diff --git a/layout/xul/base/src/nsScrollbarButtonFrame.h b/layout/xul/base/src/nsScrollbarButtonFrame.h
index 336ab37df11d..66d3de6a4ec6 100644
--- a/layout/xul/base/src/nsScrollbarButtonFrame.h
+++ b/layout/xul/base/src/nsScrollbarButtonFrame.h
@@ -33,7 +33,7 @@
 class nsSliderFrame;
 
 class nsScrollbarButtonFrame : public nsTitledButtonFrame, 
-                                      nsITimerCallback
+                               public nsITimerCallback
 {
 public: