Relanding bug 433616 part 2 to see if it's the cause of a Tp regression

content/base/src/nsFrameLoader.cpp

@@ -727,6 +727,11 @@ nsFrameLoader::EnsureDocShell()
     return NS_ERROR_UNEXPECTED;
   }
 
+  if (doc->GetDisplayDocument()) {
+    // Don't allow subframe loads in external reference documents
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
   nsCOMPtr<nsIWebNavigation> parentAsWebNav =
     do_GetInterface(doc->GetScriptGlobalObject());