I am not familiar with the browser extensions that might allow you to encrypt/decrypt emails, but I am pretty sure Thunderbird, if it still supports PGP, would encrypt your email on the client; the email providers aren't going to see the unencrypted email (except the headers).
AFAIK, secure messaging apps are the same: E2EE, with encryption/decryption done on the client, so the service providers only see the unencrypted metadata.