diff --git a/mozilla/widget/src/qt/nsLookAndFeel.cpp b/mozilla/widget/src/qt/nsLookAndFeel.cpp
index d0e59268c5b..b481ca6bc7d 100644
--- a/mozilla/widget/src/qt/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/qt/nsLookAndFeel.cpp
@@ -38,6 +38,9 @@
 #include "nsLookAndFeel.h"
 #include "nsQApplication.h"
 #include "nsXPLookAndFeel.h"
+#ifdef NS_DEBUG
+#include "nsSize.h"  // needed for nsILookAndFeel's forward declaration
+#endif
 
 #include <qpalette.h>