Alright, first things first, let's get those offline capabilities up and running on your Firebase project. This is super important because it keeps your Firebase functions available even when there's no network connection. Both Realtime Database and Firestore have features for this.

// For Firebase Realtime Database
FirebaseDatabase.getInstance().setPersistenceEnabled(true);

// For Firestore
FirebaseFirestore.getInstance().setPersistenceEnabled(true);

Now, about Firebase’s .onDisconnect feature. It's pretty handy for cleaning up data and doing certain actions when a user goes offline. But, here's the catch: it only works when a user’s connection is explicitly interrupted, like when they close their browser tab. So, it's not the best for offline-first situations. Don't rely on it for critical stuff.

Once you've got offline persistence enabled, your app can still access its data even when the user is offline. But, you need to make sure to sync your data to the local device. You do this by attaching listeners to your Firebase database references:

databaseReference.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        // Code to handle new data value
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // Code to handle read failure
    }
});

When you've got these listeners attached, all your read and write operations get cached locally. New data and updates will automatically get pushed to your Firebase database when the app goes online again, so you won't lose any data.

If your app allows multiple users to update data, you need to handle concurrent updates properly. Firebase's 'last write wins' strategy can overwrite older changes with the most recent ones, which might not be what you want, especially during an offline/online sync.

With Firestore, you can use custom conflict resolution strategies like FieldValue.serverTimestamp(), FieldValue.arrayUnion(), FieldValue.arrayRemove(), and FieldValue.increment().

Keep in mind that data synced and cached for offline use can take up a lot of local device storage space. Firestore caches up to 40MB of data before it starts removing older data. You can adjust this limit up or down based on your needs, but be careful not to use too much device storage:

FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder()
        .setCacheSizeBytes(FirebaseFirestoreSettings.CACHE_SIZE_UNLIMITED)
        .build();
db.setFirestoreSettings(settings);

And there you have it! You're all set to make your Firebase app work smoothly offline. Happy coding!