--- a/toolkit/components/places/nsNavBookmarks.cpp
+++ b/toolkit/components/places/nsNavBookmarks.cpp
@@ -2517,16 +2517,20 @@ nsNavBookmarks::RunInBatchMode(nsINavHis
 }
 
 
 NS_IMETHODIMP
 nsNavBookmarks::AddObserver(nsINavBookmarkObserver* aObserver,
                             bool aOwnsWeak)
 {
   NS_ENSURE_ARG(aObserver);
+
+  if (NS_WARN_IF(!mCanNotify))
+    return NS_ERROR_UNEXPECTED;
+
   return mObservers.AppendWeakElement(aObserver, aOwnsWeak);
 }
 
 
 NS_IMETHODIMP
 nsNavBookmarks::RemoveObserver(nsINavBookmarkObserver* aObserver)
 {
   return mObservers.RemoveWeakElement(aObserver);
@@ -2629,16 +2633,17 @@ nsNavBookmarks::Observe(nsISupports *aSu
     if (annosvc) {
       annosvc->RemoveObserver(this);
     }
   }
   else if (strcmp(aTopic, TOPIC_PLACES_CONNECTION_CLOSED) == 0) {
     // Don't even try to notify observers from this point on, the category
     // cache would init services that could try to use our APIs.
     mCanNotify = false;
+    mObservers.Clear();
   }
 
   return NS_OK;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 //// nsINavHistoryObserver
 

--- a/toolkit/components/places/nsNavHistory.cpp
+++ b/toolkit/components/places/nsNavHistory.cpp
@@ -2245,16 +2245,19 @@ nsNavHistory::GetQueryResults(nsNavHisto
 }
 
 NS_IMETHODIMP
 nsNavHistory::AddObserver(nsINavHistoryObserver* aObserver, bool aOwnsWeak)
 {
   NS_ASSERTION(NS_IsMainThread(), "This can only be called on the main thread");
   NS_ENSURE_ARG(aObserver);
 
+  if (NS_WARN_IF(!mCanNotify))
+    return NS_ERROR_UNEXPECTED;
+
   return mObservers.AppendWeakElement(aObserver, aOwnsWeak);
 }
 
 NS_IMETHODIMP
 nsNavHistory::RemoveObserver(nsINavHistoryObserver* aObserver)
 {
   NS_ASSERTION(NS_IsMainThread(), "This can only be called on the main thread");
   NS_ENSURE_ARG(aObserver);
@@ -3082,19 +3085,20 @@ nsNavHistory::Observe(nsISupports *aSubj
       strcmp(aTopic, TOPIC_PROFILE_CHANGE) == 0 ||
       strcmp(aTopic, TOPIC_SIMULATE_PLACES_SHUTDOWN) == 0) {
     // These notifications are used by tests to simulate a Places shutdown.
     // They should just be forwarded to the Database handle.
     mDB->Observe(aSubject, aTopic, aData);
   }
 
   else if (strcmp(aTopic, TOPIC_PLACES_CONNECTION_CLOSED) == 0) {
-      // Don't even try to notify observers from this point on, the category
-      // cache would init services that could try to use our APIs.
-      mCanNotify = false;
+    // Don't even try to notify observers from this point on, the category
+    // cache would init services that could try to use our APIs.
+    mCanNotify = false;
+    mObservers.Clear();
   }
 
 #ifdef MOZ_XUL
   else if (strcmp(aTopic, TOPIC_AUTOCOMPLETE_FEEDBACK_INCOMING) == 0) {
     nsCOMPtr<nsIAutoCompleteInput> input = do_QueryInterface(aSubject);
     if (!input)
       return NS_OK;