Thunderbird not trusting certificate signed with self-signed authority

Solution 1:

1. In Mozilla Thunderbird, go to Tools → Options → Advanced → tab Certificates.
2. Click on View Certificates and go to tab Servers.
3. Click on Add Exception.
4. Enter the name of your mail server and click on Get Certificate.
   Do not forget to use the port number of the mail server.
   Example: mail.provider.com:993
5. Save the exception.

Source: http://kb.kerio.com/product/kerio-connect/server-configuration/ssl-certificates/self-signed-certificates-in-mozilla-thunderbird-950.html

Solution 2:

I had the same problem. The source in my case was the CA was not created with -extensions v3_ca which Firefox does not care about but Thunderbird seems to require for a "valid" signature