Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

32 files changed, 9016 insertions, 0 deletions

diff --git a/js/xpconnect/loader/AutoMemMap.cpp b/js/xpconnect/loader/AutoMemMap.cpp
new file mode 100644
index 0000000000..f8c75ea445
--- /dev/null
+++ b/js/xpconnect/loader/AutoMemMap.cpp
@@ -0,0 +1,157 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AutoMemMap.h"
+#include "ScriptPreloader-inl.h"
+
+#include "mozilla/Unused.h"
+#include "mozilla/ipc/FileDescriptor.h"
+#include "nsIFile.h"
+
+#include <private/pprio.h>
+
+namespace mozilla {
+namespace loader {
+
+using namespace mozilla::ipc;
+
+AutoMemMap::~AutoMemMap() { reset(); }
+
+FileDescriptor AutoMemMap::cloneFileDescriptor() const {
+  if (fd.get()) {
+    auto handle =
+        FileDescriptor::PlatformHandleType(PR_FileDesc2NativeHandle(fd.get()));
+    return FileDescriptor(handle);
+  }
+  return FileDescriptor();
+}
+
+Result<Ok, nsresult> AutoMemMap::init(nsIFile* file, int flags, int mode,
+                                      PRFileMapProtect prot) {
+  MOZ_ASSERT(!fd);
+
+  MOZ_TRY(file->OpenNSPRFileDesc(flags, mode, &fd.rwget()));
+
+  return initInternal(prot);
+}
+
+Result<Ok, nsresult> AutoMemMap::init(const FileDescriptor& file,
+                                      PRFileMapProtect prot, size_t maybeSize) {
+  MOZ_ASSERT(!fd);
+  if (!file.IsValid()) {
+    return Err(NS_ERROR_INVALID_ARG);
+  }
+
+  auto handle = file.ClonePlatformHandle();
+
+  fd = PR_ImportFile(PROsfd(handle.get()));
+  if (!fd) {
+    return Err(NS_ERROR_FAILURE);
+  }
+  Unused << handle.release();
+
+  return initInternal(prot, maybeSize);
+}
+
+Result<Ok, nsresult> AutoMemMap::initInternal(PRFileMapProtect prot,
+                                              size_t maybeSize) {
+  MOZ_ASSERT(!fileMap);
+  MOZ_ASSERT(!addr);
+
+  if (maybeSize > 0) {
+    // Some OSes' shared memory objects can't be stat()ed, either at
+    // all (Android) or without loosening the sandbox (Mac) so just
+    // use the size.
+    size_ = maybeSize;
+  } else {
+    // But if we don't have the size, assume it's a regular file and
+    // ask for it.
+    PRFileInfo64 fileInfo;
+    MOZ_TRY(PR_GetOpenFileInfo64(fd.get(), &fileInfo));
+
+    if (fileInfo.size > UINT32_MAX) {
+      return Err(NS_ERROR_INVALID_ARG);
+    }
+    size_ = fileInfo.size;
+  }
+
+  fileMap = PR_CreateFileMap(fd, 0, prot);
+  if (!fileMap) {
+    return Err(NS_ERROR_FAILURE);
+  }
+
+  addr = PR_MemMap(fileMap, 0, size_);
+  if (!addr) {
+    return Err(NS_ERROR_FAILURE);
+  }
+
+  return Ok();
+}
+
+#ifdef XP_WIN
+
+Result<Ok, nsresult> AutoMemMap::initWithHandle(const FileDescriptor& file,
+                                                size_t size,
+                                                PRFileMapProtect prot) {
+  MOZ_ASSERT(!fd);
+  MOZ_ASSERT(!handle_);
+  if (!file.IsValid()) {
+    return Err(NS_ERROR_INVALID_ARG);
+  }
+
+  handle_ = file.ClonePlatformHandle().release();
+
+  MOZ_ASSERT(!addr);
+
+  size_ = size;
+
+  addr = MapViewOfFile(
+      handle_, prot == PR_PROT_READONLY ? FILE_MAP_READ : FILE_MAP_ALL_ACCESS,
+      0, 0, size);
+  if (!addr) {
+    return Err(NS_ERROR_FAILURE);
+  }
+
+  return Ok();
+}
+
+FileDescriptor AutoMemMap::cloneHandle() const {
+  return FileDescriptor(handle_);
+}
+
+#else
+
