diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 94cbfb6e1c1..adbcb8d40df 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -259,7 +259,9 @@ nsPresContext::~nsPresContext()
   NS_PRECONDITION(!mShell, "Presshell forgot to clear our mShell pointer");
   SetShell(nsnull);
 
-  delete mTransitionManager;
+  if (mTransitionManager) {
+    mTransitionManager->Disconnect();
+  }
 
   // Disconnect the refresh driver *after* the transition manager, which
   // needs it.
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 407dd37084a..564fca5c5f5 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -952,7 +952,7 @@ protected:
   nsIEventStateManager* mEventManager;  // [STRONG]
   nsILookAndFeel*       mLookAndFeel;   // [STRONG]
   nsRefPtr<nsRefreshDriver> mRefreshDriver;
-  nsTransitionManager*  mTransitionManager; // owns; it aggregates our refcount
+  nsRefPtr<nsTransitionManager> mTransitionManager;
   nsIAtom*              mMedium;        // initialized by subclass ctors;
                                         // weak pointer to static atom
 
diff --git a/layout/style/nsTransitionManager.cpp b/layout/style/nsTransitionManager.cpp
index 51a38edd648..0d070385102 100644
--- a/layout/style/nsTransitionManager.cpp
+++ b/layout/style/nsTransitionManager.cpp
@@ -333,6 +333,12 @@ nsTransitionManager::nsTransitionManager(nsPresContext *aPresContext)
 }
 
 nsTransitionManager::~nsTransitionManager()
+{
+  NS_ABORT_IF_FALSE(!mPresContext, "Disconnect should have been called");
+}
+
+void
+nsTransitionManager::Disconnect()
 {
   // Content nodes might outlive the transition manager.
   while (!PR_CLIST_IS_EMPTY(&mElementTransitions)) {
@@ -340,6 +346,8 @@ nsTransitionManager::~nsTransitionManager()
                                  PR_LIST_HEAD(&mElementTransitions));
     head->Destroy();
   }
+
+  mPresContext = nsnull;
 }
 
 static PRBool
@@ -781,9 +789,7 @@ nsTransitionManager::AddElementTransitions(ElementTransitions* aElementTransitio
  * nsISupports implementation
  */
 
-NS_IMPL_ADDREF_USING_AGGREGATOR(nsTransitionManager, mPresContext)
-NS_IMPL_RELEASE_USING_AGGREGATOR(nsTransitionManager, mPresContext)
-NS_IMPL_QUERY_INTERFACE1(nsTransitionManager, nsIStyleRuleProcessor)
+NS_IMPL_ISUPPORTS1(nsTransitionManager, nsIStyleRuleProcessor)
 
 /*
  * nsIStyleRuleProcessor implementation
@@ -885,6 +891,10 @@ nsTransitionManager::MediumFeaturesChanged(nsPresContext* aPresContext,
 /* virtual */ void
 nsTransitionManager::WillRefresh(mozilla::TimeStamp aTime)
 {
+  NS_ABORT_IF_FALSE(mPresContext,
+                    "refresh driver should not notify additional observers "
+                    "after pres context has been destroyed");
+
   // Trim transitions that have completed, and post restyle events for
   // frames that are still transitioning.
   {
diff --git a/layout/style/nsTransitionManager.h b/layout/style/nsTransitionManager.h
index c91bb81efc6..f9761c38152 100644
--- a/layout/style/nsTransitionManager.h
+++ b/layout/style/nsTransitionManager.h
@@ -52,16 +52,17 @@ class nsCSSPropertySet;
 struct nsTransition;
 struct ElementTransitions;
 
-/**
- * Must be created only as a sub-object of an nsPresContext (since its
- * reference counting methods assume that).
- */
 class nsTransitionManager : public nsIStyleRuleProcessor,
                             public nsARefreshObserver {
 public:
   nsTransitionManager(nsPresContext *aPresContext);
   ~nsTransitionManager();
 
+  /**
+   * Notify the transition manager that the pres context is going away.
+   */
+  void Disconnect();
+
   /**
    * StyleContextChanged 
    *
@@ -82,7 +83,7 @@ public:
                         nsStyleContext *aNewStyleContext);
 
   // nsISupports
-  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_ISUPPORTS
 
   // nsIStyleRuleProcessor
   NS_IMETHOD RulesMatching(ElementRuleProcessorData* aData);
@@ -120,7 +121,7 @@ private:
 			      nsCSSPseudoElements::Type aPseudoType);
 
   PRCList mElementTransitions;
-  nsPresContext *mPresContext;
+  nsPresContext *mPresContext; // weak (non-null from ctor to Disconnect)
 };
 
 #endif /* !defined(nsTransitionManager_h_) */