diff --git a/mozilla/toolkit/components/places/src/nsNavHistoryResult.cpp b/mozilla/toolkit/components/places/src/nsNavHistoryResult.cpp
index 99d2390d808..ef4aab8fc2c 100644
--- a/mozilla/toolkit/components/places/src/nsNavHistoryResult.cpp
+++ b/mozilla/toolkit/components/places/src/nsNavHistoryResult.cpp
@@ -3496,7 +3496,7 @@ nsNavHistoryResult::AddHistoryObserver(nsNavHistoryQueryResultNode* aNode)
       mIsHistoryObserver = PR_TRUE;
   }
   if (mHistoryObservers.IndexOf(aNode) != mHistoryObservers.NoIndex) {
-    NS_NOTREACHED("Attempting to register an observer twice!");
+    NS_WARNING("Attempting to register an observer twice!");
     return;
   }
   mHistoryObservers.AppendElement(aNode);
diff --git a/mozilla/toolkit/components/places/tests/unit/test_nsINavHistoryViewer.js b/mozilla/toolkit/components/places/tests/unit/test_nsINavHistoryViewer.js
new file mode 100644
index 00000000000..f1b6ef4b537
--- /dev/null
+++ b/mozilla/toolkit/components/places/tests/unit/test_nsINavHistoryViewer.js
@@ -0,0 +1,232 @@
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et: */
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is mozilla.org code.
+ *
+ * The Initial Developer of the Original Code is Google Inc.
+ * Portions created by the Initial Developer are Copyright (C) 2005
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *  Darin Fisher <darin@meer.net>
+ *  Dietrich Ayala <dietrich@mozilla.com>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+// Get history service
+try {
+  var histsvc = Cc["@mozilla.org/browser/nav-history-service;1"].getService(Ci.nsINavHistoryService);
+} catch(ex) {
+  do_throw("Could not get history service\n");
+} 
+
+// Get global history service
+try {
+  var bhist = Cc["@mozilla.org/browser/global-history;2"].getService(Ci.nsIBrowserHistory);
+} catch(ex) {
+  do_throw("Could not get history service\n");
+} 
+
+// adds a test URI visit to the database, and checks for a valid place ID
+function add_visit(aURI, aDate) {
+  var date = aDate || Date.now();
+  var placeID = histsvc.addVisit(aURI,
+                                 date,
+                                 0, // no referrer
+                                 histsvc.TRANSITION_TYPED, // user typed in URL bar
+                                 false, // not redirect
+                                 0);
+  do_check_true(placeID > 0);
+  return placeID;
+}
+
+var viewer = {
+  insertedItem: null,
+  itemInserted: function(parent, item, newIndex) {
+    this.insertedItem = item;
+  },
+  removedItem: null,
+  itemRemoved: function(parent, item, oldIndex) {
+    this.removedItem = item;
+  },
+  changedItem: null,
+  itemChanged: function(item) {
+    this.changedItem = item;
+  },
+  replacedItem: null,
+  itemReplaced: function(parent, oldItem, newItem, index) {
+    dump("itemReplaced: " + newItem.uri + "\n");
+    this.replacedItem = item;
+  },
+  openedContainer: null,
+  containerOpened: function(item) {
+    this.openedContainer = item;
+  },
+  closedContainer: null,
+  containerClosed: function(item) {
+    this.closedContainer = item;
+  },
+  invalidatedContainer: null,
+  invalidateContainer: function(item) {
+    dump("invalidateContainer()\n");
+    this.invalidatedContainer = item;
+  },
+  allInvalidated: null,
+  invalidateAll: function() {
+    this.allInvalidated = true;
+  },
+  sortingMode: null,
+  sortingChanged: function(sortingMode) {
+    this.sortingMode = sortingMode;
+  },
+  result: null,
+  addViewObserver: function(observer, ownsWeak) {},
+  removeViewObserver: function(observer) {},
+  reset: function() {
+    this.insertedItem = null;
+    this.removedItem = null;
+    this.changedItem = null;
+    this.replacedItem = null;
+    this.openedContainer = null;
+    this.closedContainer = null;
+    this.invalidatedContainer = null;
+    this.allInvalidated = null;
+    this.sortingMode = null;
+  }
+};
+
+// main
+function run_test() {
+
+  // history query
