--- a/dom/media/eme/CDMProxy.h
+++ b/dom/media/eme/CDMProxy.h
@@ -89,17 +89,18 @@ public:
   {}
 
   // Main thread only.
   // Loads the CDM corresponding to mKeySystem.
   // Calls MediaKeys::OnCDMCreated() when the CDM is created.
   virtual void Init(PromiseId aPromiseId,
                     const nsAString& aOrigin,
                     const nsAString& aTopLevelOrigin,
-                    const nsAString& aName) = 0;
+                    const nsAString& aName,
+                    nsIEventTarget* aMainThread) = 0;
 
   virtual void OnSetDecryptorId(uint32_t aId) {}
 
   // Main thread only.
   // Uses the CDM to create a key session.
   // Calls MediaKeys::OnSessionActivated() when session is created.
   // Assumes ownership of (Move()s) aInitData's contents.
   virtual void CreateSession(uint32_t aCreateSessionToken,
@@ -254,16 +255,19 @@ protected:
 
   // Our reference back to the MediaKeys object.
   // WARNING: This is a non-owning reference that is cleared by MediaKeys
   // destructor. only use on main thread, and always nullcheck before using!
   MainThreadOnlyRawPtr<dom::MediaKeys> mKeys;
 
   const nsString mKeySystem;
 
+  // The main thread associated with the root document. Must be set in Init().
+  nsCOMPtr<nsIEventTarget> mMainThread;
+
   // Onwer specified thread. e.g. Gecko Media Plugin thread.
   // All interactions with the out-of-process EME plugin must come from this thread.
   RefPtr<nsIThread> mOwnerThread;
 
   nsCString mNodeId;
 
   CDMCaps mCapabilites;
 

--- a/dom/media/eme/MediaKeys.cpp
+++ b/dom/media/eme/MediaKeys.cpp
@@ -424,17 +424,18 @@ MediaKeys::Init(ErrorResult& aRv)
   // here, and hold a self-reference until that promise is resolved or
   // rejected.
   MOZ_ASSERT(!mCreatePromiseId, "Should only be created once!");
   mCreatePromiseId = StorePromise(promise);
   AddRef();
   mProxy->Init(mCreatePromiseId,
                NS_ConvertUTF8toUTF16(origin),
                NS_ConvertUTF8toUTF16(topLevelOrigin),
-               KeySystemToGMPName(mKeySystem));
+               KeySystemToGMPName(mKeySystem),
+               top->GetExtantDoc()->EventTargetFor(TaskCategory::Other));
 
   return promise.forget();
 }
 
 void
 MediaKeys::OnCDMCreated(PromiseId aId, const uint32_t aPluginId)
 {
   RefPtr<DetailedPromise> promise(RetrievePromise(aId));

--- a/dom/media/eme/mediadrm/MediaDrmCDMProxy.cpp
+++ b/dom/media/eme/mediadrm/MediaDrmCDMProxy.cpp
@@ -41,26 +41,29 @@ MediaDrmCDMProxy::~MediaDrmCDMProxy()
 {
   MOZ_COUNT_DTOR(MediaDrmCDMProxy);
 }
 
 void
 MediaDrmCDMProxy::Init(PromiseId aPromiseId,
                        const nsAString& aOrigin,
                        const nsAString& aTopLevelOrigin,
-                       const nsAString& aName)
+                       const nsAString& aName,
+                       nsIEventTarget* aMainThread)
 {
   MOZ_ASSERT(NS_IsMainThread());
   NS_ENSURE_TRUE_VOID(!mKeys.IsNull());
 
   EME_LOG("MediaDrmCDMProxy::Init (%s, %s) %s",
           NS_ConvertUTF16toUTF8(aOrigin).get(),
           NS_ConvertUTF16toUTF8(aTopLevelOrigin).get(),
           NS_ConvertUTF16toUTF8(aName).get());
 
