Step 1: Open your firebase project, select 'Database' and click 'View' under Realtime Database.
Step 2: Select 'Rules'.
Step 3: Change the '.read' and '.write' rules to 'true' and hit enter.
Step 4: Select 'Publish'.
Step 5: Go back to the 'Data' tab. Copy the reference url, and paste it in the Arduino code.
That is it. You are all set! You can go through the example codes to get familiar with the library.