--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -327,16 +327,42 @@ NS_NewScrollbarButtonFrame (nsIPresShell
 #ifdef NOISY_FINDFRAME
 static int32_t FFWC_totalCount=0;
 static int32_t FFWC_doLoop=0;
 static int32_t FFWC_doSibling=0;
 static int32_t FFWC_recursions=0;
 static int32_t FFWC_nextInFlows=0;
 #endif
 
+#ifdef MOZ_XUL
+
+static bool
+IsXULListBox(nsIContent* aContainer)
+{
+  return aContainer->IsXULElement(nsGkAtoms::listbox);
+}
+
+static
+nsListBoxBodyFrame*
+MaybeGetListBoxBodyFrame(nsIContent* aChild)
+{
+  if (aChild->IsXULElement(nsGkAtoms::listitem) && aChild->GetParent() &&
+      IsXULListBox(aChild->GetParent())) {
+    RefPtr<nsXULElement> xulElement =
+      nsXULElement::FromNode(aChild->GetParent());
+    nsCOMPtr<nsIBoxObject> boxObject = xulElement->GetBoxObject(IgnoreErrors());
+    nsCOMPtr<nsPIListBoxObject> listBoxObject = do_QueryInterface(boxObject);
+    if (listBoxObject) {
+      return listBoxObject->GetListBoxBody(false);
+    }
+  }
+
+  return nullptr;
+}
+#endif // MOZ_XUL
 
 // Returns true if aFrame is an anonymous flex/grid item.
 static inline bool
 IsAnonymousFlexOrGridItem(const nsIFrame* aFrame)
 {
   const nsAtom* pseudoType = aFrame->Style()->GetPseudo();
   return pseudoType == nsCSSAnonBoxes::anonymousFlexItem ||
          pseudoType == nsCSSAnonBoxes::anonymousGridItem;
@@ -5527,16 +5553,19 @@ nsCSSFrameConstructor::ShouldCreateItems
                                                  nsContainerFrame* aParentFrame)
 {
   aContent->UnsetFlags(NODE_DESCENDANTS_NEED_FRAMES | NODE_NEEDS_FRAME);
   // XXX the GetContent() != aContent check is needed due to bug 135040.
   // Remove it once that's fixed.
   if (aContent->GetPrimaryFrame() &&
       aContent->GetPrimaryFrame()->GetContent() == aContent &&
       !aState.mCreatingExtraFrames) {
+    MOZ_ASSERT(MaybeGetListBoxBodyFrame(aContent),
+               "asked to create frame construction item for a node that "
+               "already has a frame");
     NS_ERROR("asked to create frame construction item for a node that already "
              "has a frame");
     return false;
   }
 
   // don't create a whitespace frame if aParent doesn't want it
   if (!NeedFrameFor(aState, aParentFrame, aContent)) {
     return false;
@@ -6729,43 +6758,16 @@ IsSpecialFramesetChild(nsIContent* aCont
 {
   // IMPORTANT: This must match the conditions in nsHTMLFramesetFrame::Init.
   return aContent->IsAnyOfHTMLElements(nsGkAtoms::frameset, nsGkAtoms::frame);
 }
 
 static void
 InvalidateCanvasIfNeeded(nsIPresShell* presShell, nsIContent* node);
 
-#ifdef MOZ_XUL
-
-static bool
-IsXULListBox(nsIContent* aContainer)
-{
-  return aContainer->IsXULElement(nsGkAtoms::listbox);
-}
-
-static
-nsListBoxBodyFrame*
-MaybeGetListBoxBodyFrame(nsIContent* aChild)
-{
-  if (aChild->IsXULElement(nsGkAtoms::listitem) && aChild->GetParent() &&
-      IsXULListBox(aChild->GetParent())) {
-    RefPtr<nsXULElement> xulElement =
-      nsXULElement::FromNode(aChild->GetParent());
-    nsCOMPtr<nsIBoxObject> boxObject = xulElement->GetBoxObject(IgnoreErrors());
-    nsCOMPtr<nsPIListBoxObject> listBoxObject = do_QueryInterface(boxObject);
-    if (listBoxObject) {
-      return listBoxObject->GetListBoxBody(false);
-    }
-  }
-
-  return nullptr;
-}
-#endif // MOZ_XUL
-
 void
 nsCSSFrameConstructor::AddTextItemIfNeeded(nsFrameConstructorState& aState,
                                            const InsertionPoint& aInsertion,
                                            nsIContent* aPossibleTextContent,
                                            FrameConstructionItemList& aItems)
 {
   NS_PRECONDITION(aPossibleTextContent, "Must have node");
   if (!aPossibleTextContent->IsNodeOfType(nsINode::eTEXT) ||