+  var options = histsvc.getNewQueryOptions();
+  options.sortingMode = options.SORT_BY_DATE_DESCENDING;
+  options.resultType = options.RESULTS_AS_VISIT;
+  var query = histsvc.getNewQuery();
+  var result = histsvc.executeQuery(query, options);
+  result.viewer = viewer;
+  var root = result.root;
+  root.containerOpen = true;
+
+  // nsINavHistoryResultViewer.containerOpened
+  do_check_neq(viewer.openedContainer, null);
+
+  // nsINavHistoryResultViewer.itemInserted
+  // add a visit
+  var testURI = uri("http://mozilla.com");
+  add_visit(testURI);
+  do_check_eq(testURI.spec, viewer.insertedItem.uri);
+
+  // nsINavHistoryResultViewer.itemChanged
+  // adding a visit causes itemChanged for the folder
+  do_check_eq(root.uri, viewer.changedItem.uri);
+
+  // nsINavHistoryResultViewer.itemChanged for a leaf node
+  bhist.addPageWithDetails(testURI, "baz", Date.now());
+  do_check_eq(viewer.changedItem.title, "baz");
+
+  // nsINavHistoryResultViewer.itemRemoved
+  var removedURI = uri("http://google.com");
+  add_visit(removedURI);
+  bhist.removePage(removedURI);
+  do_check_eq(removedURI.spec, viewer.removedItem.uri);
+
+  // XXX nsINavHistoryResultViewer.itemReplaced
+  // NHQRN.onVisit()->NHCRN.MergeResults()->NHCRN.ReplaceChildURIAt()->NHRV.ItemReplaced()
+
+  // nsINavHistoryResultViewer.invalidateContainer
+  bhist.removePagesFromHost("mozilla.com", false);
+  do_check_eq(root.uri, viewer.invalidatedContainer.uri);
+
+  // nsINavHistoryResultViewer.invalidateAll
+  // nsINavHistoryResultViewer.sortingChanged
+  result.sortingMode = options.SORT_BY_TITLE_ASCENDING;
+  do_check_true(viewer.allInvalidated);
+  do_check_eq(viewer.sortingMode, options.SORT_BY_TITLE_ASCENDING);
+
+  // nsINavHistoryResultViewer.containerClosed
+  root.containerOpen = false;
+  do_check_eq(viewer.closedContainer, viewer.openedContainer);
+  result.viewer = null;
+
+  // bookmarks query
+  
+  // reset the viewer
+  viewer.reset();
+
+  try {
+    var bmsvc = Cc["@mozilla.org/browser/nav-bookmarks-service;1"].getService(Ci.nsINavBookmarksService);
+  } catch(ex) {
+    do_throw("Could not get nav-bookmarks-service\n");
+  }
+
+  var options = histsvc.getNewQueryOptions();
+  var query = histsvc.getNewQuery();
+  query.setFolders([bmsvc.bookmarksRoot], 1);
+  var result = histsvc.executeQuery(query, options);
+  result.viewer = viewer;
+  var root = result.root;
+  root.containerOpen = true;
+
+  // nsINavHistoryResultViewer.containerOpened
+  do_check_neq(viewer.openedContainer, null);
+
+  // nsINavHistoryResultViewer.itemInserted
+  // add a bookmark
+  var testBookmark = bmsvc.insertBookmark(bmsvc.bookmarksRoot, testURI, bmsvc.DEFAULT_INDEX, "foo");
+  do_check_eq("foo", viewer.insertedItem.title);
+  do_check_eq(testURI.spec, viewer.insertedItem.uri);
+
+  // nsINavHistoryResultViewer.itemChanged
+  // adding a visit causes itemChanged for the folder
+  do_check_eq(root.uri, viewer.changedItem.uri);
+
+  // nsINavHistoryResultViewer.itemChanged for a leaf node
+  bmsvc.setItemTitle(testBookmark, "baz");
+  do_check_eq(viewer.changedItem.title, "baz");
+
+  // nsINavHistoryResultViewer.itemRemoved
+  var removedBookmark = bmsvc.insertBookmark(bmsvc.bookmarksRoot, uri("http://google.com"), bmsvc.DEFAULT_INDEX, "foo");
+  bmsvc.removeItem(removedBookmark);
+  do_check_eq(removedBookmark, viewer.removedItem.itemId);
+
+  // XXX nsINavHistoryResultViewer.itemReplaced
+  // NHQRN.onVisit()->NHCRN.MergeResults()->NHCRN.ReplaceChildURIAt()->NHRV.ItemReplaced()
+
+  // XXX nsINavHistoryResultViewer.invalidateContainer
+
+  // nsINavHistoryResultViewer.invalidateAll
+  // nsINavHistoryResultViewer.sortingChanged
+  result.sortingMode = options.SORT_BY_TITLE_ASCENDING;
+  do_check_true(viewer.allInvalidated);
+  do_check_eq(viewer.sortingMode, options.SORT_BY_TITLE_ASCENDING);
+
+  // nsINavHistoryResultViewer.containerClosed
+  root.containerOpen = false;
+  do_check_eq(viewer.closedContainer, viewer.openedContainer);
+  result.viewer = null;
+}