There is no password protection for messages already loaded into Thunderbird. There is an optional Master Password that protects access to any saved passwords and the password to your providers email account that allows you connect to their servers.

There are no Thunderbird email accounts either. Email accounts are provided by email providers.

Thank you for your reply. Did I miss something? To fix the open access to gmail, I should contact gmail for the solution and ask for a password reset?

What are you trying to accomplish? You stated you could view messages in Thunderbird without entering a password. I explained that.

Dear Airmail, I changed the password in Google, the email provider, and solved the problem. Somehow, Thunderbird allowed access to my email whenever an incorrect password was entered. I contacted Mozilla Support to stop this unauthorized access to my email. As it turns out, the password needed to be changed/reset in Google. (An account is not necessary.) Thanks for your willingness to help solve problems.

This forum is the only Thunderbird support so I have no idea who you talked to. Thunderbird is software running on your computer. I have no idea what unauthorized access is supposed to mean. I guess you finally figured out how to use gmail.