--- a/layout/xul/base/src/tree/src/nsTreeContentView.cpp
+++ b/layout/xul/base/src/tree/src/nsTreeContentView.cpp
@@ -1138,17 +1138,17 @@ nsTreeContentView::SerializeItem(nsICont
   if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
                             nsGkAtoms::_true, eCaseMatters)) {
     row->SetContainer(PR_TRUE);
     if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
                               nsGkAtoms::_true, eCaseMatters)) {
       row->SetOpen(PR_TRUE);
       nsIContent* child =
         nsTreeUtils::GetImmediateChild(aContent, nsGkAtoms::treechildren);
-      if (child) {
+      if (child && child->IsXUL()) {
         // Now, recursively serialize our child.
         PRInt32 count = aRows.Length();
         PRInt32 index = 0;
         Serialize(child, aParentIndex + *aIndex + 1, &index, aRows);
         row->mSubtreeSize += aRows.Length() - count;
       }
       else
         row->SetEmpty(PR_TRUE);
@@ -1196,17 +1196,17 @@ nsTreeContentView::GetIndexInSubtree(nsI
                                    nsGkAtoms::_true, eCaseMatters)) {
           (*aIndex)++;
           if (content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
                                    nsGkAtoms::_true, eCaseMatters) &&
               content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
                                    nsGkAtoms::_true, eCaseMatters)) {
             nsIContent* child =
               nsTreeUtils::GetImmediateChild(content, nsGkAtoms::treechildren);
-            if (child)
+            if (child && child->IsXUL())
               GetIndexInSubtree(child, aContent, aIndex);
           }
         }
       }
       else if (tag == nsGkAtoms::treeseparator) {
         if (! content->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
                                    nsGkAtoms::_true, eCaseMatters))
           (*aIndex)++;
@@ -1217,17 +1217,17 @@ nsTreeContentView::GetIndexInSubtree(nsI
 
 PRInt32
 nsTreeContentView::EnsureSubtree(PRInt32 aIndex)
 {
   Row* row = mRows[aIndex];
 
   nsIContent* child;
   child = nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treechildren);
-  if (! child) {
+  if (!child || !child->IsXUL()) {
     return 0;
   }
 
   nsAutoTArray<Row*, 8> rows;
   PRInt32 index = 0;
   Serialize(child, aIndex, &index, rows);
   mRows.InsertElementsAt(aIndex + 1, rows);
   PRInt32 count = rows.Length();