Key points: Missing API functionalities which block the release of GPG Mail as a Mail Extension
Four of the major functionalities missing in the first official release of macOS Sonoma are:
Entire message data is not always passed to the extension making processing the encrypted message impossible
Reliably encrypted drafts
Support for setting the default state of the sign and encrypt button in compose windows which can lead to dangerous side-effects
Sign and encrypt button could go out of sync with internal state, if keyring changes are detected
Probably a good time to reach out to Apple: https://www.apple.com/feedback/mail.html
You don't need to be familiar with git. Just open your browser and visit https://bugzilla.mozilla.org/enter_bug.cgi? create an account on bugzilla and file the bug against Firefox.