diff --git a/applets/appmenu/plugin/appmenumodel.h b/applets/appmenu/plugin/appmenumodel.h
--- a/applets/appmenu/plugin/appmenumodel.h
+++ b/applets/appmenu/plugin/appmenumodel.h
@@ -20,6 +20,7 @@
  ******************************************************************/
 
 #include <QAbstractListModel>
+#include <QAbstractNativeEventFilter>
 #include <QStringList>
 #include <KWindowSystem>
 #include <QPointer>
@@ -30,7 +31,7 @@
 class QDBusServiceWatcher;
 class KDBusMenuImporter;
 
-class AppMenuModel : public QAbstractListModel
+class AppMenuModel : public QAbstractListModel, public QAbstractNativeEventFilter
 {
     Q_OBJECT
 
@@ -57,6 +58,9 @@
 signals:
     void requestActivateIndex(int index);
 
+protected:
+    bool nativeEventFilter(const QByteArray &eventType, void *message, long int *result) override;
+
 private Q_SLOTS:
     void onActiveWindowChanged(WId id);
     void update();
@@ -68,6 +72,8 @@
 private:
     bool m_menuAvailable;
 
+    WId m_currentWindowId = 0;
+
     QPointer<QMenu> m_menu;
     QStringList m_activeMenu;
     QList<QAction *> m_activeActions;
diff --git a/applets/appmenu/plugin/appmenumodel.cpp b/applets/appmenu/plugin/appmenumodel.cpp
--- a/applets/appmenu/plugin/appmenumodel.cpp
+++ b/applets/appmenu/plugin/appmenumodel.cpp
@@ -31,6 +31,7 @@
 #endif
 
 #include <QAction>
+#include <QGuiApplication>
 #include <QMenu>
 #include <QDebug>
 #include <QDBusConnection>
@@ -42,6 +43,10 @@
 static const QByteArray s_x11AppMenuServiceNamePropertyName = QByteArrayLiteral("_KDE_NET_WM_APPMENU_SERVICE_NAME");
 static const QByteArray s_x11AppMenuObjectPathPropertyName = QByteArrayLiteral("_KDE_NET_WM_APPMENU_OBJECT_PATH");
 
+#if HAVE_X11
+static QHash<QByteArray, xcb_atom_t> s_atoms;
+#endif
+
 class KDBusMenuImporter : public DBusMenuImporter
 {
 
@@ -123,12 +128,18 @@
 
 void AppMenuModel::onActiveWindowChanged(WId id)
 {
+    qApp->removeNativeEventFilter(this);
+
+    if (!id) {
+        setMenuAvailable(false);
+        emit modelNeedsUpdate();
+        return;
+    }
+
 #if HAVE_X11
     if (KWindowSystem::isPlatformX11()) {
         auto *c = QX11Info::connection();
 
-        static QHash<QByteArray, xcb_atom_t> s_atoms;
-
         auto getWindowPropertyString = [c, this](WId id, const QByteArray &name) -> QByteArray {
             QByteArray value;
             if (!s_atoms.contains(name)) {
@@ -193,6 +204,11 @@
             return;
         }
 
+        // monitor whether an app menu becomes available later
+        // this can happen when an app starts, shows its window, and only later announces global menu (e.g. Firefox)
+        qApp->installNativeEventFilter(this);
+        m_currentWindowId = id;
+
         //no menu found, set it to unavailable
         setMenuAvailable(false);
         emit modelNeedsUpdate();
@@ -274,3 +290,36 @@
     });
 }
 
+bool AppMenuModel::nativeEventFilter(const QByteArray &eventType, void *message, long *result)
+{
+    Q_UNUSED(result);
+
+    if (!KWindowSystem::isPlatformX11() || eventType != "xcb_generic_event_t") {
+        return false;
+    }
+
+#if HAVE_X11
+    auto e = static_cast<xcb_generic_event_t *>(message);
+    const uint8_t type = e->response_type & ~0x80;
+    if (type == XCB_PROPERTY_NOTIFY) {
+        auto *event = reinterpret_cast<xcb_property_notify_event_t *>(e);
+        if (event->window == m_currentWindowId) {
+
+            auto serviceNameAtom = s_atoms.value(s_x11AppMenuServiceNamePropertyName);
+            auto objectPathAtom = s_atoms.value(s_x11AppMenuObjectPathPropertyName);
+
+            if (serviceNameAtom != XCB_ATOM_NONE && objectPathAtom != XCB_ATOM_NONE) { // shouldn't happen
+                if (event->atom == serviceNameAtom || event->atom == objectPathAtom) {
+                    // see if we now have a menu
+                    onActiveWindowChanged(KWindowSystem::activeWindow());
+                }
+            }
+        }
+    }
+#else
+    Q_UNUSED(message);
+#endif
+
+    return false;
+}
+