diff --git a/web/app.js b/web/app.js
index 68c37d5bf..9e83ab481 100644
--- a/web/app.js
+++ b/web/app.js
@@ -84,29 +84,52 @@ const ViewOnLoad = {
   INITIAL: 1,
 };
 
-const DefaultExternalServices = {
-  updateFindControlState(data) {},
-  updateFindMatchesCount(data) {},
-  initPassiveLoading(callbacks) {},
-  fallback(data, callback) {},
-  reportTelemetry(data) {},
-  createDownloadManager(options) {
+class DefaultExternalServices {
+  constructor() {
+    throw new Error("Cannot initialize DefaultExternalServices.");
+  }
+
+  static updateFindControlState(data) {}
+
+  static updateFindMatchesCount(data) {}
+
+  static initPassiveLoading(callbacks) {}
+
+  static fallback(data, callback) {}
+
+  static reportTelemetry(data) {}
+
+  static createDownloadManager(options) {
     throw new Error("Not implemented: createDownloadManager");
-  },
-  createPreferences() {
+  }
+
+  static createPreferences() {
     throw new Error("Not implemented: createPreferences");
-  },
-  createL10n(options) {
+  }
+
+  static createL10n(options) {
     throw new Error("Not implemented: createL10n");
-  },
-  supportsIntegratedFind: false,
-  supportsDocumentFonts: true,
-  supportsDocumentColors: true,
-  supportedMouseWheelZoomModifierKeys: {
-    ctrlKey: true,
-    metaKey: true,
-  },
-};
+  }
+
+  static get supportsIntegratedFind() {
+    return shadow(this, "supportsIntegratedFind", false);
+  }
+
+  static get supportsDocumentFonts() {
+    return shadow(this, "supportsDocumentFonts", true);
+  }
+
+  static get supportsDocumentColors() {
+    return shadow(this, "supportsDocumentColors", true);
+  }
+
+  static get supportedMouseWheelZoomModifierKeys() {
+    return shadow(this, "supportedMouseWheelZoomModifierKeys", {
+      ctrlKey: true,
+      metaKey: true,
+    });
+  }
+}
 
 const PDFViewerApplication = {
   initialBookmark: document.location.hash.substring(1),
diff --git a/web/chromecom.js b/web/chromecom.js
index 853f773e1..2853c8ccb 100644
--- a/web/chromecom.js
+++ b/web/chromecom.js
@@ -404,27 +404,30 @@ class ChromePreferences extends BasePreferences {
   }
 }
 
-const ChromeExternalServices = Object.create(DefaultExternalServices);
-ChromeExternalServices.initPassiveLoading = function(callbacks) {
-  const { overlayManager } = PDFViewerApplication;
-  // defaultUrl is set in viewer.js
-  ChromeCom.resolvePDFFile(
-    AppOptions.get("defaultUrl"),
-    overlayManager,
-    function(url, length, originalUrl) {
-      callbacks.onOpenWithURL(url, length, originalUrl);
-    }
-  );
-};
-ChromeExternalServices.createDownloadManager = function(options) {
-  return new DownloadManager(options);
-};
-ChromeExternalServices.createPreferences = function() {
-  return new ChromePreferences();
-};
-ChromeExternalServices.createL10n = function(options) {
-  return new GenericL10n(navigator.language);
-};
+class ChromeExternalServices extends DefaultExternalServices {
+  static initPassiveLoading(callbacks) {
+    // defaultUrl is set in viewer.js
+    ChromeCom.resolvePDFFile(
+      AppOptions.get("defaultUrl"),
+      PDFViewerApplication.overlayManager,
+      function(url, length, originalUrl) {
+        callbacks.onOpenWithURL(url, length, originalUrl);
+      }
+    );
+  }
+
+  static createDownloadManager(options) {
+    return new DownloadManager(options);
+  }
+
+  static createPreferences() {
+    return new ChromePreferences();
+  }
+
+  static createL10n(options) {
+    return new GenericL10n(navigator.language);
+  }
+}
 PDFViewerApplication.externalServices = ChromeExternalServices;
 
 export { ChromeCom };
diff --git a/web/firefoxcom.js b/web/firefoxcom.js
index 83db636ce..e80ef3331 100644
--- a/web/firefoxcom.js
+++ b/web/firefoxcom.js
@@ -15,9 +15,9 @@
 
 import "../extensions/firefox/tools/l10n";
 import { createObjectURL, PDFDataRangeTransport, shadow } from "pdfjs-lib";
+import { DefaultExternalServices, PDFViewerApplication } from "./app.js";
 import { BasePreferences } from "./preferences.js";
 import { DEFAULT_SCALE_VALUE } from "./ui_utils.js";
-import { PDFViewerApplication } from "./app.js";
 
 if (
   typeof PDFJSDev === "undefined" ||
@@ -241,16 +241,16 @@ class FirefoxComDataRangeTransport extends PDFDataRangeTransport {
   }
 }
 
