--- a/devtools/shim/devtools-startup.js
+++ b/devtools/shim/devtools-startup.js
@@ -183,17 +183,17 @@ DevToolsStartup.prototype = {
    */
   developerToggleCreated: false,
 
   /**
    * Flag that checks if the DevTools addon has been installed and started.
    */
   isDevToolsAvailable: false,
 
-  handle: function (cmdLine) {
+  handle(cmdLine) {
     // Update isDevToolsAvailable by checking if resource://devtools is registered.
     this.isDevToolsAvailable = Services.io.getProtocolHandler("resource")
                                .QueryInterface(Ci.nsIResProtocolHandler)
                                .hasSubstitution("devtools");
 
     let consoleFlag = cmdLine.handleFlag("jsconsole", false);
     let debuggerFlag = cmdLine.handleFlag("jsdebugger", false);
     let devtoolsFlag = cmdLine.handleFlag("devtools", false);
@@ -289,19 +289,19 @@ DevToolsStartup.prototype = {
       id: id,
       type: "view",
       viewId: "PanelUI-developer",
       shortcutId: "key_toggleToolbox",
       tooltiptext: "developer-button.tooltiptext2",
       defaultArea: AppConstants.MOZ_DEV_EDITION ?
                      CustomizableUI.AREA_NAVBAR :
                      CustomizableUI.AREA_PANEL,
-      onViewShowing: (event) => {
+      onViewShowing: async function (event) {
         // Ensure creating the menuitems in the system menu before trying to copy them.
-        this.initDevTools("HamburgerMenu");
+        await this.initDevTools("HamburgerMenu");
 
         // Populate the subview with whatever menuitems are in the developer
         // menu. We skip menu elements, because the menu panel has no way
         // of dealing with those right now.
         let doc = event.target.ownerDocument;
 
         let menu = doc.getElementById("menuWebDeveloperPopup");
 
@@ -311,17 +311,17 @@ DevToolsStartup.prototype = {
         itemsToDisplay.push(doc.getElementById("goOfflineMenuitem"));
 
         let developerItems = doc.getElementById("PanelUI-developerItems");
         // Import private helpers from CustomizableWidgets
         let { clearSubview, fillSubviewFromMenuItems } =
           Cu.import("resource:///modules/CustomizableWidgets.jsm", {});
         clearSubview(developerItems);
         fillSubviewFromMenuItems(itemsToDisplay, developerItems);
-      },
+      }.bind(this),
       onInit(anchor) {
         // Since onBeforeCreated already bails out when initialized, we can call
         // it right away.
         this.onBeforeCreated(anchor.ownerDocument);
       },
       onBeforeCreated(doc) {
         // Bug 1223127, CUI should make this easier to do.
         if (doc.getElementById("PanelUI-developerItems")) {
@@ -363,18 +363,18 @@ DevToolsStartup.prototype = {
 
     // Appending a <key> element is not always enough. The <keyset> needs
     // to be detached and reattached to make sure the <key> is taken into
     // account (see bug 832984).
     let mainKeyset = doc.getElementById("mainKeyset");
     mainKeyset.parentNode.insertBefore(keyset, mainKeyset);
   },
 
-  onKey(window, key) {
-    let require = this.initDevTools("KeyShortcut");
+  async onKey(window, key) {
+    let require = await this.initDevTools("KeyShortcut");
     let { gDevToolsBrowser } = require("devtools/client/framework/devtools-browser");
     gDevToolsBrowser.onKeyShortcut(window, key);
   },
 
   // Create a <xul:key> DOM Element
   createKey(doc, { id, toolId, shortcut, modifiers: mod }, oncommand) {
     let k = doc.createElement("key");
     k.id = "key_" + (id || toolId);
@@ -391,32 +391,39 @@ DevToolsStartup.prototype = {
 
     // Bug 371900: command event is fired only if "oncommand" attribute is set.
     k.setAttribute("oncommand", ";");
     k.addEventListener("command", oncommand);
 
     return k;
   },
 
-  initDevTools: function (reason) {
+  initDevTools(reason) {
     if (!this.isDevToolsAvailable) {
       // Under very specific circumstances, DevTools addon might only be installed but not
       // started. In this case we attempt to reload the addon.
       const { AddonManager } = Components.utils.import("resource://gre/modules/AddonManager.jsm", {});
-      AddonManager.getAddonByID("devtools@mozilla.org", (addon) => {
-        addon.enabled = true;
-        addon.active = true;
-        addon.reload();
 