+Result<Ok, nsresult> AutoMemMap::initWithHandle(const FileDescriptor& file,
+                                                size_t size,
+                                                PRFileMapProtect prot) {
+  MOZ_DIAGNOSTIC_ASSERT(size > 0);
+  return init(file, prot, size);
+}
+
+FileDescriptor AutoMemMap::cloneHandle() const { return cloneFileDescriptor(); }
+
+#endif
+
+void AutoMemMap::reset() {
+  if (addr && !persistent_) {
+    Unused << NS_WARN_IF(PR_MemUnmap(addr, size()) != PR_SUCCESS);
+    addr = nullptr;
+  }
+  if (fileMap) {
+    Unused << NS_WARN_IF(PR_CloseFileMap(fileMap) != PR_SUCCESS);
+    fileMap = nullptr;
+  }
+#ifdef XP_WIN
+  if (handle_) {
+    CloseHandle(handle_);
+    handle_ = nullptr;
+  }
+#endif
+  fd.dispose();
+}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/AutoMemMap.h b/js/xpconnect/loader/AutoMemMap.h
new file mode 100644
index 0000000000..54180d09e3
--- /dev/null
+++ b/js/xpconnect/loader/AutoMemMap.h
@@ -0,0 +1,100 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef loader_AutoMemMap_h
+#define loader_AutoMemMap_h
+
+#include "mozilla/FileUtils.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RangedPtr.h"
+#include "mozilla/Result.h"
+
+#include <prio.h>
+
+class nsIFile;
+
+namespace mozilla {
+namespace ipc {
+class FileDescriptor;
+}
+
+namespace loader {
+
+class AutoMemMap {
+  typedef mozilla::ipc::FileDescriptor FileDescriptor;
+
+ public:
+  AutoMemMap() = default;
+
+  ~AutoMemMap();
+
+  Result<Ok, nsresult> init(nsIFile* file, int flags = PR_RDONLY, int mode = 0,
+                            PRFileMapProtect prot = PR_PROT_READONLY);
+
+  Result<Ok, nsresult> init(const FileDescriptor& file,
+                            PRFileMapProtect prot = PR_PROT_READONLY,
+                            size_t maybeSize = 0);
+
+  // Initializes the mapped memory with a shared memory handle. On
+  // Unix-like systems, this is identical to the above init() method. On
+  // Windows, the FileDescriptor must be a handle for a file mapping,
+  // rather than a file descriptor.
+  Result<Ok, nsresult> initWithHandle(const FileDescriptor& file, size_t size,
+                                      PRFileMapProtect prot = PR_PROT_READONLY);
+
+  void reset();
+
+  bool initialized() const { return addr; }
+
+  uint32_t size() const { return size_; }
+
+  template <typename T = void>
+  RangedPtr<T> get() {
+    MOZ_ASSERT(addr);
+    return {static_cast<T*>(addr), size_};
+  }
+
+  template <typename T = void>
+  const RangedPtr<T> get() const {
+    MOZ_ASSERT(addr);
+    return {static_cast<T*>(addr), size_};
+  }
+
+  size_t nonHeapSizeOfExcludingThis() { return size_; }
+
+  FileDescriptor cloneFileDescriptor() const;
+  FileDescriptor cloneHandle() const;
+
+  // Makes this mapping persistent. After calling this, the mapped memory
+  // will remained mapped, even after this instance is destroyed.
+  void setPersistent() { persistent_ = true; }
+
+ private:
+  Result<Ok, nsresult> initInternal(PRFileMapProtect prot,
+                                    size_t maybeSize = 0);
+
+  AutoFDClose fd;
+  PRFileMap* fileMap = nullptr;
+
+#ifdef XP_WIN
+  // We can't include windows.h in this header, since it gets included
+  // by some binding headers (which are explicitly incompatible with
+  // windows.h). So we can't use the HANDLE type here.
