diff --git a/web/firefoxcom.js b/web/firefoxcom.js
index 113db2470..9853867b7 100644
--- a/web/firefoxcom.js
+++ b/web/firefoxcom.js
@@ -24,7 +24,7 @@ if (typeof PDFJSDev === 'undefined' ||
                   'FIREFOX and MOZCENTRAL builds.');
 }
 
-var FirefoxCom = (function FirefoxComClosure() {
+let FirefoxCom = (function FirefoxComClosure() {
   return {
     /**
      * Creates an event that the extension is listening for and will
@@ -36,17 +36,18 @@ var FirefoxCom = (function FirefoxComClosure() {
      * @return {*} The response.
      */
     requestSync(action, data) {
-      var request = document.createTextNode('');
+      let request = document.createTextNode('');
       document.documentElement.appendChild(request);
 
-      var sender = document.createEvent('CustomEvent');
+      let sender = document.createEvent('CustomEvent');
       sender.initCustomEvent('pdf.js.message', true, false,
                              { action, data, sync: true, });
       request.dispatchEvent(sender);
-      var response = sender.detail.response;
+      let response = sender.detail.response;
       document.documentElement.removeChild(request);
       return response;
     },
+
     /**
      * Creates an event that the extension is listening for and will
      * asynchronously respond by calling the callback.
@@ -56,11 +57,11 @@ var FirefoxCom = (function FirefoxComClosure() {
      * with one data argument.
      */
     request(action, data, callback) {
-      var request = document.createTextNode('');
+      let request = document.createTextNode('');
       if (callback) {
         document.addEventListener('pdf.js.response', function listener(event) {
-          var node = event.target;
-          var response = event.detail.response;
+          let node = event.target;
+          let response = event.detail.response;
 
           document.documentElement.removeChild(node);
 
@@ -70,7 +71,7 @@ var FirefoxCom = (function FirefoxComClosure() {
       }
       document.documentElement.appendChild(request);
 
-      var sender = document.createEvent('CustomEvent');
+      let sender = document.createEvent('CustomEvent');
       sender.initCustomEvent('pdf.js.message', true, false, {
         action,
         data,
@@ -82,48 +83,41 @@ var FirefoxCom = (function FirefoxComClosure() {
   };
 })();
 
-var DownloadManager = (function DownloadManagerClosure() {
-  function DownloadManager() {}
+class DownloadManager {
+  downloadUrl(url, filename) {
+    FirefoxCom.request('download', {
+      originalUrl: url,
+      filename,
+    });
+  }
 
-  DownloadManager.prototype = {
-    downloadUrl: function DownloadManager_downloadUrl(url, filename) {
-      FirefoxCom.request('download', {
-        originalUrl: url,
-        filename,
-      });
-    },
+  downloadData(data, filename, contentType) {
+    let blobUrl = createObjectURL(data, contentType, false);
 
-    downloadData: function DownloadManager_downloadData(data, filename,
-                                                        contentType) {
-      var blobUrl = createObjectURL(data, contentType, false);
+    FirefoxCom.request('download', {
+      blobUrl,
+      originalUrl: blobUrl,
+      filename,
+      isAttachment: true,
+    });
+  }
 
-      FirefoxCom.request('download', {
-        blobUrl,
-        originalUrl: blobUrl,
-        filename,
-        isAttachment: true,
-      });
-    },
+  download(blob, url, filename) {
+    let blobUrl = URL.createObjectURL(blob);
+    let onResponse = (err) => {
+      if (err && this.onerror) {
+        this.onerror(err);
+      }
+      URL.revokeObjectURL(blobUrl);
+    };
 
-    download: function DownloadManager_download(blob, url, filename) {
-      let blobUrl = window.URL.createObjectURL(blob);
-      let onResponse = (err) => {
-        if (err && this.onerror) {
-          this.onerror(err);
-        }
-        window.URL.revokeObjectURL(blobUrl);
-      };
-
-      FirefoxCom.request('download', {
-        blobUrl,
-        originalUrl: url,
-        filename,
-      }, onResponse);
-    },
-  };
-
-  return DownloadManager;
-})();
+    FirefoxCom.request('download', {
+      blobUrl,
+      originalUrl: url,
+      filename,
+    }, onResponse);
+  }
+}
 
 class FirefoxPreferences extends BasePreferences {
   _writeToStorage(prefObj) {
@@ -134,8 +128,8 @@ class FirefoxPreferences extends BasePreferences {
 
   _readFromStorage(prefObj) {
     return new Promise(function(resolve) {
-      FirefoxCom.request('getPreferences', prefObj, function (prefStr) {
-        var readPrefs = JSON.parse(prefStr);
+      FirefoxCom.request('getPreferences', prefObj, function(prefStr) {
+        let readPrefs = JSON.parse(prefStr);
         resolve(readPrefs);
       });
     });
@@ -162,13 +156,13 @@ class MozL10n {
 }
 
 (function listenFindEvents() {
-  var events = [
+  const events = [
     'find',
     'findagain',
     'findhighlightallchange',
     'findcasesensitivitychange'
   ];
-  var handleEvent = function (evt) {
+  let handleEvent = function(evt) {
     if (!PDFViewerApplication.initialized) {
       return;
     }
@@ -183,7 +177,7 @@ class MozL10n {
     });
   };
 
-  for (var i = 0, len = events.length; i < len; i++) {
+  for (let i = 0, len = events.length; i < len; i++) {
     window.addEventListener(events[i], handleEvent);
   }
 })();
@@ -209,7 +203,7 @@ PDFViewerApplication.externalServices = {
   },
 
   initPassiveLoading(callbacks) {
-    var pdfDataRangeTransport;
+    let pdfDataRangeTransport;
 
     window.addEventListener('message', function windowMessage(e) {
       if (e.source !== null) {
@@ -217,7 +211,7 @@ PDFViewerApplication.externalServices = {
         console.warn('Rejected untrusted message from ' + e.origin);
         return;
       }
-      var args = e.data;
+      let args = e.data;
 
       if (typeof args !== 'object' || !('pdfjsLoadAction' in args)) {
         return;
@@ -271,28 +265,28 @@ PDFViewerApplication.externalServices = {
   },
 
   createL10n() {
-    var mozL10n = document.mozL10n;
+    let mozL10n = document.mozL10n;
     // TODO refactor mozL10n.setExternalLocalizerServices
     return new MozL10n(mozL10n);
   },
 
   get supportsIntegratedFind() {
-    var support = FirefoxCom.requestSync('supportsIntegratedFind');
+    let support = FirefoxCom.requestSync('supportsIntegratedFind');
     return shadow(this, 'supportsIntegratedFind', support);
   },
 
   get supportsDocumentFonts() {
-    var support = FirefoxCom.requestSync('supportsDocumentFonts');
+    let support = FirefoxCom.requestSync('supportsDocumentFonts');
     return shadow(this, 'supportsDocumentFonts', support);
   },
 
   get supportsDocumentColors() {
-    var support = FirefoxCom.requestSync('supportsDocumentColors');
+    let support = FirefoxCom.requestSync('supportsDocumentColors');
     return shadow(this, 'supportsDocumentColors', support);
   },
 
   get supportedMouseWheelZoomModifierKeys() {
-    var support = FirefoxCom.requestSync('supportedMouseWheelZoomModifierKeys');
+    let support = FirefoxCom.requestSync('supportedMouseWheelZoomModifierKeys');
     return shadow(this, 'supportedMouseWheelZoomModifierKeys', support);
   },
 };