I think T-bird times-out before e-mail messages download because Yahoo now links related messages as if they are conversations which prolongs download time.

I have no idea what 'Yahoo now links related messages' means. You may post a screenshot to illustrate the problem. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Email hasn't fundamentally changed over night though. I'm pretty certain when something times out for you it has got nothing to do with 'Yahoo now links related messages'. It's much more likely you're experiencing a Yahoo server problem again.

Do I need to change to a different provider,

Up to you.

e.g. establish a "My ATT" account promoted by ATT, or is ATT doing the linking?

May be you should ask them.

Do I have to establish a new e-mail account and change my address , e.g. g-mail?

Once again, that's up to you to decide. Certainly Gmail is more reliable than Yahoo, but not everyone likes Gmail either.

It is downloading now without any change. Linking e-mail ?... A message is in my in box to multiple recipients. Before I see the message, any replies are attached to the message. Any reply I send is also added to the original message instead of being treated as a separate message. I may not want "all" to see my response. Thanks for the suggestions.

I have to admit I don't understand your problem, and what you're trying to achieve.

Whatever was preventing e-mail from downloading seems to be gone. It is now working and e-mails are coming through OK. Thank you