diff --git a/dom/media/gmp/GMPCrashHelper.cpp b/dom/media/gmp/GMPCrashHelper.cpp
index fcf9c53abd4c..f46c4e833719 100644
--- a/dom/media/gmp/GMPCrashHelper.cpp
+++ b/dom/media/gmp/GMPCrashHelper.cpp
@@ -19,7 +19,9 @@ GMPCrashHelper::Destroy()
     delete this;
   } else {
     // Don't addref, as then we'd end up releasing after the detele runs!
-    NS_DispatchToMainThread(mozilla::NewNonOwningRunnableMethod(this, &GMPCrashHelper::Destroy));
+    SystemGroup::Dispatch(
+      "GMPCrashHelper::Destroy", TaskCategory::Other,
+      NewNonOwningRunnableMethod(this, &GMPCrashHelper::Destroy));
   }
 }
 
diff --git a/dom/media/gmp/GMPParent.cpp b/dom/media/gmp/GMPParent.cpp
index 3bfd2b155feb..54e78ad4da51 100644
--- a/dom/media/gmp/GMPParent.cpp
+++ b/dom/media/gmp/GMPParent.cpp
@@ -60,7 +60,7 @@ extern LogModule* GetGMPLog();
 
 namespace gmp {
 
-GMPParent::GMPParent()
+GMPParent::GMPParent(AbstractThread* aMainThread)
   : mState(GMPStateNotLoaded)
   , mProcess(nullptr)
   , mDeleteProcessOnlyOnUnload(false)
@@ -70,6 +70,7 @@ GMPParent::GMPParent()
   , mGMPContentChildCount(0)
   , mChildPid(0)
   , mHoldingSelfRef(false)
+  , mMainThread(aMainThread)
 {
   mPluginId = GeckoChildProcessHost::GetUniqueID();
   LOGD("GMPParent ctor id=%u", mPluginId);
@@ -302,7 +303,8 @@ GMPParent::Shutdown()
 class NotifyGMPShutdownTask : public Runnable {
 public:
   explicit NotifyGMPShutdownTask(const nsAString& aNodeId)
-    : mNodeId(aNodeId)
+    : Runnable("NotifyGMPShutdownTask")
+    , mNodeId(aNodeId)
   {
   }
   NS_IMETHOD Run() override {
@@ -354,9 +356,9 @@ GMPParent::DeleteProcess()
   mProcess = nullptr;
   mState = GMPStateNotLoaded;
 
-  NS_DispatchToMainThread(
-    new NotifyGMPShutdownTask(NS_ConvertUTF8toUTF16(mNodeId)),
-    NS_DISPATCH_NORMAL);
+  nsCOMPtr<nsIRunnable> r
+    = new NotifyGMPShutdownTask(NS_ConvertUTF8toUTF16(mNodeId));
+  mMainThread->Dispatch(r.forget());
 
   if (mHoldingSelfRef) {
     Release();
@@ -517,9 +519,9 @@ GMPParent::ActorDestroy(ActorDestroyReason aWhy)
     }
 
     // NotifyObservers is mainthread-only
-    NS_DispatchToMainThread(WrapRunnableNM(&GMPNotifyObservers,
-                                           mPluginId, mDisplayName, dumpID),
-                            NS_DISPATCH_NORMAL);
+    nsCOMPtr<nsIRunnable> r = WrapRunnableNM(
+      &GMPNotifyObservers, mPluginId, mDisplayName, dumpID);
+    mMainThread->Dispatch(r.forget());
   }
 #endif
   // warn us off trying to close again
@@ -728,8 +730,7 @@ GMPParent::ReadChromiumManifestFile(nsIFile* aFile)
 