+  mMainThread = aMainThread;
+
   // Create a thread to work with cdm.
   if (!mOwnerThread) {
     nsresult rv = NS_NewNamedThread("MDCDMThread", getter_AddRefs(mOwnerThread));
     if (NS_FAILED(rv)) {
       RejectPromise(aPromiseId, NS_ERROR_DOM_INVALID_STATE_ERR,
                     NS_LITERAL_CSTRING("Couldn't create CDM thread MediaDrmCDMProxy::Init"));
       return;
     }
@@ -303,17 +306,17 @@ MediaDrmCDMProxy::RejectPromise(PromiseI
 {
   if (NS_IsMainThread()) {
     if (!mKeys.IsNull()) {
       mKeys->RejectPromise(aId, aCode, aReason);
     }
   } else {
     nsCOMPtr<nsIRunnable> task(new RejectPromiseTask(this, aId, aCode,
                                                      aReason));
-    NS_DispatchToMainThread(task);
+    mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
   }
 }
 
 void
 MediaDrmCDMProxy::ResolvePromise(PromiseId aId)
 {
   if (NS_IsMainThread()) {
     if (!mKeys.IsNull()) {
@@ -321,17 +324,17 @@ MediaDrmCDMProxy::ResolvePromise(Promise
     } else {
       NS_WARNING("MediaDrmCDMProxy unable to resolve promise!");
     }
   } else {
     nsCOMPtr<nsIRunnable> task;
     task = NewRunnableMethod<PromiseId>(this,
                                         &MediaDrmCDMProxy::ResolvePromise,
                                         aId);
-    NS_DispatchToMainThread(task);
+    mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
   }
 }
 
 const nsString&
 MediaDrmCDMProxy::KeySystem() const
 {
   return mKeySystem;
 }
@@ -403,17 +406,17 @@ MediaDrmCDMProxy::md_Init(uint32_t aProm
   MOZ_ASSERT(mCDM);
 
   mCallback.reset(new MediaDrmCDMCallbackProxy(this));
   mCDM->Init(mCallback.get());
   nsCOMPtr<nsIRunnable> task(
     NewRunnableMethod<uint32_t>(this,
                                 &MediaDrmCDMProxy::OnCDMCreated,
                                 aPromiseId));
-  NS_DispatchToMainThread(task);
+  mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
 }
 
 void
 MediaDrmCDMProxy::md_CreateSession(UniquePtr<CreateSessionData>&& aData)
 {
   MOZ_ASSERT(IsOnOwnerThread());
 
   if (!mCDM) {

--- a/dom/media/eme/mediadrm/MediaDrmCDMProxy.h
+++ b/dom/media/eme/mediadrm/MediaDrmCDMProxy.h
@@ -32,17 +32,18 @@ public:
   MediaDrmCDMProxy(dom::MediaKeys* aKeys,
                    const nsAString& aKeySystem,
                    bool aDistinctiveIdentifierRequired,
                    bool aPersistentStateRequired);
 
   void Init(PromiseId aPromiseId,
             const nsAString& aOrigin,
             const nsAString& aTopLevelOrigin,
-            const nsAString& aGMPName) override;
+            const nsAString& aGMPName,
+            nsIEventTarget* aMainThread) override;
 
   void CreateSession(uint32_t aCreateSessionToken,
                      MediaKeySessionType aSessionType,
                      PromiseId aPromiseId,
                      const nsAString& aInitDataType,
                      nsTArray<uint8_t>& aInitData) override;
 
   void LoadSession(PromiseId aPromiseId,

--- a/dom/media/gmp/GMPCDMProxy.cpp
+++ b/dom/media/gmp/GMPCDMProxy.cpp
@@ -47,25 +47,28 @@ GMPCDMProxy::~GMPCDMProxy()
 {
   MOZ_COUNT_DTOR(GMPCDMProxy);
 }
 
 void
 GMPCDMProxy::Init(PromiseId aPromiseId,
                   const nsAString& aOrigin,
                   const nsAString& aTopLevelOrigin,
-                  const nsAString& aGMPName)
+                  const nsAString& aGMPName,
+                  nsIEventTarget* aMainThread)
 {
   MOZ_ASSERT(NS_IsMainThread());
   NS_ENSURE_TRUE_VOID(!mKeys.IsNull());
 
