diff --git a/dom/messagechannel/MessagePortParent.h b/dom/messagechannel/MessagePortParent.h
index 597798242eb3..685625b227d1 100644
--- a/dom/messagechannel/MessagePortParent.h
+++ b/dom/messagechannel/MessagePortParent.h
@@ -8,13 +8,16 @@
 #define mozilla_dom_MessagePortParent_h
 
 #include "mozilla/dom/PMessagePortParent.h"
+#include "mozilla/dom/quota/CheckedUnsafePtr.h"
 
 namespace mozilla {
 namespace dom {
 
 class MessagePortService;
 
-class MessagePortParent final : public PMessagePortParent {
+class MessagePortParent final
+    : public PMessagePortParent,
+      public SupportsCheckedUnsafePtr<CheckIf<DiagnosticAssertEnabled>> {
   friend class PMessagePortParent;
 
  public:
diff --git a/dom/messagechannel/MessagePortService.cpp b/dom/messagechannel/MessagePortService.cpp
index bf3c20133f87..c94219053649 100644
--- a/dom/messagechannel/MessagePortService.cpp
+++ b/dom/messagechannel/MessagePortService.cpp
@@ -7,6 +7,7 @@
 #include "MessagePortService.h"
 #include "MessagePortParent.h"
 #include "SharedMessagePortMessage.h"
+#include "mozilla/dom/quota/CheckedUnsafePtr.h"
 #include "mozilla/ipc/BackgroundParent.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/Unused.h"
@@ -27,6 +28,23 @@ void AssertIsInMainProcess() {
 
 }  // namespace
 
+struct MessagePortService::NextParent {
+  uint32_t mSequenceID;
+  // MessagePortParent keeps the service alive, and we don't want a cycle.
+  CheckedUnsafePtr<MessagePortParent> mParent;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+// Need to call CheckedUnsafePtr's copy constructor and destructor when
+// resizing dynamic arrays containing NextParent (by calling NextParent's
+// implicit copy constructor/destructor rather than memmove-ing NextParents).
+DECLARE_USE_COPY_CONSTRUCTORS(mozilla::dom::MessagePortService::NextParent);
+
+namespace mozilla {
+namespace dom {
+
 class MessagePortService::MessagePortServiceData final {
  public:
   explicit MessagePortServiceData(const nsID& aDestinationUUID)
@@ -48,13 +66,7 @@ class MessagePortService::MessagePortServiceData final {
   nsID mDestinationUUID;
 
   uint32_t mSequenceID;
-  MessagePortParent* mParent;
-
-  struct NextParent {
-    uint32_t mSequenceID;
-    // MessagePortParent keeps the service alive, and we don't want a cycle.
-    MessagePortParent* mParent;
-  };
+  CheckedUnsafePtr<MessagePortParent> mParent;
 
   FallibleTArray<NextParent> mNextParents;
   FallibleTArray<RefPtr<SharedMessagePortMessage>> mMessages;
@@ -160,8 +172,7 @@ bool MessagePortService::RequestEntangling(MessagePortParent* aParent,
 
   // This new parent will be the next one when a Disentangle request is
   // received from the current parent.
-  MessagePortServiceData::NextParent* nextParent =
-      data->mNextParents.AppendElement(mozilla::fallible);
+  auto nextParent = data->mNextParents.AppendElement(mozilla::fallible);
   if (!nextParent) {
     CloseAll(aParent->ID());
     return false;
@@ -265,9 +276,10 @@ void MessagePortService::CloseAll(const nsID& aUUID, bool aForced) {
 
   if (data->mParent) {
     data->mParent->Close();
+    data->mParent = nullptr;
   }
 
-  for (const MessagePortServiceData::NextParent& parent : data->mNextParents) {
+  for (const auto& parent : data->mNextParents) {
     parent.mParent->CloseAndDelete();
   }
 
diff --git a/dom/messagechannel/MessagePortService.h b/dom/messagechannel/MessagePortService.h
index ea96b15de7c0..ee889adb97d8 100644
--- a/dom/messagechannel/MessagePortService.h
+++ b/dom/messagechannel/MessagePortService.h
@@ -21,6 +21,9 @@ class MessagePortService final {
  public:
   NS_INLINE_DECL_REFCOUNTING(MessagePortService)
 
+  // Needs to be public for the DECLARE_USE_COPY_CONSTRUCTORS macro.
+  struct NextParent;
+
   static MessagePortService* Get();
   static MessagePortService* GetOrCreate();