[kaiengert]

Your statement needs some clarification.

Yes, Thunderbird supports OpenPGP, with a fully integrated solution.

However, Thunderbird doesn't use gpg by default, because of license incompatibility Thunderbird cannot bundle GnuPG software.

That's why Thunderbird uses the RNP library as its default OpenPGP backend.

You may optionally configure externally installed GnuPG for secret key operations, as described at https://wiki.mozilla.org/Thunderbird:OpenPGP:Smartcards