Figured it out, not sure why Thunderbird didn't like it, but: '''Run Kleopatra, changed the expiry date of ALL certificates in the chain:''' # Double left-click the GPG key you wish to change the expiry date on, in Kleopatra's main certificates list. # Left-click the "More Details..." button in the bottom left corner of the OpenPGP Certificate window. # For each key listed in the "Subkeys Details" window, right-click "Change Expiry Date...", and determine at what point in the future you want the keys to expire (I wouldn't go past two years, you might want to regenerate your keys with more robust encryption algorithms or key sizes in the future) '''Use the `gpg` command to export your public and secret keys:''' # Run the command: `gpg --list-keys` # Run the command: `gpg --list-secret-keys` # Note the text following the term "uid" from the command outputs, you'll see something like this: uid [ultimate] John Doe <[email protected]> # Run the command: `gpg --export -a "John Doe <[email protected]>" > Documents/jdoe-pgp-public.asc` # Run the command: `gpg --export-secret-key -a "John Doe <[email protected]>" > Documents/jdoe-pgp-private.asc` # Run the command: `cat Documents/jdoe-pgp-public.asc Documents/jdoe-pgp-private.asc > Documents/jdoe-pgp-public-private.asc` Thunderbird will import that final certificate (the file "Documents/jdoe-pgp-public-private.asc") just fine. This article is helpful: https://linuxhint.com/export-import-keys-with-gpg/