I am a bit confused. Thunderbird has supported TLS 1.2 for a long time, so I don't know why you made any changes.

NetSol noticed I was still using port 465 and advised me to use 587 for the sake of compatibility with v1.2. But now I cannot send emails.

Does 465 still work? I suggest experimenting a bit. Try 465 and see if that works. If not, change the authentication (e.g., if it was ssl/tls, try starttls). ALso, try changing authentication with 587. It is not unusual for email hosts to give incomplete instructions. Let us know.

The Thunderbird email client has supported TLS 1.2 by default since Tb 78.0

https://www.thunderbird.net/thunderbird/78.0/releasenotes/

changed TLS 1.0 and 1.1 disabled

https://support.mozilla.org/en-US/kb/thunderbird-78-faq

You misread my post are comment.

You misread my post are comment.

Well I have read it all and to be blunt, I have no idea what the discussion is about. You have changed the port from from 465 to 587 and now I assume you get some sort of sending error. What is it? Clearly it has nothing to do with TLS. Without knowing what the actual error is, offering anything but platitudes is not really an option.

What I do know about the bargain basement solutions offered by network solutions is they fail entirely to offer a valid SSL certificate for the server you are asked to connect to and a manual exception has to be created for each and every host and port used. You I guess have an exception recorder for the old port but not the new. I also have seen a couple of topics suggesting a bug in the request for a certificate not occurring if the password is saved. So I suggest you go to the saved passwords in Thunderbird and delete the saved password for that outgoing account. That should allow the prompt to appear (if my reading between the lines is any good.)