Perhaps a bit of a silly question, but, is the traffic from/to firebase DB server compressed?
If so, what algorithm(s)?
What compression ratios
The communication between a Firebase client and its Database Servers goes over a secured web socket connection. The data is not compressed.
You can easily see this yourself by accessing the Firebase Database from your browser and then looking at the network tab. It'll show you exactly what data is being exchanged and in which format.