Add a public key into Enigmail Key Manager for Thunderbird
Go to the Kebab / Burger menu on the right, then go to Tools and then click on OpenPGP Key Manager
Select File then Import Public Key(s) From File
Select the public key you received from the person you need to send email to and press OK.
Check that the email address matches the one from the person you will send email to and click OK.
Once the key is successfully imported, click on the View Details and manage key acceptance
Verify that the key is the correct one. You would need to get in touch with the person by phone for example and ask the user for his key fingerprint.
Once you are sure the key is correct, select Yes, I’ve verified in person this key has the correct fingerprint and press OK.
Press OK to finish the import process