diff --git a/layout/base/nsRefreshDriver.cpp b/layout/base/nsRefreshDriver.cpp
index 0cf9fab261d2..80bb2e033593 100644
--- a/layout/base/nsRefreshDriver.cpp
+++ b/layout/base/nsRefreshDriver.cpp
@@ -44,6 +44,7 @@
 #include "nsPresContext.h"
 #include "nsComponentManagerUtils.h"
 #include "prlog.h"
+#include "nsAutoPtr.h"
 
 /*
  * TODO:
@@ -194,10 +195,22 @@ nsRefreshDriver::Notify(nsITimer *aTimer)
     return NS_OK;
   }
 
+  /*
+   * The timer holds a reference to |this| while calling |Notify|.
+   * However, implementations of |WillRefresh| are permitted to destroy
+   * the pres context, which will cause our |mPresContext| to become
+   * null.  If this happens, we must stop notifying observers.
+   */
   for (PRUint32 i = 0; i < NS_ARRAY_LENGTH(mObservers); ++i) {
     ObserverArray::EndLimitedIterator etor(mObservers[i]);
     while (etor.HasMore()) {
-      etor.GetNext()->WillRefresh(mMostRecentRefresh);
+      nsRefPtr<nsARefreshObserver> obs = etor.GetNext();
+      obs->WillRefresh(mMostRecentRefresh);
+      
+      if (!mPresContext || !mPresContext->GetPresShell()) {
+        StopTimer();
+        return NS_OK;
+      }
     }
     if (i == 0) {
       // This is the Flush_Style case.
diff --git a/layout/base/nsRefreshDriver.h b/layout/base/nsRefreshDriver.h
index 41f535d24260..a65cd44cba08 100644
--- a/layout/base/nsRefreshDriver.h
+++ b/layout/base/nsRefreshDriver.h
@@ -58,6 +58,15 @@ class nsPresContext;
  */
 class nsARefreshObserver {
 public:
+  // AddRef and Release signatures that match nsISupports.  Implementors
+  // must implement reference counting, and those that do implement
+  // nsISupports will already have methods with the correct signature.
+  //
+  // The refresh driver does NOT hold references to refresh observers
+  // except while it is notifying them.  
+  NS_IMETHOD_(nsrefcnt) AddRef(void) = 0;
+  NS_IMETHOD_(nsrefcnt) Release(void) = 0;
+
   virtual void WillRefresh(mozilla::TimeStamp aTime) = 0;
 };
 
@@ -93,6 +102,9 @@ public:
    *     painting, and, correspondingly, which get notified when there
    *     is a flush during such suppression
    * and it must be either Flush_Style, Flush_Layout, or Flush_Display.
+   *
+   * The refresh driver does NOT own a reference to these observers;
+   * they must remove themselves before they are destroyed.
    */
   PRBool AddRefreshObserver(nsARefreshObserver *aObserver,
                             mozFlushType aFlushType);