-PDFViewerApplication.externalServices = {
-  updateFindControlState(data) {
+class FirefoxExternalServices extends DefaultExternalServices {
+  static updateFindControlState(data) {
     FirefoxCom.request("updateFindControlState", data);
-  },
+  }
 
-  updateFindMatchesCount(data) {
+  static updateFindMatchesCount(data) {
     FirefoxCom.request("updateFindMatchesCount", data);
-  },
+  }
 
-  initPassiveLoading(callbacks) {
+  static initPassiveLoading(callbacks) {
     let pdfDataRangeTransport;
 
     window.addEventListener("message", function windowMessage(e) {
@@ -309,52 +309,53 @@ PDFViewerApplication.externalServices = {
       }
     });
     FirefoxCom.requestSync("initPassiveLoading", null);
-  },
+  }
 
-  fallback(data, callback) {
+  static fallback(data, callback) {
     FirefoxCom.request("fallback", data, callback);
-  },
+  }
 
-  reportTelemetry(data) {
+  static reportTelemetry(data) {
     FirefoxCom.request("reportTelemetry", JSON.stringify(data));
-  },
+  }
 
-  createDownloadManager(options) {
+  static createDownloadManager(options) {
     return new DownloadManager(options);
-  },
+  }
 
-  createPreferences() {
+  static createPreferences() {
     return new FirefoxPreferences();
-  },
+  }
 
-  createL10n(options) {
+  static createL10n(options) {
     const mozL10n = document.mozL10n;
     // TODO refactor mozL10n.setExternalLocalizerServices
     return new MozL10n(mozL10n);
-  },
+  }
 
-  get supportsIntegratedFind() {
+  static get supportsIntegratedFind() {
     const support = FirefoxCom.requestSync("supportsIntegratedFind");
     return shadow(this, "supportsIntegratedFind", support);
-  },
+  }
 
-  get supportsDocumentFonts() {
+  static get supportsDocumentFonts() {
     const support = FirefoxCom.requestSync("supportsDocumentFonts");
     return shadow(this, "supportsDocumentFonts", support);
-  },
+  }
 
-  get supportsDocumentColors() {
+  static get supportsDocumentColors() {
     const support = FirefoxCom.requestSync("supportsDocumentColors");
     return shadow(this, "supportsDocumentColors", support);
-  },
+  }
 
-  get supportedMouseWheelZoomModifierKeys() {
+  static get supportedMouseWheelZoomModifierKeys() {
     const support = FirefoxCom.requestSync(
       "supportedMouseWheelZoomModifierKeys"
     );
     return shadow(this, "supportedMouseWheelZoomModifierKeys", support);
-  },
-};
+  }
+}
+PDFViewerApplication.externalServices = FirefoxExternalServices;
 
 // l10n.js for Firefox extension expects services to be set.
 document.mozL10n.setExternalLocalizerServices({
diff --git a/web/genericcom.js b/web/genericcom.js
index fce57da66..ef81fabdf 100644
--- a/web/genericcom.js
+++ b/web/genericcom.js
@@ -37,16 +37,19 @@ class GenericPreferences extends BasePreferences {
   }
 }
 
-const GenericExternalServices = Object.create(DefaultExternalServices);
-GenericExternalServices.createDownloadManager = function(options) {
-  return new DownloadManager(options);
-};
-GenericExternalServices.createPreferences = function() {
-  return new GenericPreferences();
-};
-GenericExternalServices.createL10n = function({ locale = "en-US" }) {
-  return new GenericL10n(locale);
-};
+class GenericExternalServices extends DefaultExternalServices {
+  static createDownloadManager(options) {
+    return new DownloadManager(options);
+  }
+
+  static createPreferences() {
+    return new GenericPreferences();
+  }
+
+  static createL10n({ locale = "en-US" }) {
+    return new GenericL10n(locale);
+  }
+}
 PDFViewerApplication.externalServices = GenericExternalServices;
 
 export { GenericCom };