+  void* handle_ = nullptr;
+#endif
+
+  uint32_t size_ = 0;
+  void* addr = nullptr;
+
+  bool persistent_ = 0;
+
+  AutoMemMap(const AutoMemMap&) = delete;
+  void operator=(const AutoMemMap&) = delete;
+};
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // loader_AutoMemMap_h
diff --git a/js/xpconnect/loader/ChromeScriptLoader.cpp b/js/xpconnect/loader/ChromeScriptLoader.cpp
new file mode 100644
index 0000000000..22035d75b0
--- /dev/null
+++ b/js/xpconnect/loader/ChromeScriptLoader.cpp
@@ -0,0 +1,379 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "PrecompiledScript.h"
+
+#include "nsIIncrementalStreamLoader.h"
+#include "nsIURI.h"
+#include "nsIChannel.h"
+#include "nsNetUtil.h"
+#include "nsThreadUtils.h"
+
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/CompilationAndEvaluation.h"
+#include "js/experimental/JSStencil.h"  // JS::CompileGlobalScriptToStencil, JS::InstantiateGlobalStencil, JS::OffThreadCompileToStencil
+#include "js/SourceText.h"
+#include "js/Utility.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/SchedulerGroup.h"
+#include "mozilla/dom/ChromeUtils.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/ScriptLoader.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "nsCCUncollectableMarker.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsGlobalWindowInner.h"
+
+using namespace JS;
+using namespace mozilla;
+using namespace mozilla::dom;
+
+class AsyncScriptCompiler final : public nsIIncrementalStreamLoaderObserver,
+                                  public Runnable {
+ public:
+  // Note: References to this class are never held by cycle-collected objects.
+  // If at any point a reference is returned to a caller, please update this
+  // class to implement cycle collection.
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIINCREMENTALSTREAMLOADEROBSERVER
+  NS_DECL_NSIRUNNABLE
+
+  AsyncScriptCompiler(JSContext* aCx, nsIGlobalObject* aGlobal,
+                      const nsACString& aURL, Promise* aPromise)
+      : mozilla::Runnable("AsyncScriptCompiler"),
+        mOptions(aCx),
+        mURL(aURL),
+        mGlobalObject(aGlobal),
+        mPromise(aPromise),
+        mToken(nullptr),
+        mScriptLength(0) {}
+
+  [[nodiscard]] nsresult Start(JSContext* aCx,
+                               const CompileScriptOptionsDictionary& aOptions,
+                               nsIPrincipal* aPrincipal);
+
+  inline void SetToken(JS::OffThreadToken* aToken) { mToken = aToken; }
+
+ protected:
+  virtual ~AsyncScriptCompiler() {
+    if (mPromise->State() == Promise::PromiseState::Pending) {
+      mPromise->MaybeReject(NS_ERROR_FAILURE);
+    }
+  }
+
+ private:
+  void Reject(JSContext* aCx);
+  void Reject(JSContext* aCx, const char* aMxg);
+
+  bool StartCompile(JSContext* aCx);
+  void FinishCompile(JSContext* aCx);
+  void Finish(JSContext* aCx, RefPtr<JS::Stencil> aStencil);
+
+  OwningCompileOptions mOptions;
+  nsCString mURL;
+  nsCOMPtr<nsIGlobalObject> mGlobalObject;
+  RefPtr<Promise> mPromise;
+  nsString mCharset;
+  JS::OffThreadToken* mToken;
+  UniquePtr<Utf8Unit[], JS::FreePolicy> mScriptText;
+  size_t mScriptLength;
+};
+
+NS_IMPL_QUERY_INTERFACE_INHERITED(AsyncScriptCompiler, Runnable,
+                                  nsIIncrementalStreamLoaderObserver)
+NS_IMPL_ADDREF_INHERITED(AsyncScriptCompiler, Runnable)
+NS_IMPL_RELEASE_INHERITED(AsyncScriptCompiler, Runnable)
