The password you use is the same password you would use to access your webmail for that email address. You cannot enter a 'new' password unless you have already changed your password via webmail and are updating the password stored in Thunderbird.

Please provide info: In Thunderbird Help > TRoubleshooting Information c;lick on 'copy text to clipboard' paste info into this question in forum. You can edit all info on fonts and printers but nothing else.