How to load the Firebase Firestore with ES6 module without using the bundler tool. I use it to lazy load the Firebase Firebase in my vanilla web component. To use it you just need to import and construct an instance, then call load() and handle the promise resolve.

firestore-module.js

export default class FirestoreModule{
    constructor(apiKey,authDomain,databaseURL,projectId){
        this.promise=false;
        this.db=null;
        this.apiKey=apiKey;
        this.authDomain=authDomain;
        this.databaseURL=databaseURL;
        this.projectId=projectId;
    }
    
    load() {
        if (!this.promise) {
          this.promise = new Promise(resolve => {
            if (typeof window.firebase === 'undefined') {
              this.addScripts().then(()=>{
                firebase.initializeApp({
                    apiKey: this.apiKey,
                    authDomain: this.authDomain,
                    databaseURL: this.databaseURL,
                    projectId: this.projectId,
                  });
                  
                  // Initialize Cloud Firestore through Firebase
                  
                  this.db = firebase.firestore();
                  const settings = {/* your settings... */ timestampsInSnapshots: true};
                  this.db.settings(settings);
                  resolve(this.db);
              });
            } else {
              resolve(this.db);
            }
          });
        }
    
        return this.promise;
    }
    
    //Add script tags to document
    addScripts(){
        return new Promise(resolve=>{
            const app = document.createElement('script');
            app.src = `https://www.gstatic.com/firebasejs/5.0.4/firebase-app.js`;
            app.onload = ()=>{
                const firestore = document.createElement('script');
                firestore.src = `https://www.gstatic.com/firebasejs/5.0.4/firebase-firestore.js`;
                firestore.onload=()=>{
                    resolve();
                };
                document.body.append(firestore);
            };
            document.body.append(app);
        });
    }
    
}