+
+nsresult AsyncScriptCompiler::Start(
+    JSContext* aCx, const CompileScriptOptionsDictionary& aOptions,
+    nsIPrincipal* aPrincipal) {
+  mCharset = aOptions.mCharset;
+
+  CompileOptions options(aCx);
+  options.setFile(mURL.get()).setNoScriptRval(!aOptions.mHasReturnValue);
+
+  if (!aOptions.mLazilyParse) {
+    options.setForceFullParse();
+  }
+
+  if (NS_WARN_IF(!mOptions.copy(aCx, options))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  nsCOMPtr<nsIURI> uri;
+  nsresult rv = NS_NewURI(getter_AddRefs(uri), mURL);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIChannel> channel;
+  rv = NS_NewChannel(
+      getter_AddRefs(channel), uri, aPrincipal,
+      nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+      nsIContentPolicy::TYPE_INTERNAL_CHROMEUTILS_COMPILED_SCRIPT);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // allow deprecated HTTP request from SystemPrincipal
+  nsCOMPtr<nsILoadInfo> loadInfo = channel->LoadInfo();
+  loadInfo->SetAllowDeprecatedSystemRequests(true);
+  nsCOMPtr<nsIIncrementalStreamLoader> loader;
+  rv = NS_NewIncrementalStreamLoader(getter_AddRefs(loader), this);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return channel->AsyncOpen(loader);
+}
+
+static void OffThreadScriptLoaderCallback(JS::OffThreadToken* aToken,
+                                          void* aCallbackData) {
+  RefPtr<AsyncScriptCompiler> scriptCompiler =
+      dont_AddRef(static_cast<AsyncScriptCompiler*>(aCallbackData));
+
+  scriptCompiler->SetToken(aToken);
+
+  SchedulerGroup::Dispatch(TaskCategory::Other, scriptCompiler.forget());
+}
+
+bool AsyncScriptCompiler::StartCompile(JSContext* aCx) {
+  JS::SourceText<Utf8Unit> srcBuf;
+  if (!srcBuf.init(aCx, std::move(mScriptText), mScriptLength)) {
+    return false;
+  }
+
+  if (JS::CanCompileOffThread(aCx, mOptions, mScriptLength)) {
+    if (!JS::CompileToStencilOffThread(aCx, mOptions, srcBuf,
+                                       OffThreadScriptLoaderCallback,
+                                       static_cast<void*>(this))) {
+      return false;
+    }
+
+    NS_ADDREF(this);
+    return true;
+  }
+
+  RefPtr<Stencil> stencil =
+      JS::CompileGlobalScriptToStencil(aCx, mOptions, srcBuf);
+  if (!stencil) {
+    return false;
+  }
+
+  Finish(aCx, stencil);
+  return true;
+}
+
+NS_IMETHODIMP
+AsyncScriptCompiler::Run() {
+  AutoJSAPI jsapi;
+  if (jsapi.Init(mGlobalObject)) {
+    FinishCompile(jsapi.cx());
+  } else {
+    jsapi.Init();
+    JS::CancelOffThreadToken(jsapi.cx(), mToken);
+
+    mPromise->MaybeReject(NS_ERROR_FAILURE);
+  }
+
+  return NS_OK;
+}
+
+void AsyncScriptCompiler::FinishCompile(JSContext* aCx) {
+  RefPtr<JS::Stencil> stencil = JS::FinishOffThreadStencil(aCx, mToken);
+  if (stencil) {
+    Finish(aCx, stencil);
+  } else {
+    Reject(aCx);
+  }
+}
+
+void AsyncScriptCompiler::Finish(JSContext* aCx, RefPtr<JS::Stencil> aStencil) {
+  RefPtr<PrecompiledScript> result =
+      new PrecompiledScript(mGlobalObject, aStencil, mOptions);
+
+  mPromise->MaybeResolve(result);
+}
+
+void AsyncScriptCompiler::Reject(JSContext* aCx) {
+  RootedValue value(aCx, JS::UndefinedValue());
+  if (JS_GetPendingException(aCx, &value)) {
+    JS_ClearPendingException(aCx);
+  }
+  mPromise->MaybeReject(value);
+}
+
+void AsyncScriptCompiler::Reject(JSContext* aCx, const char* aMsg) {
