I suggest deleting the account, shutting down TB, restart and do setup again.

I did that several times last 4 days .. most recent download from FIre fox site, july 12 at 9 pm . opened it - process the start up same result ad configuration setup .end "unable to login at server probably configuration , user name or password"

Maybe if you post specifics on the account, that may help, such as email provider name (e.g., gmail)

Maybe if you post specifics on the account, that may help, such as email provider name (e.g., gmail)

This info might have helped.

Does https://support.mozilla.org/en-US/questions/1278417 help?

David your recommendation to reload the TB file has put more space between my account and my recovery .. yes i reloaded TB now there is no recovery of old version that I am aware of. ! I have Microsoft OS base PC on Win 10 , TB ver 115, and it was ver 112 on July 7 install when the account was handicapped , the email account is Bellsouth.net hosted on Yahoo as an ATT account I am guessing .. at that time .. the best advice would have been... get a secure Key from ATT and login w/ that key as password.. but the TB site failure comments sent me on a wild goose chase over configuration errors ,IMAP vs POP3 , login ID or login password..

all of which worked on the ATT mail site. 
It was a year ago when I had to get the secure key from ATT to login to my email.. and again due to a TB upgrade .

so .. maybe the default message posted for failed account login should include.. "MAY NEED A SECURE KEY FROM ACCOUNT HOST my fear now is .. because I have chased many RED HERRINGS here in 30 days .. my folders , and address book are lost in the TB reloads if you have any magic recommendations .. please let me know Tom Mc

Fundamentally ATT are not in the business of supplying an email service. They provide a very poor imitation of the yahoo service under contract with Yahoo. This "deal" comes with the obvious issue that ATT have no idea even how or if it works.

About the only thing that becomes clear as soon as you search for ATT email in the past 10 years is that they have this proprietary "secure key" thing that no one else on the planet has, and it is a password.

Not an app password. Everyone has those. Not Oauth2.0 authorization which basically the entire world is migrating to. Not two factor authentication, again very common. No a special password that they generate that they and only they call a "secure mail key" This password serves no purpose for accessing anything ATT on the web, because that would increase their support costs. No it is only for use with email applications.

ATT even go so far as to suggest that "Do you use a desktop program or a mobile app to access and manage AT&T Mail? If so, we suggest you use one with Open Authentication (OAuth) technology." Ok so Thunderbird supports OAuth. Unfortunately ATT do not.

You can force use of oAuth, by changing the server names in account settings to Yahoo ones and retaining the ATT email address as a user name. That is inbound.att.net and outbound.att.net to those server names used by Yahoo. https://help.yahoo.com/kb/SLN4724.html for POP and for IMAP https://help.yahoo.com/kb/imap-server-settings-yahoo-mail-sln4075.html

My feeling is If you do not like the service at ATT or their reliability, use another mail provider. You could even use Yahoo which is currently providing the limited ATT service, as I described. You do not even need to change your email address. But the internet has many many free mail providers and few are as difficult to deal with as ATT/Yahoo arrangement.