The expiration of my GPG keys happens on January, 1st.

In ‘Passwords and Keys’ it is easy to update the expiration date. In Preferences, add ‘hkp://keys.openpgp.net’.

gpg --list-keys rclobus@rclobus.nl

gpg --send-key 50554B33905D8FB1922446FE62C57C6AA61495BD
The default is hkp://hkps.pool.sks-keyservers.net

gpg --keyserver keys.openpgp.org --send-key 50554B33905D8FB1922446FE62C57C6AA61495BD

As instructed by openpgp.org:

gpg --export rclobus@rclobus.nl | curl -T - https://keys.openpgp.org

As instructed by https://anweshadas.in/how-to-use-yubikey-or-any-gpg-smartcard-in-thunderbird-78/

Preferences | Config Editor… | I Accept the risk

mail.openpgp.allow_external_gnupg -> true

Restart Thunderbird

Account Settings | End-to-end encryption | Use GnuPG | 50554B33905D8FB1922446FE62C57C6AA61495BD

New 2023:

Thunderbird requires the key to be uploaded here: https://keys.openpgp.org

Debian information: https://wiki.debian.org/Keysigning