All of that already works without the need for making your own HTTPRequests. :) That's all handled under the hood. The project has an example built-in that should show how to use that stuff. Here's a link to where I listen for chats:
https://github.com/WolfgangSenff/GodotFirebase/blob/master/Demo/Chat.gd
Here's a link to where I submit the chats to Firebase:
https://github.com/WolfgangSenff/GodotFirebase/blob/master/Demo/ChatControl.gd#L26
All the CRUD operations are handled through the Firebase.Database global. You can get references there (that's usually what you'd do - it essentially gives back the position in the database you want to point at to look for updates or send data), and access to the Auth variable.
https://github.com/WolfgangSenff/GodotFirebase/blob/master/Demo/addons/GDFirebase/Firebase.gd
Ignore Firestore and Storage for now - those are still works in progress, although I found out at I/O this year that Storage cannot be implemented because there's no REST interface. Annoying, but I guess it makes some sense, since it's primarily for sending/receiving large files.