-        // DevTools should now be ready to be used.
-        this.isDevToolsAvailable = true;
+      return new Promise((resolve, reject) => {
+        AddonManager.getAddonByID("devtools@mozilla.org", (addon) => {
+          if (!addon) {
+            reject("Firefox DevTools addon had to be reloaded , please try again." +
+                   "Force addon scan via `--setpref extensions.startupScanScopes=15`");
+          }
+          addon.enabled = true;
+          addon.active = true;
+          addon.reload();
+
+          // DevTools should now be ready to be used.
+          this.isDevToolsAvailable = true;
+
+          // Call initDevTools again, which should now resolve correctly.
+          resolve(this.initDevTools(reason));
+        });
       });
-
-      throw new Error("Firefox DevTools addon had to be reloaded , please try again." +
-                      "Force addon scan via `--setpref extensions.startupScanScopes=15`");
     }
 
     if (reason && !this.recorded) {
       // Only save the first call for each firefox run as next call
       // won't necessarely start the tool. For example key shortcuts may
       // only change the currently selected tool.
       try {
         Services.telemetry.getHistogramById("DEVTOOLS_ENTRY_POINT")
@@ -429,41 +436,41 @@ DevToolsStartup.prototype = {
     if (!this.initialized) {
       Services.prefs.setBoolPref("devtools.enabled", true);
     }
     this.initialized = true;
     let { require } = Cu.import("resource://devtools/shared/Loader.jsm", {});
     // Ensure loading main devtools module that hooks up into browser UI
     // and initialize all devtools machinery.
     require("devtools/client/framework/devtools-browser");
-    return require;
+    return Promise.resolve(require);
   },
 
-  handleConsoleFlag: function (cmdLine) {
+  async handleConsoleFlag(cmdLine) {
     let window = Services.wm.getMostRecentWindow("devtools:webconsole");
     if (!window) {
-      this.initDevTools("CommandLine");
+      await this.initDevTools("CommandLine");
 
       let { require } = Cu.import("resource://devtools/shared/Loader.jsm", {});
       let { HUDService } = require("devtools/client/webconsole/hudservice");
       let { console } = Cu.import("resource://gre/modules/Console.jsm", {});
       HUDService.toggleBrowserConsole().catch(console.error);
     } else {
       // the Browser Console was already open
       window.focus();
     }
 
     if (cmdLine.state == Ci.nsICommandLine.STATE_REMOTE_AUTO) {
       cmdLine.preventDefault = true;
     }
   },
 
   // Open the toolbox on the selected tab once the browser starts up.
-  handleDevToolsFlag: function (window) {
-    const require = this.initDevTools("CommandLine");
+  async handleDevToolsFlag(window) {
+    const require = await this.initDevTools("CommandLine");
     const {gDevTools} = require("devtools/client/framework/devtools");
     const {TargetFactory} = require("devtools/client/framework/target");
     let target = TargetFactory.forTab(window.gBrowser.selectedTab);
     gDevTools.showToolbox(target);
   },
 
   _isRemoteDebuggingEnabled() {
     let remoteDebuggingEnabled = false;
@@ -483,17 +490,17 @@ DevToolsStartup.prototype = {
       console.error(new Error(errorMsg));
       // Dump as well, as we're doing this from a commandline, make sure people
       // don't miss it:
       dump(errorMsg + "\n");
     }
     return remoteDebuggingEnabled;
   },
 
-  handleDebuggerFlag: function (cmdLine) {
+  handleDebuggerFlag(cmdLine) {
     if (!this._isRemoteDebuggingEnabled()) {
       return;
     }
 
     let devtoolsThreadResumed = false;
     let pauseOnStartup = cmdLine.handleFlag("wait-for-jsdebugger", false);
     if (pauseOnStartup) {
       let observe = function (subject, topic, data) {
@@ -533,17 +540,17 @@ DevToolsStartup.prototype = {
    *   Start the server on a Unix domain socket.
    *
    * --start-debugger-server ws:6789
    *   Start the WebSocket server on port 6789.
    *
    * --start-debugger-server ws:
    *   Start the WebSocket server on the default port (taken from d.d.remote-port)
    */
-  handleDebuggerServerFlag: function (cmdLine, portOrPath) {
+  handleDebuggerServerFlag(cmdLine, portOrPath) {
     if (!this._isRemoteDebuggingEnabled()) {
       return;
     }
 
     let webSocket = false;
     let defaultPort = Services.prefs.getIntPref("devtools.debugger.remote-port");
     if (portOrPath === true) {
       // Default to pref values if no values given on command line
@@ -614,17 +621,17 @@ DevToolsStartup.prototype = {
 
 /**
  * Singleton object that represents the JSON View in-content tool.
  * It has the same lifetime as the browser.
  */
 const JsonView = {
   initialized: false,
 
-  initialize: function () {
+  initialize() {
     // Prevent loading the frame script multiple times if we call this more than once.
     if (this.initialized) {
       return;
     }
     this.initialized = true;
 
     // Load JSON converter module. This converter is responsible
     // for handling 'application/json' documents and converting
@@ -642,17 +649,17 @@ const JsonView = {
   },
 
   // Message handlers for events from child processes
 
   /**
    * Save JSON to a file needs to be implemented here
    * in the parent process.
    */
-  onSave: function (message) {
+  onSave(message) {
     let chrome = Services.wm.getMostRecentWindow("navigator:browser");
     let browser = chrome.gBrowser.selectedBrowser;
     if (message.data.url === null) {
       // Save original contents
       chrome.saveBrowser(browser, false, message.data.windowID);
     } else {
       // The following code emulates saveBrowser, but:
       // - Uses the given blob URL containing the custom contents to save.