After downloading the Integrate Firebase PRO plugin, please follow the video in order to have better review of how to install and make use of this plugin in your WordPress site.
The current version of the plugin works best in the situation that you want to take advantage of Firebase features without connecting with the default WordPress users database.
List of features
Login to WordPress with Firebase Authentication
A headless CMS solution from WordPress
Firebase User Management in WordPress Dashboard
Save data from WordPress to Firebase
Retrieve data from Realtime / Firestore
Dynamically display Realtime / Firestore Data