--- a/layout/generic/nsHTMLParts.h
+++ b/layout/generic/nsHTMLParts.h
@@ -18,16 +18,19 @@ class nsIDocument;
 class nsIFrame;
 class nsIHTMLContentSink;
 class nsIFragmentContentSink;
 class nsStyleContext;
 class nsIURI;
 class nsIPresShell;
 class nsIChannel;
 class nsTableColFrame;
+namespace mozilla {
+class ViewportFrame;
+} // namespace mozilla
 
 // These are all the block specific frame bits, they are copied from
 // the prev-in-flow to a newly created next-in-flow, except for the
 // NS_BLOCK_FLAGS_NON_INHERITED_MASK bits below.
 #define NS_BLOCK_FLAGS_MASK (NS_BLOCK_MARGIN_ROOT              | \
                              NS_BLOCK_FLOAT_MGR                | \
                              NS_BLOCK_CLIP_PAGINATED_OVERFLOW  | \
                              NS_BLOCK_HAS_FIRST_LETTER_STYLE   | \
@@ -76,18 +79,17 @@ NS_NewCommentFrame(nsIPresShell* aPresSh
 
 // <frame> and <iframe> 
 nsIFrame*
 NS_NewSubDocumentFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
 // <frameset>
 nsIFrame*
 NS_NewHTMLFramesetFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
 
-class ViewportFrame;
-ViewportFrame*
+mozilla::ViewportFrame*
 NS_NewViewportFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
 class nsCanvasFrame;
 nsCanvasFrame*
 NS_NewCanvasFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
 nsIFrame*
 NS_NewImageFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
 class nsInlineFrame;
 nsInlineFrame*

--- a/layout/generic/nsPageContentFrame.h
+++ b/layout/generic/nsPageContentFrame.h
@@ -6,17 +6,17 @@
 #define nsPageContentFrame_h___
 
 #include "mozilla/Attributes.h"
 #include "nsViewportFrame.h"
 class nsPageFrame;
 class nsSharedPageData;
 
 // Page frame class used by the simple page sequence frame
-class nsPageContentFrame : public ViewportFrame {
+class nsPageContentFrame : public mozilla::ViewportFrame {
 
 public:
   NS_DECL_FRAMEARENA_HELPERS
 
   friend nsPageContentFrame* NS_NewPageContentFrame(nsIPresShell* aPresShell,
                                                     nsStyleContext* aContext);
   friend class nsPageFrame;
 

--- a/layout/generic/nsViewportFrame.h
+++ b/layout/generic/nsViewportFrame.h
@@ -11,16 +11,18 @@
 #ifndef nsViewportFrame_h___
 #define nsViewportFrame_h___
 
 #include "mozilla/Attributes.h"
 #include "nsContainerFrame.h"
 
 class nsPresContext;
 
+namespace mozilla {
+
 /**
   * ViewportFrame is the parent of a single child - the doc root frame or a scroll frame
   * containing the doc root frame. ViewportFrame stores this child in its primary child
   * list.
   */
 class ViewportFrame : public nsContainerFrame {
 public:
   NS_DECL_QUERYFRAME_TARGET(ViewportFrame)
@@ -99,10 +101,11 @@ protected:
    * determining if the viewport is scrollable and whether the vertical and/or
    * horizontal scrollbars are visible.  Adjust the computed width/height and
    * available width for aReflowInput accordingly.
    * @return the current scroll position, or 0,0 if not scrollable
    */
   nsPoint AdjustReflowInputForScrollbars(ReflowInput* aReflowInput) const;
 };
 
+} // namespace mozilla
 
 #endif // nsViewportFrame_h___