+  nsAutoString msg;
+  msg.AppendASCII(aMsg);
+  msg.AppendLiteral(": ");
+  AppendUTF8toUTF16(mURL, msg);
+
+  RootedValue exn(aCx);
+  if (xpc::NonVoidStringToJsval(aCx, msg, &exn)) {
+    JS_SetPendingException(aCx, exn);
+  }
+
+  Reject(aCx);
+}
+
+NS_IMETHODIMP
+AsyncScriptCompiler::OnIncrementalData(nsIIncrementalStreamLoader* aLoader,
+                                       nsISupports* aContext,
+                                       uint32_t aDataLength,
+                                       const uint8_t* aData,
+                                       uint32_t* aConsumedData) {
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+AsyncScriptCompiler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
+                                      nsISupports* aContext, nsresult aStatus,
+                                      uint32_t aLength, const uint8_t* aBuf) {
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobalObject)) {
+    mPromise->MaybeReject(NS_ERROR_FAILURE);
+    return NS_OK;
+  }
+
+  JSContext* cx = jsapi.cx();
+
+  if (NS_FAILED(aStatus)) {
+    Reject(cx, "Unable to load script");
+    return NS_OK;
+  }
+
+  nsresult rv = ScriptLoader::ConvertToUTF8(
+      nullptr, aBuf, aLength, mCharset, nullptr, mScriptText, mScriptLength);
+  if (NS_FAILED(rv)) {
+    Reject(cx, "Unable to decode script");
+    return NS_OK;
+  }
+
+  if (!StartCompile(cx)) {
+    Reject(cx);
+  }
+
+  return NS_OK;
+}
+
+namespace mozilla {
+namespace dom {
+
+/* static */
+already_AddRefed<Promise> ChromeUtils::CompileScript(
+    GlobalObject& aGlobal, const nsAString& aURL,
+    const CompileScriptOptionsDictionary& aOptions, ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  MOZ_ASSERT(global);
+
+  RefPtr<Promise> promise = Promise::Create(global, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  NS_ConvertUTF16toUTF8 url(aURL);
+  RefPtr<AsyncScriptCompiler> compiler =
+      new AsyncScriptCompiler(aGlobal.Context(), global, url, promise);
+
+  nsresult rv = compiler->Start(aGlobal.Context(), aOptions,
+                                aGlobal.GetSubjectPrincipal());
+  if (NS_FAILED(rv)) {
+    promise->MaybeReject(rv);
+  }
+
+  return promise.forget();
+}
+
+PrecompiledScript::PrecompiledScript(nsISupports* aParent,
+                                     RefPtr<JS::Stencil> aStencil,
+                                     JS::ReadOnlyCompileOptions& aOptions)
+    : mParent(aParent),
+      mStencil(aStencil),
+      mURL(aOptions.filename()),
+      mHasReturnValue(!aOptions.noScriptRval) {
+  MOZ_ASSERT(aParent);
+  MOZ_ASSERT(aStencil);
+#ifdef DEBUG
+  JS::InstantiateOptions options(aOptions);
+  options.assertDefault();
+#endif
+};
+
+void PrecompiledScript::ExecuteInGlobal(JSContext* aCx, HandleObject aGlobal,
+                                        const ExecuteInGlobalOptions& aOptions,
+                                        MutableHandleValue aRval,
+                                        ErrorResult& aRv) {
+  {
+    RootedObject targetObj(aCx, JS_FindCompilationScope(aCx, aGlobal));
+    // Use AutoEntryScript for its ReportException method call.
+    // This will ensure notified any exception happening in the content script
+    // directly to the console, so that exceptions are flagged with the right
+    // innerWindowID. It helps these exceptions to appear in the page's web
+    // console.
