how do I find what my icloud.com settings are so I can provide that information to Mozilla-Thunderbird in order to sent and receive emails

See the following Apple Support article for information on setting up iCloud email in a 3rd party email app such as Thunderbird: iCloud Mail server settings for other email client apps - Apple Support

Note in particular that you will need to turn on two-factor authentication for your Apple ID, if not already on, and that you will need to use an app-specific password instead of your normal Apple ID password. See: Two-factor authentication for Apple ID - Apple Support

and

Sign in to apps with your Apple ID using app-specific passwords - Apple Support