const first = firestore.collection("collectionName")
  .orderBy("population")
  .limit(25)
  .get()
  .then(function (documentSnapshots) {
    // Get the last visible document
    const lastVisible = documentSnapshots.docs[documentSnapshots.docs.length-1];

    const next = firestore.collection("collectionName")
      .orderBy("population")
      .startAfter(lastVisible)
      .limit(25)
      .get()
      .then(data => {
        console.log(data.docs.length);
      });
  });