Andrew David Wong:
Many of us have recently upgraded to Thunderbird 78, which changes the way OpenPGP keys are handled. Thanks to Frédéric, the Split GPG documentation was updated a little over a week ago with detailed new instructions, including a full screenshot walk-through, for how to use Split GPG with Thunderbird 78 and higher. If you haven't already seen it, take a look:
By the way, if anyone figures out how to automatically sign all emails, please let me know.
Answering my own question:
Account Settings -> End-To-End Encryption -> Default settings for sending messages -> [x] Add my digital signature by default
There's also an option here to require encryption by default.
However, I don't see an option to use inline signatures for compatibility with our mailing lists.
> -- unknown schedule --
> sending an INLINE cleartext signed message without attachments (we don't intend to support sending other kinds of inline OpenPGP messages)
Also there is no possibility to select what type of encryption I would like to use OpenPGP or S/MIME.
In the Account Settings End To End Encryption, at the bottom there is Preferrend Encryption Technology with options for both OpenPGP and S/MIME, but the selection is disabled.
It is disabled as well in the Security button -> Encryption Technology. The Open PGP is selected and S/MIME is impossible to select.
Does the OpenPGP encrypt attachments and Subject like S/MIME?
Also btw it is not possible to use more than one PGP key in one account.