Thunderbird 78 uses a different setup than Enigmail. With Enigmail, you can enable OpenPGP for an email account, but have it automatically select the key to use. Thunderbird 78 combines these settings. To enable OpenPGP for an email account, you must explicitly specify the personal key to use.
Therefore, if you used automatic selection before, the migration may not have selected the key. After migration, you should manually check the configuration of all email accounts and identities and manually select the corresponding keys if necessary.
The Enigmail migration was completed successfully, but I still can't use OpenPGP.
If you enabled Enigmail for your email accounts on Thunderbird 68, and you enabled the preference "Use this homeowner database identity's email address to identify OpenPGP keys", OpenPGP may not be automatically enabled in Thunderbird 78. You need to use the account settings to manually select an OpenPGP key for each account and identity you want to use with OpenPGP. Unfortunately, the Enigmail migration will not automatically select them for you.
If there are any problems with the migration, you can repeat it. For example, if you get errors in Thunderbird or you forgot the passwords to all your personal keys and only did a partial migration, the migration may fail. To repeat the migration, you need to access the command from the top menu bar. If you are using Windows or Linux and the top menu bar is not visible, right-click with your mouse in the top area of the Thunderbird main window and enable the menu bar. Then use the "Tools" menu, which contains the "Migrate Enigmail settings" command.
I tried importing the file using the public key but I got an error message saying the file was too large.
See the answers to the following questions.
I previously used OpenPGP with GnuPG, but with different email software. How do I migrate my keys to Thunderbird 78?
You will need to export your keys from the other software first, then re-import them into Thunderbird.
As a way to export personal keys (also called private or secret keys), you can export them to a file using commands in the Command Prompt. To export a key managed by GnuPG, you can use the following command:
gpg –export-secret-keys –armor > my-secret-keys.asc
You can then import them into Thunderbird. Use the Add Keys and Import feature in Thunderbird Account Settings for end-to-end encryption. Or use the Global Menu Bar to open the Tools menu which offers the OpenPGP Key Manager. Use File to import keys and select the file you created above. You probably only have a small number of personal keys, so this method should work.