diff -up firefox-69.0/widget/gtk/nsWindow.cpp.1568569 firefox-69.0/widget/gtk/nsWindow.cpp
--- firefox-69.0/widget/gtk/nsWindow.cpp.1568569	2019-08-29 09:42:10.484631750 +0200
+++ firefox-69.0/widget/gtk/nsWindow.cpp	2019-08-29 09:49:35.371121273 +0200
@@ -3252,6 +3252,8 @@ void nsWindow::OnWindowStateEvent(GtkWid
     return;
   }
 
+  nsSizeMode lastSizeState = mSizeState;
+
   if (aEvent->new_window_state & GDK_WINDOW_STATE_ICONIFIED) {
     LOG(("\tIconified\n"));
     mSizeState = nsSizeMode_Minimized;
@@ -3275,6 +3277,18 @@ void nsWindow::OnWindowStateEvent(GtkWid
 #endif  // ACCESSIBILITY
   }
 
+  // Fullscreen video playback may generate bogus alpha values which blends
+  // with desktop background in fullscreen video playback (Bug 1568569).
+  // As a workaround enable to draw mShell background in fullscreen mode
+  // if we draw to mContainer.
+  if (gtk_widget_get_has_window(GTK_WIDGET(mContainer))) {
+    if (mSizeState == nsSizeMode_Fullscreen) {
+      gtk_widget_set_app_paintable(mShell, FALSE);
+    } else if (lastSizeState == nsSizeMode_Fullscreen) {
+      gtk_widget_set_app_paintable(mShell, TRUE);
+    }
+  }
+
   if (mWidgetListener) {
     mWidgetListener->SizeModeChanged(mSizeState);
     if (aEvent->changed_mask & GDK_WINDOW_STATE_FULLSCREEN) {
@@ -3800,6 +3814,11 @@ nsresult nsWindow::Create(nsIWidget* aPa
       gtk_widget_add_events(eventWidget, kEvents);
       if (drawToContainer) {
         gtk_widget_add_events(mShell, GDK_PROPERTY_CHANGE_MASK);
+        // Don't paint mShell background when we draw to mContainer.
+        // Otherwise we see mShell backround to shine through
+        // mContainer (Bug 1507608).
+        // But it may also lead to various bugs where mContainer has unintended
+        // transparent parts which blend with underlying desktop (Bug 1516224).
         gtk_widget_set_app_paintable(mShell, TRUE);
       }
       if (mTransparencyBitmapForTitlebar) {