+    AutoEntryScript aes(targetObj, "pre-compiled-script execution");
+    JSContext* cx = aes.cx();
+
+    // See assertion in constructor.
+    JS::InstantiateOptions options;
+    Rooted<JSScript*> script(
+        cx, JS::InstantiateGlobalStencil(cx, options, mStencil));
+    if (!script) {
+      aRv.NoteJSContextException(aCx);
+      return;
+    }
+
+    if (!JS_ExecuteScript(cx, script, aRval)) {
+      JS::RootedValue exn(cx);
+      if (aOptions.mReportExceptions) {
+        // Note that ReportException will consume the exception.
+        aes.ReportException();
+      } else {
+        // Set the exception on our caller's cx.
+        aRv.MightThrowJSException();
+        aRv.StealExceptionFromJSContext(cx);
+      }
+      return;
+    }
+  }
+
+  JS_WrapValue(aCx, aRval);
+}
+
+void PrecompiledScript::GetUrl(nsAString& aUrl) { CopyUTF8toUTF16(mURL, aUrl); }
+
+bool PrecompiledScript::HasReturnValue() { return mHasReturnValue; }
+
+JSObject* PrecompiledScript::WrapObject(JSContext* aCx,
+                                        HandleObject aGivenProto) {
+  return PrecompiledScript_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+bool PrecompiledScript::IsBlackForCC(bool aTracingNeeded) {
+  return (nsCCUncollectableMarker::sGeneration && HasKnownLiveWrapper() &&
+          (!aTracingNeeded || HasNothingToTrace(this)));
+}
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(PrecompiledScript, mParent)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PrecompiledScript)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_BEGIN(PrecompiledScript)
+  return tmp->IsBlackForCC(false);
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_END
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_BEGIN(PrecompiledScript)
+  return tmp->IsBlackForCC(true);
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_END
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_BEGIN(PrecompiledScript)
+  return tmp->IsBlackForCC(false);
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(PrecompiledScript)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(PrecompiledScript)
+
+}  // namespace dom
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/ComponentModuleLoader.cpp b/js/xpconnect/loader/ComponentModuleLoader.cpp
new file mode 100644
index 0000000000..9f293fdcc0
--- /dev/null
+++ b/js/xpconnect/loader/ComponentModuleLoader.cpp
@@ -0,0 +1,272 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ComponentModuleLoader.h"
+
+#include "nsISupportsImpl.h"
+
+#include "js/loader/ModuleLoadRequest.h"
+#include "js/RootingAPI.h"          // JS::Rooted
+#include "js/PropertyAndElement.h"  // JS_SetProperty
+#include "js/Value.h"               // JS::Value, JS::NumberValue
+#include "mozJSModuleLoader.h"
+
+using namespace JS::loader;
+
+namespace mozilla {
+namespace loader {
+
+//////////////////////////////////////////////////////////////
+// ComponentScriptLoader
+//////////////////////////////////////////////////////////////
+
+NS_IMPL_ISUPPORTS0(ComponentScriptLoader)
+
+nsIURI* ComponentScriptLoader::GetBaseURI() const { return nullptr; }
+
+void ComponentScriptLoader::ReportErrorToConsole(ScriptLoadRequest* aRequest,
+                                                 nsresult aResult) const {}
+
+void ComponentScriptLoader::ReportWarningToConsole(
+    ScriptLoadRequest* aRequest, const char* aMessageName,
+    const nsTArray<nsString>& aParams) const {}
+
+nsresult ComponentScriptLoader::FillCompileOptionsForRequest(
+    JSContext* cx, ScriptLoadRequest* aRequest, JS::CompileOptions* aOptions,
+    JS::MutableHandle<JSScript*> aIntroductionScript) {
+  return NS_OK;
+}
+
