have you created a mailkey as required by ATT? See https://www.att.com/support/article/email-support/KM1240308/

Once you have it, that is your password for use in Thunderbird.

Why isn't my att password working. In works in yahoo.mail.att

Because ATT or more to the point Yahoo have decided that is the way it is to be. use of normal passwords will no longer be accepted for IMAP POP or SMTP services. they still use it for the web, but not for email.

i havent created a mail key. will I have to change my password?

When did this happen. I haven't been able to receive emails in Thunderbird for about two weeks now.

It started about a year ago. but the implementation appears to be staggered. But you really are asking the wrong person. I live in Australia and I am not an ATT customer so what I have gleaned is from this forum and the hundreds of ATT customers that have been here over the past year or so asking the same questions and getting the same answers.. I suggest you ask ATT when they invented the policy and all your other questions about it as it is their policy and their rules. All Thunderbird can do is connect using their advertised method. You are paying ATT for the service so I would assume it is up to them to explain it to you. Rather that fellow Thunderbird users.