FirebaseArduino is a library to simplify connecting to the Firebase database from arduino clients.
It is a full abstraction of Firebase's REST API exposed through C++ calls in a wiring friendly way.
ArduinoJson is no longer part of this library and you will have to install latest version in Arduino environment yourself. (through Board manager or download+unpack from master: https://github.com/bblanchon/ArduinoJson)