The long string is probably a secure mail key that is required for AT&T. A separate key must be created for each computer, even for the same account.

Change the servers to inbound.att.net and outbound.att.net, remove the passwords from Saved Passwords in Preferences, restart TB, enter the secure mail key when asked for a password, and check the box in the prompt to have it stored in Saved Passwords.

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

The servers are already set to these in my Account viewed on my new computer. I don't see anywhere else to change the servers to these. To clarify, there are no saved passwords to be removed on the new computer's Tbird Preferences > Passwords > Saved Passwords. I don't see a way to get a new ATT string of letters on this new computer.

I'm going to leave the old computer Saved Passwords as is so I can continue to get emails thru Yahoo there. It is the same letter string for mailbox://pop.att.yahoo.com and smtp://smtp.att.yahoo.com

My goal is to save a Tbird password set up in the new computer so I can use Tbird seamlessly from Yahoo. If there has to be a string of letters, I would like to edit it to my choice of password if possible for easy recall. Further suggestions are welcome. Thank you.

Do you not see a checkbox in the password prompt? You cannot edit the secure mail key to an easier to remember form. But if you check the box, it's not necessary to enter it again.

The server settings can be edited in Tools/Account Settings.

The servers are already set to inbound.att.net and outbound.att.net in my Account. The default ports are accurate. Connection security is SSL/TLS. Authentication method: Normal password.

The only "check box" I see is Preferences > Passwords > Use a Primary Password. When I click it, "(not set)" shows up in the Current Password box. I don't see a specific password "prompt". The Primary Password checkbox and the "Saved Passwords ...." box are the only things I can activate under the Passwords section.

There are no saved passwords to be removed going to Preferences > Passwords > Saved Passwords. The next screen titled Saved Logins shows up completely empty: No provider, username, last changed entries. Show passwords is grayed out. I don't see a way to enter that same ATT string of letters into the Saved Passwords box.

So it seems I don't have a Current Password either set or saved anywhere or a way to create one. Further thoughts? Thank you.

If you don't have any stored passwords, then a prompt appears when TB connects to the mail server. In that prompt, there should be a box to check that stores the password (or key for AT&T) in Saved Passwords. If there's no checkbox, look for the preference signon.rememberSignons in Preferences/General/Indexing, Config. editor.

http://kb.mozillazine.org/Password_not_remembered_-_Thunderbird#No_checkbox_to_remember_passwords

If the preference is already true, you can reset all password data by deleting or renaming these files from the profile folder, with TB closed:

logins.json, cert9.db, key4.db, pkcs11.txt

Help/More Troubleshooting, Profile Folder, Open Folder, to open the profile folder.

When TB restarts, you should see a correct password prompt for the account.

Thanks for your patience and hanging in there with me. I can log-in using that long string of random letters (AT&T key) as a password. I hadn't paid attention before but there is a checkbox below the password field that says "Use Password Manager to remember this password." Is this the same checkbox you refer to as "..... stores the password (AT&T key) in Saved Passwords"?

If so and since the wording is a bit different, I want to be perfectly sure these are the same thing. If they are, and I check the box and log in with the AT&T key in the Password field, I know my Inbox will show up. Seeing my Inbox then exiting and restarting TB, will my Inbox show up automatically from now on without a Password prompt and therefore not having to ever enter the AT&T key again?

The preference is True for signon.rememberSignons in Config.editor. Should I just check the box now, log in, see my Inbox, exit, and restart TB to see if my Inbox comes up without the Password field prompt?

If checking this box is all I need to do, it appears I've misinterpreted your guidance by me focusing so much on Preferences > Password > Saved Passwords.

If this doesn't work as desired, I'll leave a question about deleting those four files to a follow up message. Thank you.

Yes, the 'Use Password Manager....' box is the one I meant, and if you check the box you should see the key in Saved Passwords, and you shouldn't be asked for a password (key) when TB restarts. You might be asked the first time you send a message, but checking the box will add another entry in Saved Passwords for smtp://.

Thank you so much! The automatic appearance of my inbox and the sending of messages works as desired. And the mailto: and smtp: passwords show up in Saved Passwords on separate lines. I'm sorry I made a mountain out of a mole hill only focused on the Preferences > Passwords path rather than the checkbox on the log-in screen. I appreciate your time and knowledge