diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 9988c40acfd4..8288978ead53 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -11630,21 +11630,6 @@ nsGlobalModalWindow::SetReturnValue(nsIVariant *aRetVal)
   return NS_OK;
 }
 
-nsresult
-nsGlobalModalWindow::SetNewDocument(nsIDocument *aDocument,
-                                    nsISupports *aState,
-                                    bool aForceReuseInnerWindow)
-{
-  MOZ_ASSERT(aDocument);
-
-  // If we're loading a new document into a modal dialog, clear the
-  // return value that was set, if any, by the current document.
-  mReturnValue = nullptr;
-
-  return nsGlobalWindow::SetNewDocument(aDocument, aState,
-                                        aForceReuseInnerWindow);
-}
-
 void
 nsGlobalWindow::SetHasAudioAvailableEventListeners()
 {
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 2d42581a9db1..a5d71e571a8d 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -1332,10 +1332,6 @@ public:
 
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(nsGlobalModalWindow, nsGlobalWindow)
 
-  virtual NS_HIDDEN_(nsresult) SetNewDocument(nsIDocument *aDocument,
-                                              nsISupports *aState,
-                                              bool aForceReuseInnerWindow);
-
 protected:
   // For use by outer windows only.
   nsRefPtr<DialogValueHolder> mReturnValue;
diff --git a/dom/tests/mochitest/bugs/test_bug504862.html b/dom/tests/mochitest/bugs/test_bug504862.html
index 63e06e1e241e..5d61a0b4eec2 100644
--- a/dom/tests/mochitest/bugs/test_bug504862.html
+++ b/dom/tests/mochitest/bugs/test_bug504862.html
@@ -27,7 +27,10 @@ function runTest() {
   window.addEventListener("message", onMsgRcv, false);
 
   var result = window.showModalDialog("file_bug504862.html", "my args");
-  is(result, null, "window sees previous dialog documents return value.");
+  // NB: We used to clear returnValue on each navigation, but now we do a
+  // security check on access, so we can safely make returnValue live on
+  // the browsing context, per spec.
+  is(result, 3, "window sees previous dialog documents return value.");
 
   result = window.showModalDialog("http://test1.example.com/tests/dom/tests/mochitest/bugs/file_bug504862.html", "my args");