You didnt give your o/s

Windows - Tools / Account Setting Linux - Edit / Account Setting

Highlight the account you want gone Down bottom there is a dropdown menu - Account Actions / Remove Account

Sorry for the lack of info. I run Thunderbird 78 on top of Windows 10. att.com POP server, I set the correct password. But Thunderbird could not reach those servers, I suspect the Yahoo, which provide the email services to the ATT subscribers from the beginning, changed or migrated their email servers with new addresses. That's why i need to delete this email client account from Thunderbird. Account Setting and management is the biggest change/improvement from Thunderbird 68 to 78. I could not find any reference to "Delete", "Remove", or "Cancel" email Account in the Account setting tab. I still need help for this, or how to set new email servers in the email account when I find their new Yahoo addresses. Thank for your support. BernardG

The settings for AT&T accounts are here:

https://www.att.com/support/article/dsl-high-speed/KM1010523/

POP uses inbound.att.net on 995, outbound.att.net on 465; SSL/TLS, normal password authentication, User Name = email address.

Correct the current settings, and enter a secure mail key instead of the account password:

See https://support.mozilla.org/en-US/questions/1313640