You can watch the video below for a tutorial.
- Browse to your certificate and enter the PFX certificate password to import it to Thunderbird.
- Click OK to exit the Certificate Manager.
- Click the Security tab on the left-hand side. Click Select next to the Digital Signing field.
- Select the certificate that was just imported to digitally sign and encrypt emails.
Note: If you wish to digitally sign every email, please check the Digitally Sign Messages (by default) box.
- Click OK when all of the settings have been adjusted. Your PersonalSign certificate is now ready for use to sign and encrypt email.