+//////////////////////////////////////////////////////////////
+// ComponentModuleLoader
+//////////////////////////////////////////////////////////////
+
+NS_IMPL_ADDREF_INHERITED(ComponentModuleLoader, JS::loader::ModuleLoaderBase)
+NS_IMPL_RELEASE_INHERITED(ComponentModuleLoader, JS::loader::ModuleLoaderBase)
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(ComponentModuleLoader,
+                                   JS::loader::ModuleLoaderBase, mLoadRequests)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ComponentModuleLoader)
+NS_INTERFACE_MAP_END_INHERITING(JS::loader::ModuleLoaderBase)
+
+ComponentModuleLoader::ComponentModuleLoader(
+    ComponentScriptLoader* aScriptLoader, nsIGlobalObject* aGlobalObject)
+    : ModuleLoaderBase(aScriptLoader, aGlobalObject, new SyncEventTarget()) {}
+
+ComponentModuleLoader::~ComponentModuleLoader() {
+  MOZ_ASSERT(mLoadRequests.isEmpty());
+}
+
+already_AddRefed<ModuleLoadRequest> ComponentModuleLoader::CreateStaticImport(
+    nsIURI* aURI, ModuleLoadRequest* aParent) {
+  RefPtr<ComponentLoadContext> context = new ComponentLoadContext();
+  RefPtr<ModuleLoadRequest> request = new ModuleLoadRequest(
+      aURI, aParent->mFetchOptions, dom::SRIMetadata(), aParent->mURI, context,
+      false, /* is top level */
+      false, /* is dynamic import */
+      this, aParent->mVisitedSet, aParent->GetRootModule());
+  return request.forget();
+}
+
+already_AddRefed<ModuleLoadRequest> ComponentModuleLoader::CreateDynamicImport(
+    JSContext* aCx, nsIURI* aURI, LoadedScript* aMaybeActiveScript,
+    JS::Handle<JS::Value> aReferencingPrivate, JS::Handle<JSString*> aSpecifier,
+    JS::Handle<JSObject*> aPromise) {
+  return nullptr;  // Not yet implemented.
+}
+
+bool ComponentModuleLoader::CanStartLoad(ModuleLoadRequest* aRequest,
+                                         nsresult* aRvOut) {
+  return mozJSModuleLoader::IsTrustedScheme(aRequest->mURI);
+}
+
+nsresult ComponentModuleLoader::StartFetch(ModuleLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->HasLoadContext());
+
+  aRequest->mBaseURL = aRequest->mURI;
+
+  // Loading script source and compilation are intertwined in
+  // mozJSModuleLoader. Perform both operations here but only report load
+  // failures. Compilation failure is reported in CompileFetchedModule.
+
+  dom::AutoJSAPI jsapi;
+  if (!jsapi.Init(GetGlobalObject())) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JSContext* cx = jsapi.cx();
+  JS::RootedScript script(cx);
+  nsresult rv =
+      mozJSModuleLoader::LoadSingleModuleScript(this, cx, aRequest, &script);
+  MOZ_ASSERT_IF(jsapi.HasException(), NS_FAILED(rv));
+  MOZ_ASSERT(bool(script) == NS_SUCCEEDED(rv));
+
+  // Check for failure to load script source and abort.
+  bool threwException = jsapi.HasException();
+  if (NS_FAILED(rv) && !threwException) {
+    nsAutoCString uri;
+    nsresult rv2 = aRequest->mURI->GetSpec(uri);
+    NS_ENSURE_SUCCESS(rv2, rv2);
+
+    JS_ReportErrorUTF8(cx, "Failed to load %s", PromiseFlatCString(uri).get());
+
+    // Remember the error for MaybeReportLoadError.
+    if (!mLoadException.initialized()) {
+      mLoadException.init(cx);
+    }
+    if (!jsapi.StealException(&mLoadException)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    if (mLoadException.isObject()) {
+      // Expose `nsresult`.
+      JS::Rooted<JS::Value> resultVal(cx, JS::NumberValue(uint32_t(rv)));
+      JS::Rooted<JSObject*> exceptionObj(cx, &mLoadException.toObject());
+      if (!JS_SetProperty(cx, exceptionObj, "result", resultVal)) {
+        // Ignore the error and keep reporting the exception without the result
+        // property.
