TB = Thunderbird . so you have to configure TB properly to access AT&T Yahoo emails:

first you should see this answer and other answers in that webpage , though the answers are for AT&T ISP's broadband users , but it mentions about some features (Secure-Mail-Key=SMK, Permitted App List, etc) which are very similar to what is used by AT&T-Yahoo mail , so please check that page first.

Then please also check/see this answer on Setting up Yahoo mail with OAuth2 in TB, then also see (part-2) of my answer, there.

EDIT : added few more sentences.

did you check the pages in the LINKs mentioned earlier ?

ISP, MSP are upgrading their system, so, different area's users and/or different type of (mail-server) user's account, etc have different policy set by ISP, MSP, etc.

TB = Thunderbird.

your account policy might be that, that you have to use SMK code with your TB (3rd-party email-client) app & also approve TB app , so please also TRY with that, find SMK generation/create page in your AT&T-Yahoo, get a SMK code, use that in TB (with auth-method "Normal-Password"), AT&T-Yahoo will ask you to approve/verify that you're indeed using TB to access emails, once you do those TB will begin to function.

if above steps did not work or options are not available, then try OAuth2 based auth-method (shown in above mentioned LINKs) & allow Cookies in TB . in OAuth2 based solution you will have to use AT&T-Yahoo's web-access password, then AT&T-Yahoo will ask you to approve/verify that you're indeed using TB to access emails, once you do those TB will begin to function.

I deleted my tb acct and created a new one with my existing email address with IMAP 993 but "authentication timed out" trying to get my at&t emails. Could I get a phone # for a tech assistant to resolve this by phone on Monday (tomorrow) because your instructions are way beyond me. Thanks Dawn Watkins

You have to delete the password from Saved Passwords in Options, restart TB, and enter a secure mail key that's created on the AT&T site:

https://support.mozilla.org/en-US/questions/1316825

I'm sorry; I feel like a complete dummy. I got an at&t keycode. When I went to delete my original password, there was none listed so I think I already did that step. So I went back to TB to re-create my existing account using the at&t code but no luck. "Authentication could not be confirmed". Again is there someone I can talk to that can walk me through analyzing the problem and through a solution to correct my lost email function on mozilla/thunderbird. Call me at [removed] eastern time Thanks.

edited phone# from public and search/spam bots view as this is a public forum and nobody here does support by phone.

inside the TB, first you have to delete old password for that mail-server, look for "Saved Passwords", then exit from TB, restart TB, when you will click on any Inbox or Sent, etc for that AT&T mail-account, TB will ask you to enter PASSWORD, here use the mail-key (aka: SMK), instead of AT&T mail web-access password.

SMK are generated/created in MSP's mail web-access website, then user can use that on 3rd-party email-client apps.

sometime some ISP/MSP will also send you TEXT code or email code or send notification in mobile app, etc to verify that you, yourself indeed just attempted to add email account in an email-client software (TB), once you approve/verify that , then TB will begin to function. please see the linked page's PAL section for more info.

Erik, I tried many times to delete the original password as you suggest. I don't have Edit/Preferences/Privacy/Passwords/Saved passwords. But I do have Options/Saved Passwords....which is now empty except the heading says "Saved Logins". So I reopen TB and click on Inbox but TB thinks a while and "times out". I never get a request to enter a password. So my original password is saved somewhere else that I can't find in order to delete.. Now What?? Thanks , Dawn

TB passwords are only stored in the Saved Passwords section of Options. But if you can't connect to the server to enter your key, you might be blocked by an interfering antivirus app. What is the antivirus? I recommend you remove it if it isn't the built-in Windows Defender.

You might also be blocked if you have the wrong server settings. Consult this article for the correct ones:

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