Hint:
Maybe the manual ist not on nowadays technical standards. Pleas use the manual of FAU Erlangen: https://www.anleitungen.rrze.fau.de/e-mail/e2ee/smime/s-mime-zertifikat-in-thunderbird-einbinden/
Importing the SMIME certificate into Thunderbird
In Thunderbird, you must select the options under the „Tools“ menu item. In the window that opens, select „Advanced“ and click on „View certificates“:
Normally, the field under the „Own certificates“ tab is empty. Import your certificate here from the backup file you have created. When importing, you must enter the certificate password that you used to protect your backup.
When importing, you must enter the certificate password that you used to protect your backup.
Attention: If you already have older certificates in your memory that you used in the past, do not delete them! Otherwise, the messages encrypted with them can never be read again!
After successful import, the certificate appears in the certificate store:
If you have not already done so, assign a master password for your Thunderbird. This is done in the options:
Under the „Security“ tab:
This master password is requested if you need access to the private key of the certificate when signing a message. During use, the private key in Thunderbird is stored in memory for a while, so you do not have to re-enter it for every email.
Now you need to set up the imported certificate for use in your account. To do this, go to the „Security“ section in your account settings:
Select the certificate that you have previously imported into the certificate store. You also use the same certificate for encryption. We recommend ticking the „Sign messages by default“ box.
Done. You can now send signed messages.
When creating a new message, you will find the options for encrypting and signing under „Security“. Signing should be set by default, so you don't need to do anything else. To encrypt a message, you still need the public key of your communication partner.
Deutsche Version: Zertifikatsimport in Thunderbird