   // DOM JSON parsing needs to run on the main thread.
   return InvokeAsync<nsString&&>(
-    // Non DocGroup-version of AbstractThread::MainThread for the task in parent.
-    AbstractThread::MainThread(), this, __func__,
+    mMainThread, this, __func__,
     &GMPParent::ParseChromiumManifest, NS_ConvertUTF8toUTF16(json));
 }
 
diff --git a/dom/media/gmp/GMPParent.h b/dom/media/gmp/GMPParent.h
index 091d26386f0b..9d9cbf758830 100644
--- a/dom/media/gmp/GMPParent.h
+++ b/dom/media/gmp/GMPParent.h
@@ -73,7 +73,7 @@ class GMPParent final : public PGMPParent
 public:
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(GMPParent)
 
-  GMPParent();
+  explicit GMPParent(AbstractThread* aMainThread);
 
   RefPtr<GenericPromise> Init(GeckoMediaPluginServiceParent* aService, nsIFile* aPluginDir);
   nsresult CloneFrom(const GMPParent* aOther);
@@ -228,6 +228,8 @@ private:
 #ifdef MOZ_CRASHREPORTER
   UniquePtr<ipc::CrashReporterHost> mCrashReporter;
 #endif
+
+  const RefPtr<AbstractThread> mMainThread;
 };
 
 } // namespace gmp
diff --git a/dom/media/gmp/GMPService.cpp b/dom/media/gmp/GMPService.cpp
index 9139b6d536d2..f3f4a059e257 100644
--- a/dom/media/gmp/GMPService.cpp
+++ b/dom/media/gmp/GMPService.cpp
@@ -79,12 +79,11 @@ public:
     if (NS_IsMainThread()) {
       service = GetOrCreateOnMainThread();
     } else {
-      nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
-      MOZ_ASSERT(mainThread);
-
       RefPtr<GMPServiceCreateHelper> createHelper = new GMPServiceCreateHelper();
 
-      mozilla::SyncRunnable::DispatchToThread(mainThread, createHelper, true);
+      mozilla::SyncRunnable::DispatchToThread(
+        SystemGroup::EventTargetFor(mozilla::TaskCategory::Other),
+        createHelper, true);
 
       service = createHelper->mService.forget();
     }
@@ -94,6 +93,7 @@ public:
 
 private:
   GMPServiceCreateHelper()
+    : Runnable("GMPServiceCreateHelper")
   {
   }
 
diff --git a/dom/media/gmp/GMPServiceChild.cpp b/dom/media/gmp/GMPServiceChild.cpp
index 8982bd449320..25c7e4b1f9f9 100644
--- a/dom/media/gmp/GMPServiceChild.cpp
+++ b/dom/media/gmp/GMPServiceChild.cpp
@@ -305,8 +305,9 @@ GeckoMediaPluginServiceChild::GetServiceChild()
     MozPromiseHolder<GetServiceChildPromise>* holder = mGetServiceChildPromises.AppendElement();
     RefPtr<GetServiceChildPromise> promise = holder->Ensure(__func__);
     if (mGetServiceChildPromises.Length() == 1) {
-        NS_DispatchToMainThread(WrapRunnable(contentChild,
-                                             &dom::ContentChild::SendCreateGMPService));
+      nsCOMPtr<nsIRunnable> r = WrapRunnable(
+        contentChild, &dom::ContentChild::SendCreateGMPService);
+      SystemGroup::Dispatch("SendCreateGMPService", TaskCategory::Other, r.forget());
     }
     return promise;
   }
diff --git a/dom/media/gmp/GMPServiceParent.cpp b/dom/media/gmp/GMPServiceParent.cpp
index 4f3b6d908855..f22e616ee3f5 100644
--- a/dom/media/gmp/GMPServiceParent.cpp
+++ b/dom/media/gmp/GMPServiceParent.cpp
@@ -94,6 +94,7 @@ GeckoMediaPluginServiceParent::GeckoMediaPluginServiceParent()
   , mInitPromiseMonitor("GeckoMediaPluginServiceParent::mInitPromiseMonitor")
   , mLoadPluginsFromDiskComplete(false)
   , mServiceUserCount(0)
+  , mMainThread(SystemGroup::AbstractMainThreadFor(TaskCategory::Other))
 {
   MOZ_ASSERT(NS_IsMainThread());
   mInitPromise.SetMonitor(&mInitPromiseMonitor);
@@ -462,9 +463,10 @@ GeckoMediaPluginServiceParent::UnloadPlugins()
     plugin->CloseActive(true);
   }
 
