When you receive an email with a digital signature you can add that digital signature to your contacts

Thunderbird maintains it's own certificate store. Certificates from other people are stored there too. Thunderbird does not store people certificates in the address book, where contacts are managed.

When receiving a signed message click on the S/MIME button in the top right corner of the message window, to verify the signature. That should already add the senders cert to your Thunderbird certificate store, so that you can then use the stored cert to send an encrypted reply.

With the latest level of Thunderbird the certificate icon does NOT open up so that you can incorporate it, it needs to be a .cer file the latest Thunderbird version opens it as a .pem file which does not work for adding it to a file or installing it in the program.

I don't understand what you were doing here.

there are pem to cer converter command: https://unix.stackexchange.com/a/131338/367237

and cer/der to pem: https://serverfault.com/a/254630/217110

openssl tool for Windows: https://indy.fulgan.com/SSL/ https://github.com/curl/curl-for-win#binary-package-downloads https://sourceforge.net/projects/openssl-for-windows/