This guide describes how to integrate a personal certificate for signing and encrypting emails in Thunderbird. To do this, you need a certificate, which must be available as a .p12 file.
How do I request a personal certificate?

Please note the information on Email Encryption.
 

1. Open the "Account Settings" in Thunderbird.

   

2. Select "End-to-end encryption" in the desired mailbox.

   

3. Click on "Manage S/MIME certificates" to import the certificate.

   

4. Click on "Import".

   

5. Select your certificate and confirm the selection with "Open".

   

6. Enter the certificate password and click on "Sign in".

   

7. Close the certificate management with "OK".

   

8. Now click on "Select" under "S/MIME".

   

9. Select your current certificate and confirm with "OK".

   

10. Confirm the following dialog with "Yes".

    

11. Under Default settings for sending messages please choose the option "Enable encryption for new messages" and select "Sign unencrypted messages".

    

12. If you now write a new e-mail, it will be signed and encrypted by default. If the recipient does not have a certificate, you can adjust the sending settings under "S/MIME" for the corresponding e-mail and send the message unencrypted.