--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -660,22 +660,16 @@
 #ifdef XP_MACOSX
       <hbox class="titlebar-placeholder" type="fullscreen-button"
             id="titlebar-placeholder-on-TabsToolbar-for-fullscreen-button" persist="width"
             skipintoolbarset="true"/>
 #endif
 #endif
     </toolbar>
 
-    <!--
-           CAVEAT EMPTOR
-           Should you need to add items to the toolbar here, make sure to also add them
-           to the default placements of buttons in CustomizableUI.jsm, so the
-           customization code doesn't get confused.
-      -->
     <toolbar id="nav-bar"
              aria-label="&navbarCmd.label;"
              fullscreentoolbar="true" mode="icons" customizable="true"
              iconsize="small"
              customizationtarget="nav-bar-customization-target"
              overflowable="true"
              overflowbutton="nav-bar-overflow-button"
              overflowtarget="widget-overflow-list"

--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -721,46 +721,48 @@ var CustomizableUIInternal = {
         }
 
         let [provider, node] = this.getWidgetNode(id, window);
         if (!node) {
           log.debug("Unknown widget: " + id);
           continue;
         }
 
+        let widget = null;
         // If the placements have items in them which are (now) no longer removable,
         // we shouldn't be moving them:
         if (provider == CustomizableUI.PROVIDER_API) {
-          let widgetInfo = gPalette.get(id);
-          if (!widgetInfo.removable && aArea != widgetInfo.defaultArea) {
+          widget = gPalette.get(id);
+          if (!widget.removable && aArea != widget.defaultArea) {
             placementsToRemove.add(id);
             continue;
           }
         } else if (provider == CustomizableUI.PROVIDER_XUL &&
                    node.parentNode != container && !this.isWidgetRemovable(node)) {
           placementsToRemove.add(id);
           continue;
         } // Special widgets are always removable, so no need to check them
 
-        if (inPrivateWindow && provider == CustomizableUI.PROVIDER_API) {
-          let widget = gPalette.get(id);
-          if (!widget.showInPrivateBrowsing && inPrivateWindow) {
-            continue;
-          }
+        if (inPrivateWindow && widget && !widget.showInPrivateBrowsing) {
+          continue;
         }
 
         this.ensureButtonContextMenu(node, aAreaNode);
         if (node.localName == "toolbarbutton") {
           if (areaIsPanel) {
             node.setAttribute("wrap", "true");
           } else {
             node.removeAttribute("wrap");
           }
         }
 
+        // This needs updating in case we're resetting / undoing a reset.
+        if (widget) {
+          widget.currentArea = aArea;
+        }
         this.insertWidgetBefore(node, currentNode, container, aArea);
         if (gResetting) {
           this.notifyListeners("onWidgetReset", node, container);
         } else if (gUndoResetting) {
           this.notifyListeners("onWidgetUndoMove", node, container);
         }
       }
 

--- a/browser/components/customizableui/CustomizableWidgets.jsm
+++ b/browser/components/customizableui/CustomizableWidgets.jsm
@@ -743,16 +743,23 @@ const CustomizableWidgets = [
 
         onWidgetReset: function(aWidgetNode) {
           if (aWidgetNode != node)
             return;
           updateCombinedWidgetStyle(node, this.currentArea, true);
           updateZoomResetButton();
         }.bind(this),
 
+        onWidgetUndoMove: function(aWidgetNode) {
+          if (aWidgetNode != node)
+            return;
+          updateCombinedWidgetStyle(node, this.currentArea, true);
+          updateZoomResetButton();
+        }.bind(this),
+
         onWidgetMoved: function(aWidgetId, aArea) {
           if (aWidgetId != this.id)
             return;
           updateCombinedWidgetStyle(node, aArea, true);
           updateZoomResetButton();
         }.bind(this),
 
         onWidgetInstanceRemoved: function(aWidgetId, aDoc) {
@@ -854,16 +861,22 @@ const CustomizableWidgets = [
         }.bind(this),
 
         onWidgetReset: function(aWidgetNode) {
           if (aWidgetNode != node)
             return;
           updateCombinedWidgetStyle(node, this.currentArea);
         }.bind(this),
 
+        onWidgetUndoMove: function(aWidgetNode) {
+          if (aWidgetNode != node)
+            return;
+          updateCombinedWidgetStyle(node, this.currentArea);
+        }.bind(this),
+
         onWidgetMoved: function(aWidgetId, aArea) {
           if (aWidgetId != this.id)
             return;
           updateCombinedWidgetStyle(node, aArea);
         }.bind(this),
 
         onWidgetInstanceRemoved: function(aWidgetId, aDoc) {
           if (aWidgetId != this.id || aDoc != aDocument)