How to skip initial data and trigger only new updates in Firestore Firebase?

There is an option to check if the querySnapshot is from a cache, changes return false

if(querySnapshot.getMetadata().isFromCache()) return

When you are listening for changes in Cloud Firestore for realtime changes, using Firestore Query's addSnapshotListener() method, it:

Starts listening to this query.

Which basically means that first time you attach the listener, you get all documents that correspond to that particular query. Furthermore, everytime a property within a document changes, you are notified according to that change. Obviously, this is happening only if the listener remains active and is not removed.

Unfortunately, Firestore listeners don't work that way, so you cannot skip that "case ADDED". What you can do instead, is to add add under each user object a Date property (this is how you can add it) and query your database on client, according to this new property, for all documents that have changed since a previous time.

According to Nick Cardoso's comment, for future visitors that might ask why this behaviour happens, is because the reason he mentioned in his comment. I also recommend see Doug Stevenson's answer from this post, for a better understanding.