2 possible causes. 1. You told Thunderbird to remember the password and enter it for you. 2. You have a gmail account using Oauth authorization and gmail issues a token in lew of asking for your password.

If you go to Options-Security-Passwords-Saved Passwords Click show passwords. What do you see? Do not give exactly what is there! Do you see your email password saved? Are you a gmail user and do you see a long random string of characters?

It is possible that I inadvertently ticked the "remember password" check box. That being the case, can this be reversed?

Follow the instructions above to get to the saved password screen and right click the password and remove it.

Sorry, I'm not following your answer. I don't want to remove the password, just remove the automatic login when I start Thunderbird.

Your email password is administered with your email provider. That is the only place you can change it. You are complaining that you no longer have to enter a password. If you told Thunderbird to remember it and enter it for you that is why. You asked how to fix it. I told you where to go in Thunderbird to delete the saved password. What part are you having a problem with.