   EME_LOG("GMPCDMProxy::Init (%s, %s)",
           NS_ConvertUTF16toUTF8(aOrigin).get(),
           NS_ConvertUTF16toUTF8(aTopLevelOrigin).get());
 
+  mMainThread = aMainThread;
+
   nsCString pluginVersion;
   if (!mOwnerThread) {
     nsCOMPtr<mozIGeckoMediaPluginService> mps =
       do_GetService("@mozilla.org/gecko-media-plugin-service;1");
     if (!mps) {
       RejectPromise(aPromiseId, NS_ERROR_DOM_INVALID_STATE_ERR,
                     NS_LITERAL_CSTRING("Couldn't get MediaPluginService in GMPCDMProxy::Init"));
       return;
@@ -136,17 +139,17 @@ GMPCDMProxy::gmp_InitDone(GMPDecryptorPr
 void GMPCDMProxy::OnSetDecryptorId(uint32_t aId)
 {
   MOZ_ASSERT(mCreatePromiseId);
   mDecryptorId = aId;
   nsCOMPtr<nsIRunnable> task(
     NewRunnableMethod<uint32_t>(this,
                                 &GMPCDMProxy::OnCDMCreated,
                                 mCreatePromiseId));
-  NS_DispatchToMainThread(task);
+  mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
 }
 
 class gmp_InitDoneCallback : public GetGMPDecryptorCallback
 {
 public:
   gmp_InitDoneCallback(GMPCDMProxy* aGMPCDMProxy,
                        UniquePtr<GMPCDMProxy::InitData>&& aData)
     : mGMPCDMProxy(aGMPCDMProxy),
@@ -513,17 +516,17 @@ GMPCDMProxy::RejectPromise(PromiseId aId
 {
   if (NS_IsMainThread()) {
     if (!mKeys.IsNull()) {
       mKeys->RejectPromise(aId, aCode, aReason);
     }
   } else {
     nsCOMPtr<nsIRunnable> task(new RejectPromiseTask(this, aId, aCode,
                                                      aReason));
-    NS_DispatchToMainThread(task);
+    mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
   }
 }
 
 void
 GMPCDMProxy::ResolvePromise(PromiseId aId)
 {
   if (NS_IsMainThread()) {
     if (!mKeys.IsNull()) {
@@ -531,17 +534,17 @@ GMPCDMProxy::ResolvePromise(PromiseId aI
     } else {
       NS_WARNING("GMPCDMProxy unable to resolve promise!");
     }
   } else {
     nsCOMPtr<nsIRunnable> task;
     task = NewRunnableMethod<PromiseId>(this,
                                         &GMPCDMProxy::ResolvePromise,
                                         aId);
-    NS_DispatchToMainThread(task);
+    mMainThread->Dispatch(task.forget(), NS_DISPATCH_NORMAL);
   }
 }
 
 const nsCString&
 GMPCDMProxy::GetNodeId() const
 {
   return mNodeId;
 }

--- a/dom/media/gmp/GMPCDMProxy.h
+++ b/dom/media/gmp/GMPCDMProxy.h
@@ -26,17 +26,18 @@ public:
               const nsAString& aKeySystem,
               GMPCrashHelper* aCrashHelper,
               bool aDistinctiveIdentifierRequired,
               bool aPersistentStateRequired);
 
   void Init(PromiseId aPromiseId,
             const nsAString& aOrigin,
             const nsAString& aTopLevelOrigin,
-            const nsAString& aGMPName) override;
+            const nsAString& aGMPName,
+            nsIEventTarget* aMainThread) override;
 
   void OnSetDecryptorId(uint32_t aId) override;
 
   void CreateSession(uint32_t aCreateSessionToken,
                      dom::MediaKeySessionType aSessionType,
                      PromiseId aPromiseId,
                      const nsAString& aInitDataType,
                      nsTArray<uint8_t>& aInitData) override;