https://help.blacknight.com/hc/en-us/articles/360011553098-Issues-with-Thunderbird-78-

  * Options/Config Editor
  * Skip past the warning
  * Scroll down until you find security.tls.version.min
  * Double click on it, and set the value to 1
  * Close Thunderbird, and open it again

How to find Config Editor
https://support.mozilla.org/en-US/kb/config-editor