--- a/dom/media/ipc/RDDProcessHost.cpp
+++ b/dom/media/ipc/RDDProcessHost.cpp
@@ -142,25 +142,30 @@ void RDDProcessHost::OnChannelErrorTask(
 
 static uint64_t sRDDProcessTokenCounter = 0;
 
 void RDDProcessHost::InitAfterConnect(bool aSucceeded) {
   MOZ_ASSERT(mLaunchPhase == LaunchPhase::Waiting);
   MOZ_ASSERT(!mRDDChild);
 
   mLaunchPhase = LaunchPhase::Complete;
-  mPrefSerializer = nullptr;
 
   if (aSucceeded) {
     mProcessToken = ++sRDDProcessTokenCounter;
     mRDDChild = MakeUnique<RDDChild>(this);
     DebugOnly<bool> rv =
         mRDDChild->Open(GetChannel(), base::GetProcId(GetChildProcessHandle()));
     MOZ_ASSERT(rv);
 
+    // Only clear mPrefSerializer in the success case to avoid a
+    // possible race in the case case of a timeout on Windows launch.
+    // See Bug 1555076 comment 7:
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1555076#c7
+    mPrefSerializer = nullptr;
+
     bool startMacSandbox = false;
 
 #if defined(XP_MACOSX) && defined(MOZ_SANDBOX)
     // If the sandbox was started at launch time,
     // do not start the sandbox again.
     startMacSandbox = !sLaunchWithMacSandbox;
 #endif