Installation steps if you want to use PGP keys in Thunderbird 78+
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Starting with Thunderbird 78, Enigmail will no longer let you manage your PGP
keys but will only provide a migration tool.  Thunderbird will no longer use
system keyring and GnuPG but it will handle PGP keys internally.

To use PGP keys with Thunderbird 78 and above, you can use the migration tool
from Enigmail or follow these steps:
1. Import your PGP (public and private) key.
2. Edit end-to-end encryption inside Accounts preferences and select your
   existing openpgp key.
3. Import public PGP key from existing recipients.

In all cases, to protect your keys, you should also define a master password
in Thunderbird.