Firebase does not support random picking directly. User needs to fetch all items then picking in local. But this solution might not efficient since it fetch all items back.
So that I made another plugin for picking with conditions (expression) - rex_firebase_itemfilter, it saved each key of item and the properties into individual nodes. Call "Action:Get random items" to get keys of random items (dataset), then fetch these items by keys in other plugin like rex_firebase. (rex_firebase_itemfilter only used to get keys)
There is another rex_firebase_itemtable for storing item (dataset) in 1d hash table.