--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -4298,16 +4298,22 @@ nsWindow::SetTransparencyMode(nsTranspar
 
         nsWindow *topWindow = get_window_for_gtk_widget(topWidget);
         if (!topWindow)
             return;
 
         topWindow->SetTransparencyMode(aMode);
         return;
     }
+
+    if (mWindowType != eWindowType_popup) {
+        NS_WARNING("Cannot set transparency mode on non-popup windows.");
+        return;
+    }
+
     bool isTransparent = aMode == eTransparencyTransparent;
 
     if (mIsTransparent == isTransparent)
         return;
 
     if (!isTransparent) {
         ClearTransparencyBitmap();
     } // else the new default alpha values are "all 1", so we don't