--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -1225,18 +1225,17 @@ protected:
                                             // since there is no dependency
                                             // from gfx back to layout.
   nsRefPtr<mozilla::EventStateManager> mEventManager;
   nsRefPtr<nsRefreshDriver> mRefreshDriver;
   nsRefPtr<nsTransitionManager> mTransitionManager;
   nsRefPtr<nsAnimationManager> mAnimationManager;
   nsRefPtr<mozilla::RestyleManager> mRestyleManager;
   nsRefPtr<mozilla::CounterStyleManager> mCounterStyleManager;
-  nsIAtom*              mMedium;        // initialized by subclass ctors;
-                                        // weak pointer to static atom
+  nsIAtom* MOZ_UNSAFE_REF("always a static atom") mMedium; // initialized by subclass ctors
   nsCOMPtr<nsIAtom> mMediaEmulated;
 
   nsILinkHandler*       mLinkHandler;   // [WEAK]
 
   // Formerly mLangGroup; moving from charset-oriented langGroup to
   // maintaining actual language settings everywhere (see bug 524107).
   // This may in fact hold a langGroup such as x-western rather than
   // a specific language, however (e.g, if it is inferred from the