<link href="https://unpkg.com/grapesjs/dist/css/grapes.min.css" rel="stylesheet"/>
<script src="https://unpkg.com/grapesjs"></script>
<script src="path/to/grapesjs-firestore.min.js"></script>

<script src="https://www.gstatic.com/firebasejs/8.10.1/firebase.js"></script>
<script src="https://www.gstatic.com/firebasejs/8.10.1/firebase-firestore.js"></script>

<div id="gjs"></div>

<script type="text/javascript">
  var editor = grapesjs.init({
      container : '#gjs',
      ...
      storageManager: { type: 'firestore' },
      plugins: ['grapesjs-firestore'],
      pluginsOpts: {
        'grapesjs-firestore': {
          docId: 'someID',
          apiKey: '<API_KEY>',
          authDomain: '<PROJECT_ID>.firebaseapp.com',
          projectId: '<PROJECT_ID>',
        }
      }
  });
</script>