--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -189,46 +189,34 @@ const PanelUI = {
    * Opens the menu panel. If the event target has a child with the
    * toolbarbutton-icon attribute, the panel will be anchored on that child.
    * Otherwise, the panel is anchored on the event target itself.
    *
    * @param aEvent the event (if any) that triggers showing the menu.
    */
   show(aEvent) {
     this._ensureShortcutsShown();
-    return new Promise(resolve => {
-      this.ensureReady().then(() => {
-        if (this.panel.state == "open" ||
-            document.documentElement.hasAttribute("customizing")) {
-          resolve();
-          return;
-        }
+    (async () => {
+      await this.ensureReady();
+
+      if (this.panel.state == "open" ||
+          document.documentElement.hasAttribute("customizing")) {
+        return;
+      }
 
-        let anchor;
-        let domEvent = null;
-        if (!aEvent ||
-            aEvent.type == "command") {
-          anchor = this.menuButton;
-        } else {
-          domEvent = aEvent;
-          anchor = aEvent.target;
-        }
+      let domEvent = null;
+      if (aEvent && aEvent.type != "command") {
+        domEvent = aEvent;
+      }
 
-        this.panel.addEventListener("popupshown", function() {
-          resolve();
-        }, {once: true});
-
-        anchor = this._getPanelAnchor(anchor);
-        PanelMultiView.openPopup(this.panel, anchor, {
-          triggerEvent: domEvent,
-        }).catch(Cu.reportError);
-      }, (reason) => {
-        console.error("Error showing the PanelUI menu", reason);
+      let anchor = this._getPanelAnchor(this.menuButton);
+      await PanelMultiView.openPopup(this.panel, anchor, {
+        triggerEvent: domEvent,
       });
-    });
+    })().catch(Cu.reportError);
   },
 
   /**
    * If the menu panel is being shown, hide it.
    */
   hide() {
     if (document.documentElement.hasAttribute("customizing")) {
       return;

--- a/browser/tools/mozscreenshots/mozscreenshots/extension/configurations/AppMenu.jsm
+++ b/browser/tools/mozscreenshots/mozscreenshots/extension/configurations/AppMenu.jsm
@@ -54,17 +54,17 @@ var AppMenu = {
 
   },
 };
 
 async function reopenAppMenu(browserWindow) {
   browserWindow.PanelUI.hide();
   let view = browserWindow.document.getElementById("appMenu-mainView");
   let promiseViewShown = BrowserTestUtils.waitForEvent(view, "ViewShown");
-  await browserWindow.PanelUI.show();
+  browserWindow.PanelUI.show();
   await promiseViewShown;
 }
 
 function verifyConfigHelper() {
   if (isCustomizing()) {
     return "navigator:browser has the customizing attribute";
   }
   return undefined;