Javascript – Firestore emulator | FirebaseError: Expected first argument to collection() to be a CollectionReference, a DocumentReference or FirebaseFirestore

When i make a query to my firestore database i got this error

FirebaseError: Expected first argument to collection() to be a CollectionReference, a DocumentReference or FirebaseFirestore

Apparenly i havent pass the right argument to getFirestore()

Here is my db initialization code:

import { initializeApp } from "firebase/app";
import { getFirestore, connectFirestoreEmulator } from "@firebase/firestore/lite";

class Db {
static firebaseApp = null;

constructor() {
    if (!Db.firebaseApp) {
        Db.firebaseApp = initializeApp({
            apiKey: "",
            authDomain: "",
            projectId: "",
            storageBucket: "",
            messagingSenderId: "",
            appId: "",
        });

        const useEmulator = process.env.NODE_ENV === "development";
        this.db = getFirestore();
        if (useEmulator) {
            const emulatorHost = "192.168.1.51";
            const emulatorPort = 8080;
            connectFirestoreEmulator(this.db, emulatorHost, emulatorPort);
        }
     }
   }
 }

export default Db;

the static property is for executing initializeApp one time at the instantation of the class and not getting this error:

FirebaseError: Firestore has already been started and its settings can no longer be changed. You can only modify settings before calling any other methods on a Firestore object.

Here is the queries i make:

import { collection, getDocs, query, where } from "@firebase/firestore/lite";
    import Db from "./DataBase.js";

class SignUpClientSide extends Db {
    async checkUsernameUniqueness(username) {
        const usersRef = collection(this.db, "users");
        console.log(usersRef);
        const lowercaseQuerySnapshot = await getDocs(query(usersRef, where("username", "==", username.toLowerCase())));
        console.log(lowercaseQuerySnapshot);
        const uppercaseQuerySnapshot = await getDocs(query(usersRef, where("username", "==", username.toUpperCase())));

        if (!lowercaseQuerySnapshot.empty || !uppercaseQuerySnapshot.empty) {
            return false;
        }

        return true;
    }
  }