Please clarify what you mean by "sign in to mozilla". Do you mean this support web site? Or do you mean when you try to send email, in which case, any such blacklist notifications would be from your email provider, not from Thunderbird (though Thunderbird would display them for you,) and you would need to contact them to be removed from the list.

But in the meantime, you can verify that your server settings are correct. They are stored in Account Settings. Your email provider should have their recommended settings available on their help/support web page.