diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index 141e7c3043..99e26e8511 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -877,22 +877,27 @@ ExtractByteStreamFromBody(const fetch::ResponseBodyInit& aBodyInit,
 
 
 template <class Derived>
-FetchBody<Derived>::FetchBody()
+FetchBody<Derived>::FetchBody(nsIGlobalObject* aOwner)
   : mWorkerPrivate(nullptr)
+  , mOwner(aOwner)
   , mReadableStreamBody(nullptr)
   , mBodyUsed(false)
 {
+  MOZ_ASSERT(aOwner);
+
   if (!NS_IsMainThread()) {
     mWorkerPrivate = GetCurrentThreadWorkerPrivate();
     MOZ_ASSERT(mWorkerPrivate);
+  } else {
+    mWorkerPrivate = nullptr;
   }
 }
 
 template
-FetchBody<Request>::FetchBody();
+FetchBody<Request>::FetchBody(nsIGlobalObject* aOwner);
 
 template
-FetchBody<Response>::FetchBody();
+FetchBody<Response>::FetchBody(nsIGlobalObject* aOwner);
 
 template <class Derived>
 FetchBody<Derived>::~FetchBody()
diff --git a/dom/fetch/Fetch.h b/dom/fetch/Fetch.h
index d3a7ac598e..b16dac3bf1 100644
--- a/dom/fetch/Fetch.h
+++ b/dom/fetch/Fetch.h
@@ -20,6 +20,7 @@
 #include "mozilla/dom/RequestBinding.h"
 
 class nsIGlobalObject;
+class nsIEventTarget;
 
 namespace mozilla {
 namespace dom {
@@ -208,7 +209,9 @@ public:
   GetSignal() const = 0;
 
 protected:
-  FetchBody();
+  nsCOMPtr<nsIGlobalObject> mOwner;
+
+  explicit FetchBody(nsIGlobalObject* aOwner);
 
   // Always set whenever the FetchBody is created on the worker thread.
   workers::WorkerPrivate* mWorkerPrivate;
@@ -252,6 +255,9 @@ private:
   // Only ever set once, always on target thread.
   bool mBodyUsed;
   nsCString mMimeType;
+
+  // The main-thread event target for runnable dispatching.
+  nsCOMPtr<nsIEventTarget> mMainThreadEventTarget;
 };
 
 } // namespace dom
diff --git a/dom/fetch/Request.cpp b/dom/fetch/Request.cpp
index 03df3021c5..eec4828131 100644
--- a/dom/fetch/Request.cpp
+++ b/dom/fetch/Request.cpp
@@ -54,8 +54,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Request)
 NS_INTERFACE_MAP_END
 
 Request::Request(nsIGlobalObject* aOwner, InternalRequest* aRequest, AbortSignal* aSignal)
-  : FetchBody<Request>()
-  , mOwner(aOwner)
+  : FetchBody<Request>(aOwner)
   , mRequest(aRequest)
   , mSignal(aSignal)
 {
diff --git a/dom/fetch/Request.h b/dom/fetch/Request.h
index 8a22f1340b..d5fc09fe25 100644
--- a/dom/fetch/Request.h
+++ b/dom/fetch/Request.h
@@ -164,7 +164,6 @@ public:
 private:
   ~Request();
 
-  nsCOMPtr<nsIGlobalObject> mOwner;
   RefPtr<InternalRequest> mRequest;
   
   // Lazily created.
diff --git a/dom/fetch/Response.cpp b/dom/fetch/Response.cpp
index d69a199076..b92e918e7c 100644
--- a/dom/fetch/Response.cpp
+++ b/dom/fetch/Response.cpp
@@ -57,8 +57,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Response)
 NS_INTERFACE_MAP_END
 
 Response::Response(nsIGlobalObject* aGlobal, InternalResponse* aInternalResponse, AbortSignal* aSignal)
-  : FetchBody<Response>()
-  , mOwner(aGlobal)
+  : FetchBody<Response>(aGlobal)
   , mInternalResponse(aInternalResponse)
   , mSignal(aSignal)
 {
diff --git a/dom/fetch/Response.h b/dom/fetch/Response.h
index 9b29dff998..d2f885fbc7 100644
--- a/dom/fetch/Response.h
+++ b/dom/fetch/Response.h
@@ -144,7 +144,6 @@ public:
 private:
   ~Response();
 
-  nsCOMPtr<nsIGlobalObject> mOwner;
   RefPtr<InternalResponse> mInternalResponse;
 
   // Lazily created