Guide on how to configure Qubes's split GPG to work in Thunderbird 78 as a "smartcard". This is based on a post in r/Qubes, but the OP deleted it. Also see the Thunderbird split GPG documentation and migration notes.
- Upgrade your TemplateVM so it installs Thunderbird 78
- Open Thunderbird, go to Preferences, scroll all the way down and click on "Config Editor". Accept the warning.
- Restart Thunderbird
- Go to Account settings -> End-To-End encryption. You'll see something like "Thunderbird doesn't have an OpenPGP key for <address>"
- Click "Add key", select "Use your external key through GnuPG (e.g. from a smartcard)" and then "Continue"
- Enter the full key ID of the associated GPG key. You can run
gpg --list-secret-keysin your split GPG vm to get the full key ID.
- Click "Save Key ID" and then you should be set.
Supposedly there's a way to get Enigmail to import your previous GPG keyring into Thunderbird, but I didn't figure that part out.