diff -up firefox-128.0/widget/gtk/nsWindow.cpp.mozilla-bmo1636168-fscreen firefox-128.0/widget/gtk/nsWindow.cpp
--- firefox-128.0/widget/gtk/nsWindow.cpp.mozilla-bmo1636168-fscreen	2024-07-04 18:20:43.000000000 +0200
+++ firefox-128.0/widget/gtk/nsWindow.cpp	2024-07-16 14:54:21.026716936 +0200
@@ -174,7 +174,8 @@ const gint kEvents = GDK_TOUCHPAD_GESTUR
                      GDK_ENTER_NOTIFY_MASK | GDK_LEAVE_NOTIFY_MASK |
                      GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK |
                      GDK_SMOOTH_SCROLL_MASK | GDK_TOUCH_MASK | GDK_SCROLL_MASK |
-                     GDK_POINTER_MOTION_MASK | GDK_PROPERTY_CHANGE_MASK;
+                     GDK_POINTER_MOTION_MASK | GDK_PROPERTY_CHANGE_MASK |
+                     GDK_FOCUS_CHANGE_MASK;
 
 /* utility functions */
 static bool is_mouse_in_window(GdkWindow* aWindow, gdouble aMouseX,
@@ -430,7 +431,8 @@ nsWindow::nsWindow()
       mResizedAfterMoveToRect(false),
       mConfiguredClearColor(false),
       mGotNonBlankPaint(false),
-      mNeedsToRetryCapturingMouse(false) {
+      mNeedsToRetryCapturingMouse(false),
+      mPendingFullscreen(false) {
   mWindowType = WindowType::Child;
   mSizeConstraints.mMaxSize = GetSafeWindowSize(mSizeConstraints.mMaxSize);
 
@@ -5374,6 +5376,19 @@ void nsWindow::OnWindowStateEvent(GtkWid
       ClearTransparencyBitmap();
     }
   }
+
+  // Hack to ensure window switched to fullscreen - avoid to fail when starting
+  // in kiosk mode
+  if (mPendingFullscreen &&
+      !(aEvent->new_window_state & GDK_WINDOW_STATE_FULLSCREEN)) {
+    LOG(
+        "  Window should be fullscreen, but it's not, retrying set to "
+         "fullscreen.\n");
+    MakeFullScreen(true);
+  } else {
+    LOG("  Window successfully switched to fullscreen, happy now\n");
+    mPendingFullscreen = false;
+  }
 }
 
 void nsWindow::OnDPIChanged() {
@@ -7526,6 +7541,7 @@ nsresult nsWindow::MakeFullScreen(bool a
 
     if (mKioskMonitor.isSome()) {
       KioskLockOnMonitor();
+      mPendingFullscreen = true;
     } else {
       gtk_window_fullscreen(GTK_WINDOW(mShell));
     }
diff -up firefox-128.0/widget/gtk/nsWindow.h.mozilla-bmo1636168-fscreen firefox-128.0/widget/gtk/nsWindow.h
--- firefox-128.0/widget/gtk/nsWindow.h.mozilla-bmo1636168-fscreen	2024-07-04 18:20:43.000000000 +0200
+++ firefox-128.0/widget/gtk/nsWindow.h	2024-07-16 14:25:51.636952919 +0200
@@ -758,6 +758,7 @@ class nsWindow final : public nsBaseWidg
    * move-to-rect callback we set mMovedAfterMoveToRect/mResizedAfterMoveToRect.
    */
   bool mWaitingForMoveToRectCallback : 1;
+  bool mPendingFullscreen : 1;
   bool mMovedAfterMoveToRect : 1;
   bool mResizedAfterMoveToRect : 1;