The help section on the firebase console says that
Firebase app can have up to 25 uniquely named user properties (case-sensitive). You should use properties for non-variable attributes, such as “handedness=right”, “spender=true”.
For every User Property named k, you need to register an entry in the "User Properties" tab in Firebase Analytics. For every user with User Property value k=v, you need to call
setUserProperty(k,v) (and after logging some events), you will be able to filter your Analytics reports by applying a User Property filter for k=v (favorite_food=pizza) .