If you have forgotten the password, you probably need to contact email provider for a new one. That is, if you are being prompted for a password, that indicates any password stored in Thunderbird is not working. When you click settings>privacy&security, the password being requested there is your WINDOWS password - the one you enter when you boot the PC.

If you are trying to view saved passwords and are unable to get past the "Primary Password" prompt. Try to remember your old master password. The Primpary password in Thunderbird is a local security measure and cannot be bypassed without knowing it. If you don’t have that but still control your email account, resetting the account password via the provider is the fastest route.