This is Development release. Although working successfully with some live Drupal projects.
I suppose you can use this module as it is, Please post any issue bug you found I will try to resolve them on high priority..
Module configuration is 2 steps configuration for web and for mobile you need to configuration mobile app on https://console.firebase.google.com/ and ned to install and configure services with REST API server module before using web service to register FCM token.
README.md file helps you to setup module.