For Firebase tips, see
Beware this danger of adding data to Firebase directly from a non-AI2 program:
Add some data to your Firebase data store using AI2 to send some sample data, then inspect it in the Firebase dashboard. You will see extra wrappers added by AI2 to let it know the data type(s) of the data for when AI2 later retrieves the data.
You will need to send those wrappers from your ESP.