SSL certificates are used to sign and encrypt/decrypt mails in thunderbird via S/MIME. If you have got a certificate that you want to use to validate your identity, import it as follows:
Open Thunderbird and go to Edit > Preferences:
Click on Advanced and go to the Certificates tab. There, click on View Certificates:
On the Your Certificates tab, click on Import and browse to the directory where you have saved your certificate, which must be saved as .p12 file (see here to learn how to produce that format). Your certificate will then be shown in the list.
Please notice that your certificate must be valid and trusted to be used in Thunderbird! You will have problems with most self-signed certificates.