What is your antivirus, and is the TB profile folder excluded? Does performance change if you run OS X in safe mode?

After recent updates to both macOS Catalina and Thunderbird the problem is gone. I was never able to pin it down to anything specific. It didn't happen after every sent email, but it was more often than not. If I sent an email, then closed the window, the application would stop responding and I would have to force quit it. It didn't matter which account I send from. Disabling add-ons made no difference, nor did safe mode. Whatever the issue, it appears to have been resolved.

So I figured out to fix the email in Thunder Bird. Yes you guys have no clue what your talking about. Took me less than a minute once I figured it out. I was unable after a week to get help from anyone , so I researched it myself. Bring up thunderbird, hit ALT A, go to tools then security, then passwords. You need to change the password with a security key off ATT and walla it will work! Have a great day!

So I figured out to fix the email in Thunder Bird. Yes you guys have no clue what your talking about. Took me less than a minute once I figured it out. I was unable after a week to get help from anyone , so I researched it myself. Bring up thunderbird, hit ALT A, go to tools then security, then passwords. You need to change the password with a security key off ATT and walla it will work! Have a great day!

Joylee - you never provided any information nor actually asked your own question despite being advised to do so - I did locate where you tried to hijack someone elses question. So it is hardly unsurprising you did not get help. You have posted several comments all identical to the above in several of other peoples questions. Your comment that AT&T needs an app password has no relevance to this question nor any of the others. Please do not post obviously irrelevant comments as it will cause much confusion.