the problem is the symmetric encrypted private key cannot be decrypt directly in thunderbird.
This looks like a general problem from tb with symmetric keys.
So before importing the key in newer tb you need first to decrypt the key.
By the way the in html enveloped attachment it makes it more complex, because gnupg does not find a key directly in the file so the ascii part of the encrypted key must be firstly stored to a legal file:
- Send the private key to yourself by using the button in dc below -settings- -> -advanced-. Leave the window open because you need the 4x9 code to decrypt the key.
- In thunderbird open the inbox and identify and open the new autocrypt message which contains a autocrypt….html file
- Store the file on local filesystem.
- Edit the file and remove all the html stuff so only the lines beginning
-----BEGIN PGP MESSAGE----- … -----END PGP MESSAGE-----is left. Save the file with .asc.asc (double .asc) extension.
- Use in a terminal to decrypt the file to yourfilename.asc. As encryption key you must yuse the setup code from your dc with all minus-signs like 1234-5678-9012-2345-…-7890. This will create the yourfilename.asc which contains your unprotected private key.
- Use tb and open the PGP KeyManager in Tools menu.
- Here in File -> Import Secret Key File choose the unprotected yourfilename.asc to import the key to thunderbird 78.
- Check preferences.
You should now be able to decrypt the mails in deltachat imap folder.
If you are using other gpg frontends like gpa maybe it is easier to use them by using the clipboard. You can open the html attachement and copy simply the whole pgp part in the clipboard, then decrypt the content by the tool.
But before importing to tb mostly you must use a file where you save the encrypted private key.
It is also possible for a direct import of an decrypted key as attachement (with asc ending) is more complex because you must sent a new mail with the encrypted file to yourself ant this is more unsafe.