--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1122,16 +1122,21 @@ addEventListener("DOMContentLoaded", fun
     if (tabToOpen instanceof XULElement && tabToOpen.hasAttribute("usercontextid")) {
       initBrowser.setAttribute("usercontextid", tabToOpen.getAttribute("usercontextid"));
     }
   }
 
   gBrowser.updateBrowserRemoteness(initBrowser, gMultiProcessBrowser);
 });
 
+let _resolveDelayedStartup;
+var delayedStartupPromise = new Promise(resolve => {
+  _resolveDelayedStartup = resolve;
+});
+
 var gBrowserInit = {
   delayedStartupFinished: false,
 
   onLoad() {
     gBrowser.addEventListener("DOMUpdatePageReport", gPopupBlockerObserver);
 
     Services.obs.addObserver(gPluginHandler.NPAPIPluginCrashed, "plugin-crashed");
 
@@ -1601,16 +1606,17 @@ var gBrowserInit = {
         if (panel.state == "open") {
           panel.hidePopup();
         }
       }
     });
 
     this.delayedStartupFinished = true;
 
+    _resolveDelayedStartup();
     Services.obs.notifyObservers(window, "browser-delayed-startup-finished");
     TelemetryTimestamps.add("delayedStartupFinished");
   },
 
   // Returns the URI(s) to load at startup.
   _getUriToLoad() {
     // window.arguments[0]: URI to load (string), or an nsIArray of
     //                      nsISupportsStrings to load, or a xul:tab of

--- a/browser/modules/ExtensionsUI.jsm
+++ b/browser/modules/ExtensionsUI.jsm
@@ -30,24 +30,26 @@ const BRAND_PROPERTIES = "chrome://brand
 const HTML_NS = "http://www.w3.org/1999/xhtml";
 
 this.ExtensionsUI = {
   sideloaded: new Set(),
   updates: new Set(),
   sideloadListener: null,
   histogram: null,
 
-  init() {
+  async init() {
     this.histogram = Services.telemetry.getHistogramById("EXTENSION_INSTALL_PROMPT_RESULT");
 
     Services.obs.addObserver(this, "webextension-permission-prompt");
     Services.obs.addObserver(this, "webextension-update-permissions");
     Services.obs.addObserver(this, "webextension-install-notify");
     Services.obs.addObserver(this, "webextension-optional-permission-prompt");
 
+    await RecentWindow.getMostRecentBrowserWindow().delayedStartupPromise;
+
     this._checkForSideloaded();
   },
 
   _checkForSideloaded() {
     AddonManager.getAllAddons(addons => {
       // Check for any side-loaded addons that the user is allowed
       // to enable.
       let sideloaded = addons.filter(