diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
index 2bf27a461a3c..9d8367f728b8 100644
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -5715,6 +5715,11 @@ nsresult nsWindow::MakeFullScreen(bool aFullScreen, nsIScreen* aTargetScreen) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
+  bool wasFullscreen = mSizeState == nsSizeMode_Fullscreen;
+  if (aFullScreen != wasFullscreen && mWidgetListener) {
+    mWidgetListener->FullscreenWillChange(aFullScreen);
+  }
+
   if (aFullScreen) {
     if (mSizeMode != nsSizeMode_Fullscreen) mLastSizeMode = mSizeMode;