I've lost my Thunderbird password.

There is no such thing as a "Thunderbird password". Thunderbird is an application run on your device, so it does not provide email addresses, nor the service to transmit emails. But Thunderbird can be used with your existing email address and service provider to send, receive, sort and search your email messages. Hence the password is needed for your email provider's server, and Thunderbird is just prompting you for it.

If you told Thunderbird to remember the password, it may be stored in the Thunderbird password manager, unless your email account uses OAuth2 authentication. In that case Thunderbird will only store an authentication token, and not the actual account password.

So you'd have to check with your email provider about password reset options.

Thanks for your help. I didn't understand that "Thunderbird password" didn't mean that at all. Maybe someday they could rephrase that to, "password of the account you are referencing", or something. Anyway, good to go.