Need to change password for new computer

You do not need a password to use Thunderbird. Thunderbird is a mail client. The message that it requires is your email account password. You can reset your email account password via your email's web interface.

Understood. But it's my email account password that I don't remember. How do I reset that?

You must reset your password at your email provider's web site. Once you do that, you can update your saved password in Thunderbird at <Settings | Privacy & Security |Passwords | Saved Passwords>.