--- a/dom/media/platforms/PDMFactory.cpp
+++ b/dom/media/platforms/PDMFactory.cpp
@@ -24,16 +24,17 @@
 #ifdef MOZ_WIDGET_ANDROID
 #include "AndroidDecoderModule.h"
 #endif
 #include "GMPDecoderModule.h"
 
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/SharedThreadPool.h"
 #include "mozilla/StaticPtr.h"
+#include "mozilla/SyncRunnable.h"
 #include "mozilla/TaskQueue.h"
 
 #include "MediaInfo.h"
 #include "MediaPrefs.h"
 #include "FuzzingWrapper.h"
 #include "H264Converter.h"
 
 #include "AgnosticDecoderModule.h"
@@ -82,27 +83,41 @@ PDMFactory::PDMFactory()
 
 PDMFactory::~PDMFactory()
 {
 }
 
 void
 PDMFactory::EnsureInit() const
 {
-  StaticMutexAutoLock mon(sMonitor);
-  if (!sInstance) {
-    sInstance = new PDMFactoryImpl();
+  {
+    StaticMutexAutoLock mon(sMonitor);
+    if (sInstance) {
+      // Quick exit if we already have an instance.
+      return;
+    }
     if (NS_IsMainThread()) {
+      // On the main thread and holding the lock -> Create instance.
+      sInstance = new PDMFactoryImpl();
       ClearOnShutdown(&sInstance);
-    } else {
-      nsCOMPtr<nsIRunnable> runnable =
-        NS_NewRunnableFunction([]() { ClearOnShutdown(&sInstance); });
-      NS_DispatchToMainThread(runnable);
+      return;
     }
   }
+
+  // Not on the main thread -> Sync-dispatch creation to main thread.
+  nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
+  nsCOMPtr<nsIRunnable> runnable =
+    NS_NewRunnableFunction([]() {
+      StaticMutexAutoLock mon(sMonitor);
+      if (!sInstance) {
+        sInstance = new PDMFactoryImpl();
+        ClearOnShutdown(&sInstance);
+      }
+    });
+  SyncRunnable::DispatchToThread(mainThread, runnable);
 }
 
 already_AddRefed<MediaDataDecoder>
 PDMFactory::CreateDecoder(const CreateDecoderParams& aParams)
 {
   const TrackInfo& config = aParams.mConfig;
   bool isEncrypted = mEMEPDM && config.mCrypto.mValid;
 

--- a/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp
+++ b/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp
@@ -8,17 +8,16 @@
 #include "DecoderDoctorDiagnostics.h"
 #include "GMPAudioDecoder.h"
 #include "GMPVideoDecoder.h"
 #include "MediaDataDecoderProxy.h"
 #include "MediaPrefs.h"
 #include "mozIGeckoMediaPluginService.h"
 #include "nsServiceManagerUtils.h"
 #include "mozilla/StaticMutex.h"
-#include "mozilla/SyncRunnable.h"
 #include "gmp-audio-decode.h"
 #include "gmp-video-decode.h"
 #ifdef XP_WIN
 #include "WMFDecoderModule.h"
 #endif
 
 namespace mozilla {
 
@@ -164,23 +163,17 @@ GMPDecoderModule::UpdateUsableCodecs()
                              nsDependentCString(gmp.mKeySystem));
   }
 }
 
 /* static */
 void
 GMPDecoderModule::Init()
 {
-  if (!NS_IsMainThread()) {
-    nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
-    nsCOMPtr<nsIRunnable> runnable =
-      NS_NewRunnableFunction([]() { Init(); });
-    SyncRunnable::DispatchToThread(mainThread, runnable);
-    return;
-  }
+  MOZ_ASSERT(NS_IsMainThread());
   // GMPService::HasPluginForAPI is main thread only, so to implement
   // SupportsMimeType() we build a table of the codecs which each whitelisted
   // GMP has and update it when any GMPs are removed or added at runtime.
   UpdateUsableCodecs();
 }
 
 /* static */
 const Maybe<nsCString>