I use Torbirdy, and I noticed repeatedly that Enigmail would not import GPG keys after I clicked the button. I read the Torbirty Torbirdy page(s) and learned that although Enigmail is fortunately compatible, GPG functionality requires a shim or some mysterious Curl package that is only packaged in Debian. I also followed the GPG onion URL and found my new favorite source of public keys. The site sends keys in their ASCII form, but it's real easy to import them after saving as a .key file. The command is just 'gpg --import *.key'.