This is ATT... unfortunately the offering from ATT/Yahoo is something of a junkyard dog. A product delivered under contract for ATT by Yahoo.

About the only useful information here is that the firewall is not blocking Thunderbird as it would not be able to check it's database of provider settings if the firewall was the issue. [removed image due to exposed password]

Although what you doing trying to add the account again I have no idea. If it was not working, adding it again will never result in a better outcome. It fixes nothing.

The ATT junkyard dog comment I made means that you never use your ATT password with ATT mail in a mail client. You must use your mailkey. But the ATT support folks fall back onto some sort of service obligation in their terms of service telling you that if web mail works then they have met their obligation to supply email. This other process is also theirs entirely, but they rarely if ever advise their own customers that they require you to use this procedure. They recommend using oauth in their web pages, but that will only work if you use Yahoo server names not ATT ones.

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

Dennis, thanks for being a supporter. Please post here whether Matt's advice helps.

THanks Matt for posting the advice.