If you are setting up Thunderbird to use your Gmail account you may find that when Thunderbird opens a new window to a Google web portal into which you are to provide your email address and password that it will keep asking you over and over again for your email and never enable to you to enter the password.
This occurs when Thunderbird’s privacy settings do not allow it to store cookies.
First, ensure that your gmail account has Allow insecure apps off. Unfortunately, it may take some time for this setting to propagate to your account.
Than, go to Preferences > Privacy and under Web Content check the Accept cookies from sites checkbox.
Return to your account settings when when prompted you should now be able to enter your credentials to grant Thunderbird access to your account.