Firebase database is a NoSQL database that stores its data in the form of hierarchal JSON objects. There are no tables or records of any form as an SQL database would normally have, just nodes that make up a key-value structure.
Data Normalization
In order to have a properly designed database structure, the data requirements must be thoroughly outlined and forethought. The structure in this case should be normalized; the more flat the JSON tree, the faster data-access is.