-  nsCOMPtr<nsIRunnable> task(NewRunnableMethod(
-    this, &GeckoMediaPluginServiceParent::NotifySyncShutdownComplete));
-  NS_DispatchToMainThread(task);
+  nsCOMPtr<nsIRunnable> task = NewRunnableMethod(
+    "GeckoMediaPluginServiceParent::NotifySyncShutdownComplete",
+    this, &GeckoMediaPluginServiceParent::NotifySyncShutdownComplete);
+  mMainThread->Dispatch(task.forget());
 }
 
 void
@@ -520,7 +522,8 @@ GeckoMediaPluginServiceParent::LoadFromEnvironment()
 class NotifyObserversTask final : public mozilla::Runnable {
 public:
   explicit NotifyObserversTask(const char* aTopic, nsString aData = EmptyString())
-    : mTopic(aTopic)
+    : Runnable(aTopic)
+    , mTopic(aTopic)
     , mData(aData)
   {}
   NS_IMETHOD Run() override {
@@ -553,9 +556,10 @@ void
 GeckoMediaPluginServiceParent::UpdateContentProcessGMPCapabilities()
 {
   if (!NS_IsMainThread()) {
-    nsCOMPtr<nsIRunnable> task =
-      NewRunnableMethod(this, &GeckoMediaPluginServiceParent::UpdateContentProcessGMPCapabilities);
-    NS_DispatchToMainThread(task);
+    nsCOMPtr<nsIRunnable> task = NewRunnableMethod(
+      "GeckoMediaPluginServiceParent::UpdateContentProcessGMPCapabilities",
+      this, &GeckoMediaPluginServiceParent::UpdateContentProcessGMPCapabilities);
+    mMainThread->Dispatch(task.forget());
     return;
   }
 
@@ -617,7 +621,7 @@ GeckoMediaPluginServiceParent::AsyncAddPluginDirectory(const nsAString& aDirecto
            thread, this, __func__,
            &GeckoMediaPluginServiceParent::AddOnGMPThread, dir)
     ->Then(
-      AbstractThread::MainThread(), // Non DocGroup-version for the task in parent.
+      mMainThread,
       __func__,
       [dir, self]() -> void {
         LOGD(("GeckoMediaPluginServiceParent::AsyncAddPluginDirectory %s succeeded",
@@ -779,7 +783,7 @@ GeckoMediaPluginServiceParent::SelectPluginForAPI(const nsACString& aNodeId,
 }
 
 RefPtr<GMPParent>
-CreateGMPParent()
+CreateGMPParent(AbstractThread* aMainThread)
 {
 #if defined(XP_LINUX) && defined(MOZ_GMP_SANDBOX)
   if (!SandboxInfo::Get().CanSandboxMedia()) {
@@ -790,7 +794,7 @@ CreateGMPParent()
     NS_WARNING("Loading media plugin despite lack of sandboxing.");
   }
 #endif
-  return new GMPParent();
+  return new GMPParent(aMainThread);
 }
 
 already_AddRefed<GMPParent>
@@ -798,7 +802,7 @@ GeckoMediaPluginServiceParent::ClonePlugin(const GMPParent* aOriginal)
 {
   MOZ_ASSERT(aOriginal);
 
-  RefPtr<GMPParent> gmp = CreateGMPParent();
+  RefPtr<GMPParent> gmp = CreateGMPParent(mMainThread);
   nsresult rv = gmp ? gmp->CloneFrom(aOriginal) : NS_ERROR_NOT_AVAILABLE;
 
   if (NS_FAILED(rv)) {
@@ -837,7 +841,7 @@ GeckoMediaPluginServiceParent::AddOnGMPThread(nsString aDirectory)
     return GenericPromise::CreateAndReject(NS_ERROR_FAILURE, __func__);
   }
 
