The good solution for this is to move Thunderbird 60 to stretch-backports instead of being at normal repository.
Normal users will keep current Enigmail 2:1.9 , current Thunderbird 1:52 and current GnuPG 2.1 and not unstable repositories. Other users will be allowed to use stretch-backports to upgrade to Thunderbird 1:60 , Enigmail 2:2.2 and GnuPG 2.2 when all available. No packages break, no Enigmail loss for unexperienced users. -- __________ I'm using this express-made address because personal addresses aren't masked enough at this mail public archive. Public archive administrator should fix this against automated addresses collectors.