There is no such thing as a 'Mozilla Thunderbird sign in 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. So you'd have to talk to your email provider about password recovery options.

Thank you for the input. However, attached is an image of the window that appears when I start Thunderbird. How do you explain this and how do I proceed to open Thunderbird?

That is the Primary password. Use of it is optional and, at some point in the past, you created a Primary Password. If you have forgotten it, you can do this:

Go to tools>Developertools>error console - enter this text

   openDialog("chrome://pippki/content/resetpassword.xhtml")   

- the Reset button should appear. All passwords for accounts will be deleted