-  RefPtr<GMPParent> gmp = CreateGMPParent();
+  RefPtr<GMPParent> gmp = CreateGMPParent(mMainThread);
   if (!gmp) {
     NS_WARNING("Can't Create GMPParent");
     return GenericPromise::CreateAndReject(NS_ERROR_FAILURE, __func__);
@@ -920,9 +924,9 @@ GeckoMediaPluginServiceParent::RemoveOnGMPThread(const nsAString& aDirectory,
     }
     if (NS_SUCCEEDED(directory->Remove(true))) {
       mPluginsWaitingForDeletion.RemoveElement(aDirectory);
-      NS_DispatchToMainThread(new NotifyObserversTask("gmp-directory-deleted",
-                                                      nsString(aDirectory)),
-                              NS_DISPATCH_NORMAL);
+      nsCOMPtr<nsIRunnable> task = new NotifyObserversTask(
+        "gmp-directory-deleted", nsString(aDirectory));
+      mMainThread->Dispatch(task.forget());
     }
   }
 }
@@ -1537,7 +1541,9 @@ GeckoMediaPluginServiceParent::ClearRecentHistoryOnGMPThread(PRTime aSince)
 
   ClearNodeIdAndPlugin(filter);
 
-  NS_DispatchToMainThread(new NotifyObserversTask("gmp-clear-storage-complete"), NS_DISPATCH_NORMAL);
+  nsCOMPtr<nsIRunnable> task
+    = new NotifyObserversTask("gmp-clear-storage-complete");
+  mMainThread->Dispatch(task.forget());
 }
 
 NS_IMETHODIMP
@@ -1647,7 +1653,9 @@ GeckoMediaPluginServiceParent::ClearStorage()
   // Clear private-browsing storage.
   mTempGMPStorage.Clear();
 
-  NS_DispatchToMainThread(new NotifyObserversTask("gmp-clear-storage-complete"), NS_DISPATCH_NORMAL);
+  nsCOMPtr<nsIRunnable> task
+    = new NotifyObserversTask("gmp-clear-storage-complete");
+  mMainThread->Dispatch(task.forget());
 }
 
 already_AddRefed<GMPParent>
@@ -1664,9 +1672,10 @@ GeckoMediaPluginServiceParent::GetById(uint32_t aPluginId)
 
 GMPServiceParent::~GMPServiceParent()
 {
-  NS_DispatchToMainThread(
-    NewRunnableMethod(mService.get(),
-                      &GeckoMediaPluginServiceParent::ServiceUserDestroyed));
+  nsCOMPtr<nsIRunnable> task = NewRunnableMethod(
+    "GeckoMediaPluginServiceParent::ServiceUserDestroyed",
+    mService.get(), &GeckoMediaPluginServiceParent::ServiceUserDestroyed);
+  mService->MainThread()->Dispatch(task.forget());
 }
 
 mozilla::ipc::IPCResult
diff --git a/dom/media/gmp/GMPServiceParent.h b/dom/media/gmp/GMPServiceParent.h
index ea3ccf243656..35c3dc0b14d9 100644
--- a/dom/media/gmp/GMPServiceParent.h
+++ b/dom/media/gmp/GMPServiceParent.h
@@ -65,6 +65,8 @@ public:
 
   void UpdateContentProcessGMPCapabilities();
 
+  AbstractThread* MainThread() const { return mMainThread; }
+
 private:
   friend class GMPServiceParent;
 
@@ -202,6 +204,8 @@ private:
   // Tracks how many users are running (on the GMP thread). Only when this count
   // drops to 0 can we safely shut down the thread.
   MainThreadOnly<int32_t> mServiceUserCount;
+
+  const RefPtr<AbstractThread> mMainThread;
 };
 
 nsresult ReadSalt(nsIFile* aPath, nsACString& aOutData);