https://support.mozilla.org/kb/cannot-receive-messages

On TB v78 the minimum requirement for SSL has increased and probably your email service provider still uses deprecated security for the mail server. Can you try to set lowest security by following the below steps and let us know how it works? From the menu at the top right, go to Options. Scroll all the way to the bottom and click on Config Editor -> Skip past the warning. Scroll down until you find security.tls.version.min (or paste security.tls.version.min to upper frame) Change the value 3 to 2. Restart TB. If it's still not working then change the same option again but this time to 1. Restart TB.