Ah, it looks like Tbird uses its own cert store, maybe. https://manuals.gfi.com/en/kerio/connect/content/server-configuration/ssl-certificates/self-signed-certi

Ah, it looks like Tbird uses its own cert store, maybe. https://manuals.gfi.com/en/kerio/connect/content/server-configuration/ssl-certificates/self-signed-certificates-in-mozilla-thunderbird-950.html https://manuals.gfi.com/en/kerio/connect/content/server-conf... describes a pretty unique process for manually applying a trust exception to an arbitrary server cert - maybe worth a try if you didn't know about it, as I also did not until five minutes ago despite having used Thunderbird for what must now be a couple of decades.