--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -4430,29 +4430,21 @@ void Document::SetContainer(nsDocShell* 
 
   // IsTopLevelWindowInactive depends on the docshell, so
   // update the cached value now that it's available.
   UpdateDocumentStates(NS_DOCUMENT_STATE_WINDOW_INACTIVE);
   if (!aContainer) {
     return;
   }
 
-  // Get the Docshell
-  if (aContainer->ItemType() == nsIDocShellTreeItem::typeContent) {
-    // check if same type root
-    nsCOMPtr<nsIDocShellTreeItem> sameTypeRoot;
-    aContainer->GetSameTypeRootTreeItem(getter_AddRefs(sameTypeRoot));
-    NS_ASSERTION(
-        sameTypeRoot,
-        "No document shell root tree item from document shell tree item!");
-
-    if (sameTypeRoot == aContainer) {
+  BrowsingContext* context = aContainer->GetBrowsingContext();
+  if (context && context->IsContent()) {
+    if (!context->GetParent()) {
       SetIsTopLevelContentDocument(true);
     }
-
     SetIsContentDocument(true);
   }
 
   mAncestorPrincipals = aContainer->AncestorPrincipals();
   mAncestorOuterWindowIDs = aContainer->AncestorOuterWindowIDs();
 }
 
 nsISupports* Document::GetContainer() const {