The password is the same password you would use to access your webmail account using a browser. Thunderbird needs to know the password you have set up to access your webmail. You do not create mail account passwords in Thunderbird; it only 'remembers' passwords to faciltate quicker access to viewing, sending and downloading emails. If you cannot access your webmail account because you have forgotten the password, then you need to use the forgotten password link to reset/modify the password. Logging on to webmail is the place with the 'forgotten password' link. Then use that new password when creating an account in thunderbird.