App.js

import Firebase from "../Config/Firebase";


Firebase.firestore()
          .collection("Requests")
          .doc()
          .get();

Firebase.js

import firebase from "firebase/app";
import "firebase/firestore";

var firebaseConfig = {
  apiKey: "XXX",
  authDomain: "XXX",
  databaseURL: "XXX",
  projectId: "XXX",
  storageBucket: "XXX",
  messagingSenderId: "XXX",
  appId: "XXX",
  measurementId: "XXX"
};

const Firebase = firebase.initializeApp(firebaseConfig);
export default !firebase.apps.length ? Firebase : firebase.app();

@RazaShehryar

import firebase from 'firebase';
import 'firebase/firestore';

I think you don't need the /app prefix, only import firebase from 'firebase'