--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -1769,16 +1769,21 @@ JS_PUBLIC_API JSObject* JS_NewObjectForC
 
 JS_PUBLIC_API bool JS_IsNative(JSObject* obj) { return obj->isNative(); }
 
 JS_PUBLIC_API void JS::AssertObjectBelongsToCurrentThread(JSObject* obj) {
   JSRuntime* rt = obj->compartment()->runtimeFromAnyThread();
   MOZ_RELEASE_ASSERT(CurrentThreadCanAccessRuntime(rt));
 }
 
+JS_PUBLIC_API void SetHelperThreadTaskCallback(
+    void (*callback)(js::RunnableTask*)) {
+  HelperThreadTaskCallback = callback;
+}
+
 /*** Standard internal methods **********************************************/
 
 JS_PUBLIC_API bool JS_GetPrototype(JSContext* cx, HandleObject obj,
                                    MutableHandleObject result) {
   cx->check(obj);
   return GetPrototype(cx, obj, result);
 }
 

--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -390,16 +390,22 @@ JS_PUBLIC_API bool InitSelfHostedCode(JS
 /**
  * Asserts (in debug and release builds) that `obj` belongs to the current
  * thread's context.
  */
 JS_PUBLIC_API void AssertObjectBelongsToCurrentThread(JSObject* obj);
 
 } /* namespace JS */
 
+/**
+ * Set callback to send tasks to XPCOM thread pools
+ */
+JS_PUBLIC_API void SetHelperThreadTaskCallback(
+    void (*callback)(js::RunnableTask*));
+
 extern JS_PUBLIC_API const char* JS_GetImplementationVersion(void);
 
 extern JS_PUBLIC_API void JS_SetDestroyCompartmentCallback(
     JSContext* cx, JSDestroyCompartmentCallback callback);
 
 extern JS_PUBLIC_API void JS_SetSizeOfIncludingThisCompartmentCallback(
     JSContext* cx, JSSizeOfIncludingThisCompartmentCallback callback);
 

--- a/js/src/vm/Runtime.cpp
+++ b/js/src/vm/Runtime.cpp
@@ -66,16 +66,18 @@ using mozilla::PodZero;
 using mozilla::PositiveInfinity;
 
 /* static */ MOZ_THREAD_LOCAL(JSContext*) js::TlsContext;
 /* static */
 Atomic<size_t> JSRuntime::liveRuntimesCount;
 Atomic<JS::LargeAllocationFailureCallback> js::OnLargeAllocationFailure;
 
 namespace js {
+void (*HelperThreadTaskCallback)(js::RunnableTask*);
+
 bool gCanUseExtraThreads = true;
 }  // namespace js
 
 void js::DisableExtraThreads() { gCanUseExtraThreads = false; }
 
 const JSSecurityCallbacks js::NullSecurityCallbacks = {};
 
 static const JSWrapObjectCallbacks DefaultWrapObjectCallbacks = {

--- a/js/src/vm/Runtime.h
+++ b/js/src/vm/Runtime.h
@@ -1078,11 +1078,15 @@ extern const JSSecurityCallbacks NullSec
 // and may be null. See comment in jsapi.h.
 extern mozilla::Atomic<JS::LargeAllocationFailureCallback>
     OnLargeAllocationFailure;
 
 // This callback is set by JS::SetBuildIdOp and may be null. See comment in
 // jsapi.h.
 extern mozilla::Atomic<JS::BuildIdOp> GetBuildId;
 
+// This callback is set by js::SetHelperThreadTaskCallback and may be null.
+// See comment in jsapi.h.
+extern void (*HelperThreadTaskCallback)(js::RunnableTask*);
+
 } /* namespace js */
 
 #endif /* vm_Runtime_h */

--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -3080,18 +3080,20 @@ void XPCJSRuntime::Initialize(JSContext*
   JS_SetAccumulateTelemetryCallback(cx, AccumulateTelemetryCallback);
   JS_SetSetUseCounterCallback(cx, SetUseCounterCallback);
   js::SetWindowProxyClass(cx, &OuterWindowProxyClass);
   js::SetXrayJitInfo(&gXrayJitInfo);
   JS::SetProcessLargeAllocationFailureCallback(
       OnLargeAllocationFailureCallback);
   JS::SetProcessBuildIdOp(GetBuildId);
 
-  // Initialize a helper thread pool for JS offthread tasks.
+  // Initialize a helper thread pool for JS offthread tasks. Set the
+  // task callback to divert tasks to the helperthreads.
   InitializeHelperThreadPool();
+  SetHelperThreadTaskCallback(&DispatchOffThreadTask);
 
   // The JS engine needs to keep the source code around in order to implement
   // Function.prototype.toSource(). It'd be nice to not have to do this for
   // chrome code and simply stub out requests for source on it. Life is not so
   // easy, unfortunately. Nobody relies on chrome toSource() working in core
   // browser code, but chrome tests use it. The worst offenders are addons,
   // which like to monkeypatch chrome functions by calling toSource() on them
   // and using regular expressions to modify them. We avoid keeping most browser