diff --git a/dom/base/nsCCUncollectableMarker.cpp b/dom/base/nsCCUncollectableMarker.cpp
index 2b97003233d8..96baf5eb7855 100644
--- a/dom/base/nsCCUncollectableMarker.cpp
+++ b/dom/base/nsCCUncollectableMarker.cpp
@@ -169,8 +169,8 @@ MarkMessageManagers()
   if (nsFrameMessageManager::sSameProcessParentManager) {
     nsFrameMessageManager::sSameProcessParentManager->MarkForCC();
   }
-  if (nsFrameMessageManager::sChildProcessManager) {
-    nsFrameMessageManager::sChildProcessManager->MarkForCC();
+  if (nsFrameMessageManager::GetChildProcessManager()) {
+    nsFrameMessageManager::GetChildProcessManager()->MarkForCC();
   }
 }
 
diff --git a/dom/base/nsFrameMessageManager.cpp b/dom/base/nsFrameMessageManager.cpp
index f983cee42bf6..86fc597badfe 100644
--- a/dom/base/nsFrameMessageManager.cpp
+++ b/dom/base/nsFrameMessageManager.cpp
@@ -1656,7 +1656,7 @@ public:
 
   NS_IMETHOD Run()
   {
-    nsFrameMessageManager* ppm = nsFrameMessageManager::sChildProcessManager;
+    nsFrameMessageManager* ppm = nsFrameMessageManager::GetChildProcessManager();
     ReceiveMessage(static_cast<nsIContentFrameMessageManager*>(ppm), ppm);
     return NS_OK;
   }
@@ -1920,7 +1920,7 @@ nsFrameMessageManager::NewProcessMessageManager(mozilla::dom::nsIContentParent*
 nsresult
 NS_NewChildProcessMessageManager(nsISyncMessageSender** aResult)
 {
-  NS_ASSERTION(!nsFrameMessageManager::sChildProcessManager,
+  NS_ASSERTION(!nsFrameMessageManager::GetChildProcessManager(),
                "Re-creating sChildProcessManager");
 
   MessageManagerCallback* cb;
@@ -1933,7 +1933,7 @@ NS_NewChildProcessMessageManager(nsISyncMessageSender** aResult)
   nsFrameMessageManager* mm = new nsFrameMessageManager(cb,
                                                         nullptr,
                                                         MM_PROCESSMANAGER | MM_OWNSCALLBACK);
-  nsFrameMessageManager::sChildProcessManager = mm;
+  nsFrameMessageManager::SetChildProcessManager(mm);
   return CallQueryInterface(mm, aResult);
 }
 
diff --git a/dom/base/nsFrameMessageManager.h b/dom/base/nsFrameMessageManager.h
index 366f7b1d5ef4..59fb3ecacc6a 100644
--- a/dom/base/nsFrameMessageManager.h
+++ b/dom/base/nsFrameMessageManager.h
@@ -274,6 +274,10 @@ public:
   {
     return sChildProcessManager;
   }
+  static void SetChildProcessManager(nsFrameMessageManager* aManager)
+  {
+    sChildProcessManager = aManager;
+  }
 private:
   nsresult SendMessage(const nsAString& aMessageName,
                        JS::Handle<JS::Value> aJSON,
@@ -307,10 +311,10 @@ protected:
                           nsFrameMessageManager* aChildMM);
 public:
   static nsFrameMessageManager* sParentProcessManager;
-  static nsFrameMessageManager* sChildProcessManager;
   static nsFrameMessageManager* sSameProcessParentManager;
   static nsTArray<nsCOMPtr<nsIRunnable> >* sPendingSameProcessAsyncMessages;
 private:
+  static nsFrameMessageManager* sChildProcessManager;
   enum ProcessCheckerType {
     PROCESS_CHECKER_PERMISSION,
     PROCESS_CHECKER_MANIFEST_URL,
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index d91b2f787be1..4cf538d6e5e9 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -1919,7 +1919,7 @@ ContentChild::RecvAsyncMessage(const nsString& aMsg,
                                InfallibleTArray<CpowEntry>&& aCpows,
                                const IPC::Principal& aPrincipal)
 {
-    nsRefPtr<nsFrameMessageManager> cpm = nsFrameMessageManager::sChildProcessManager;
+    nsRefPtr<nsFrameMessageManager> cpm = nsFrameMessageManager::GetChildProcessManager();
     if (cpm) {
         StructuredCloneData cloneData = ipc::UnpackClonedMessageDataForChild(aData);
         CrossProcessCpowHolder cpows(this, aCpows);
diff --git a/dom/ipc/nsIContentChild.cpp b/dom/ipc/nsIContentChild.cpp
index 25acddea2932..929ea9490faa 100644
--- a/dom/ipc/nsIContentChild.cpp
+++ b/dom/ipc/nsIContentChild.cpp
@@ -113,7 +113,7 @@ nsIContentChild::RecvAsyncMessage(const nsString& aMsg,
                                   InfallibleTArray<CpowEntry>&& aCpows,
                                   const IPC::Principal& aPrincipal)
 {
-  nsRefPtr<nsFrameMessageManager> cpm = nsFrameMessageManager::sChildProcessManager;
+  nsRefPtr<nsFrameMessageManager> cpm = nsFrameMessageManager::GetChildProcessManager();
   if (cpm) {
     StructuredCloneData cloneData = ipc::UnpackClonedMessageDataForChild(aData);
     CrossProcessCpowHolder cpows(this, aCpows);