diff -up firefox-140.0/widget/gtk/nsWindow.cpp.mozilla-bmo1636168-fscreen firefox-140.0/widget/gtk/nsWindow.cpp
--- firefox-140.0/widget/gtk/nsWindow.cpp.mozilla-bmo1636168-fscreen	2025-06-02 15:27:00.000000000 +0200
+++ firefox-140.0/widget/gtk/nsWindow.cpp	2025-06-12 10:23:04.789578675 +0200
@@ -196,7 +196,7 @@ constexpr gint kEvents =
     GDK_VISIBILITY_NOTIFY_MASK | 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_PROPERTY_CHANGE_MASK | GDK_FOCUS_CHANGE_MASK;
 
 /* utility functions */
 static bool is_mouse_in_window(GdkWindow* aWindow, gdouble aMouseX,
@@ -454,7 +454,8 @@ nsWindow::nsWindow()
       mResizedAfterMoveToRect(false),
       mConfiguredClearColor(false),
       mGotNonBlankPaint(false),
-      mNeedsToRetryCapturingMouse(false) {
+      mNeedsToRetryCapturingMouse(false),
+      mPendingFullscreen(false) {
   mSizeConstraints.mMaxSize = GetSafeWindowSize(mSizeConstraints.mMaxSize);
 
   if (!gGlobalsInitialized) {
@@ -5424,6 +5425,19 @@ void nsWindow::OnWindowStateEvent(GtkWid
   if (mSizeMode != oldSizeMode && mWidgetListener) {
     mWidgetListener->SizeModeChanged(mSizeMode);
   }
+
+  // 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() {
@@ -7239,6 +7253,7 @@ nsresult nsWindow::MakeFullScreen(bool a
 
     if (mKioskMonitor.isSome()) {
       KioskLockOnMonitor();
+      mPendingFullscreen = true;
     } else {
       gtk_window_fullscreen(GTK_WINDOW(mShell));
     }
diff -up firefox-140.0/widget/gtk/nsWindow.h.mozilla-bmo1636168-fscreen firefox-140.0/widget/gtk/nsWindow.h
--- firefox-140.0/widget/gtk/nsWindow.h.mozilla-bmo1636168-fscreen	2025-06-02 15:27:00.000000000 +0200
+++ firefox-140.0/widget/gtk/nsWindow.h	2025-06-12 10:18:42.354025723 +0200
@@ -786,6 +786,7 @@ 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;