diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index ed3f449e4d3a..b037d9af7b4a 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -14524,18 +14524,15 @@ nsGlobalWindow::GetGroupMessageManager(const nsAString& aGroup,
 
   nsGlobalChromeWindow* myself = static_cast<nsGlobalChromeWindow*>(this);
   nsCOMPtr<nsIMessageBroadcaster> messageManager =
-    myself->mGroupMessageManagers.Get(aGroup);
-
-  if (!messageManager) {
-    nsFrameMessageManager* parent =
-      static_cast<nsFrameMessageManager*>(GetMessageManager(aError));
-
-    messageManager = new nsFrameMessageManager(nullptr,
-                                               parent,
-                                               MM_CHROME | MM_BROADCASTER);
-    myself->mGroupMessageManagers.Put(aGroup, messageManager);
-  }
+    myself->mGroupMessageManagers.LookupForAdd(aGroup).OrInsert(
+      [this, &aError] () {
+        nsFrameMessageManager* parent =
+          static_cast<nsFrameMessageManager*>(GetMessageManager(aError));
 
+        return new nsFrameMessageManager(nullptr,
+                                         parent,
+                                         MM_CHROME | MM_BROADCASTER);
+      });
   return messageManager;
 }