN4075.html or POP https://help.yahoo.com/kb/pop-access-settings-instructions-yahoo-mail-sln4724.html

For the yahoo case the authentication method is oauth and your user name must be your ATT email address. Because of the changes ATT make to DNS to mess up what everyone else gets on certain sites I also suggest you change Thunderbird to use DNS over HTTPS in settings as you will then have the same DNS resolution as the rest of the internet, and Yahoo properties will appear as such, without ATT overlays.

Settle on a se5tup method, and the protocol and try it. then report any errors verbatim so I and other can be constructive with pointed information instead of generalities.

Note that account configuration is often completely messed up by antivirus programs. Common offenders include McAfee and Norton.

Thanks for your help. I was able to find the article on the mailkey and that resolved the problem. I don't know why At&T has to make things so difficult. Mozilla doesn't help with lack of information. I have had this e-mail address for 20 years so I don't really want to switch to gmail or some other. Again, thanks.