Bugzilla

I see a number of odd issues here.
1. There has never been an approved OAuth for ATT accounts.  Hence, I filed bug 11698316 four years ago.
2. The user can not select the OAuth "secret" to be used to self select an OAuth provider that Thunderbird has but does not offer for the domain provided.  Hence, I filled bug 1591782 six years ago.

For these reasons, I am struggling with most of the content in this bug.  Fundamentally, you are talking about authentication methods that are simply not available to ATT customers according to the instructions provided by ATT for the setting up of a mail client. 
There is, or was ,a way to use OAuth with all yahoo contracted providers.by using the mail.yahoo.com servers that Thunderbird offers OAuth using the Yahoo OAuth secret and choosing the OAuth option Thunderbird offers for Yahoo.  

However, there are limitations.  
It relies on the user's email address being registered with yahoo as a valid user.  I do not know if this is still the case for all ATT customers.  It used to be.
You must enable, DNS over HTTPS in options or choose custom DNS servers so you no longer use the poisoned DNS of the ISP which simply redirects some domains to their own "hosted" offering.  
You also need to close the manual setup/ account settings page/tab to force Thunderbird to refresh the offered authentication methods to match the selected server name. As Thunderbird is incapable of refreshing the list items based on the server name change until the entire construct is reloaded the visible pane needs to go away.  You only really have to change from server settings to any of the other options,  but it does make any sort of batch updating of account settings a mammoth task instead of just a few clicks.

Some other decade old facts.
The only password you can use in a mail account with "normal password" is an ATT mailkey.  Use of your ATT password will fail, and will fail by ATT/Yahoo design.  It is not an accident or a bug
Using OAuth and Yahoo server names never uses the mailkey as the authentication occurs over HTTPS not one of the mail protocols. The ATT password used on their website is the one used in all circumstances.   By the nature of OAuth, the ATT password is not stored in Thunderbird only the OAuth token is stored in the password manager.  The reality is the OAuth authentication pages are web pages and served by Yahoo,  so Thunderbird never actually has the ATT password in this scenario to store, only the resulting token from a successful authentication and grant of permissions is actually returned to Thunderbird.

I see a number of odd issues here.
1. There has never been an approved OAuth for ATT accounts.  Hence, I filed bug 1698316 four years ago.
2. The user can not select the OAuth "secret" to be used to self select an OAuth provider that Thunderbird has but does not offer for the domain provided.  Hence, I filled bug 1591782 six years ago.

For these reasons, I am struggling with most of the content in this bug.  Fundamentally, you are talking about authentication methods that are simply not available to ATT customers according to the instructions provided by ATT for the setting up of a mail client. 
There is, or was ,a way to use OAuth with all yahoo contracted providers.by using the mail.yahoo.com servers that Thunderbird offers OAuth using the Yahoo OAuth secret and choosing the OAuth option Thunderbird offers for Yahoo.  

However, there are limitations.  
It relies on the user's email address being registered with yahoo as a valid user.  I do not know if this is still the case for all ATT customers.  It used to be.
You must enable, DNS over HTTPS in options or choose custom DNS servers so you no longer use the poisoned DNS of the ISP which simply redirects some domains to their own "hosted" offering.  
You also need to close the manual setup/ account settings page/tab to force Thunderbird to refresh the offered authentication methods to match the selected server name. As Thunderbird is incapable of refreshing the list items based on the server name change until the entire construct is reloaded the visible pane needs to go away.  You only really have to change from server settings to any of the other options,  but it does make any sort of batch updating of account settings a mammoth task instead of just a few clicks.

Some other decade old facts.
The only password you can use in a mail account with "normal password" is an ATT mailkey.  Use of your ATT password will fail, and will fail by ATT/Yahoo design.  It is not an accident or a bug
Using OAuth and Yahoo server names never uses the mailkey as the authentication occurs over HTTPS not one of the mail protocols. The ATT password used on their website is the one used in all circumstances.   By the nature of OAuth, the ATT password is not stored in Thunderbird only the OAuth token is stored in the password manager.  The reality is the OAuth authentication pages are web pages and served by Yahoo,  so Thunderbird never actually has the ATT password in this scenario to store, only the resulting token from a successful authentication and grant of permissions is actually returned to Thunderbird.