Thunderbird is the free and open source email client by Mozilla Foundation. I have been using it for some years now. Till now the Thunderbird users had to use an extension Enigmail to use GnuPG. Thunderbird 78 now uses a different implementation of OpenPGP called RNP.

Since RNP library still does not support the use of secret key on smartcards, to use Yubikey or any other GnuPG enabled smartcards, we need manually configure Thunderbird with GnuPG. The steps as said are the following :

This article details my experiment on the matter. It is partly based on first hand experience, but also links to various other tutorials that helped me along the way. Especially thanks to folks on various IRC channels that really helped me out in understanding this.