I tried to install the enigmail extension in the thunderbird snap, but initialization failed:
Enigmail initialization failed.
You are using GnuPG version 1.4.20, which is not supported anymore. Enigmail requires GnuPG version 2.0.14 or newer. Please upgrade your GnuPG installation, or Enigmail will not work.
So this needs to be sorted out. And I guess we will need to add the gpg-keys plug too.
This other thread might be relevant.