Not sure what you mean when you say the "password for Thunderbird", but this being a new installation I guess you're referring to the password needed to log into your email account, not to any Thunderbird-specific password… You cannot reset the password of your email account using a mail client such as Thunderbird. All that Thunderbird can do is <i>use</i> the password you tell it to use. If Thunderbird (or any other mail client) could reset the password needed to get access to your email account without knowing what the current password is, then anyone would be able to get access to your mail account by merely knowing your email address and resetting the password… It's the email provider that hosts your email account who knows the password and may allow you to change it. It's there where you have to go (and be able to authenticate or prove your identity somehow) to request a password reset.