Your key/certificate don't have a .pem extension but they are PEM-encoded files. You can confirm that using the '''file''' command. If you're interested - here's some documentation about [http://wiki2.dovecot.org/SSL/DovecotConfiguration Dovecot SSL configuration].

Use netstat to confirm you're only listening on the imaps port, and not the plain imap port. Then reconfigure your account settings in Thunderbird to use SSL/TLS connection security with your IMAP server. You'll get a warning because you're using a self-signed certificate, authorise the exception.