Qel
The next generation of rex_firebase_itemtable is rex_firebase_itembook, which contains multiple tables and it can modify multiple items across these tables in an atomic write process. i.e. values will be changed at the same time, and it has querying function to pick items by a single condition.
Therefor you could use itembook to query items by a single key condition.
Querying in firebase is limited, uses backendless if you need powerful querying features.