Вы можете использовать FirebaseFirestore следующим образом
mFirestore = FirebaseFirestore.getInstance()
mFirestore.collection("YOUR COLLECTION")
.get()
.addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<"YOUR OBJECT">()
for (document in task.result) {
val yourobjectType = document.toObject(YOUR OBJECT::class.java)
list.add(yourobjectType)
}
} else {
Log.d(TAG, "Error getting documents: ", task.exception)
}
}