I also want to ask a question. I noticed that all open source developers do this. I don't know if you do it for security reasons, or if you do it to avoid legal problems. what I'm wondering about is the signature issue. when I do gpg verification, I see that the signature of the key is “good”, it says “WARNING: This key has not been certified with a trust-grade signature!”, ‘There is no indication that this signature is good’. What is your reason for not certifying this signature? This information...
Difficulty in verification due to lack of public key
The public key is already available on the web site and linked on the instructions web page - see the text " (alternatively, the public key is also available from here)".
If you want to decrypt a message sent to yourself you need the same private key in both email clients. Did you import they OpenPGP key you're using in Postbox to Thunderbird?
Sender's Autocrypt key is not imported
I released Enigmail 3.1.8, which fixes the issue. Autocrypt keys are now automatically imported again when needed. Using Enigmail -> Sender's Key -> Import Public Key will not work - such functionality is not intended.
I debugged this and found that Autocrypt doesn't work indeed on SeaMonkey. I'll fix this.
Sender's Autocrypt key is not imported