[kaiengert]

Yes, you can use smartcards with S/MIME.

You, you can use smartcards with OpenPGP, if you configure Thunderbird for optional GnuPG support. This way Thunderbird can use your secret key that is managed by GnuPG, for secret key operations, including keys on smartcards.

How to do that is documented here: https://wiki.mozilla.org/Thunderbird:OpenPGP:Smartcards