+        JS_ClearPendingException(cx);
+      }
+    }
+
+    return rv;
+  }
+
+  // Otherwise remember the results in this context so we can report them later.
+  ComponentLoadContext* context = aRequest->GetComponentLoadContext();
+  context->mRv = rv;
+  if (threwException) {
+    context->mExceptionValue.init(cx);
+    if (!jsapi.StealException(&context->mExceptionValue)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+  }
+  if (script) {
+    context->mScript.init(cx);
+    context->mScript = script;
+  }
+
+  mLoadRequests.AppendElement(aRequest);
+
+  return NS_OK;
+}
+
+nsresult ComponentModuleLoader::CompileFetchedModule(
+    JSContext* aCx, JS::Handle<JSObject*> aGlobal, JS::CompileOptions& aOptions,
+    ModuleLoadRequest* aRequest, JS::MutableHandle<JSObject*> aModuleOut) {
+  // Compilation already happened in StartFetch. Report the result here.
+  ComponentLoadContext* context = aRequest->GetComponentLoadContext();
+  nsresult rv = context->mRv;
+  if (context->mScript) {
+    aModuleOut.set(JS::GetModuleObject(context->mScript));
+    context->mScript = nullptr;
+  }
+  if (NS_FAILED(rv)) {
+    JS_SetPendingException(aCx, context->mExceptionValue);
+    context->mExceptionValue = JS::UndefinedValue();
+  }
+
+  MOZ_ASSERT(JS_IsExceptionPending(aCx) == NS_FAILED(rv));
+  MOZ_ASSERT(bool(aModuleOut) == NS_SUCCEEDED(rv));
+
+  return rv;
+}
+
+void ComponentModuleLoader::MaybeReportLoadError(JSContext* aCx) {
+  if (JS_IsExceptionPending(aCx)) {
+    // Do not override.
+    return;
+  }
+
+  if (mLoadException.isUndefined()) {
+    return;
+  }
+
+  JS_SetPendingException(aCx, mLoadException);
+  mLoadException = JS::UndefinedValue();
+}
+
+void ComponentModuleLoader::OnModuleLoadComplete(ModuleLoadRequest* aRequest) {}
+
+nsresult ComponentModuleLoader::ProcessRequests() {
+  // Work list to drive module loader since this is all synchronous.
+  while (!mLoadRequests.isEmpty()) {
+    RefPtr<ScriptLoadRequest> request = mLoadRequests.StealFirst();
+    nsresult rv = OnFetchComplete(request->AsModuleRequest(), NS_OK);
+    if (NS_FAILED(rv)) {
+      mLoadRequests.CancelRequestsAndClear();
+      return rv;
+    }
+  }
+
+  return NS_OK;
+}
+
+//////////////////////////////////////////////////////////////
+// ComponentModuleLoader::SyncEventTarget
+//////////////////////////////////////////////////////////////
+
+NS_IMPL_ADDREF(ComponentModuleLoader::SyncEventTarget)
+NS_IMPL_RELEASE(ComponentModuleLoader::SyncEventTarget)
+
+NS_INTERFACE_MAP_BEGIN(ComponentModuleLoader::SyncEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsISerialEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsIEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::DispatchFromScript(
+    nsIRunnable* aRunnable, uint32_t aFlags) {
+  nsCOMPtr<nsIRunnable> event(aRunnable);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::Dispatch(
+    already_AddRefed<nsIRunnable> aRunnable, uint32_t aFlags) {
+  MOZ_ASSERT(IsOnCurrentThreadInfallible());
+
+  nsCOMPtr<nsIRunnable> runnable(aRunnable);
+  runnable->Run();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::DelayedDispatch(
+    already_AddRefed<nsIRunnable>, uint32_t) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::RegisterShutdownTask(
+    nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::UnregisterShutdownTask(
+    nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+ComponentModuleLoader::SyncEventTarget::IsOnCurrentThread(
+    bool* aIsOnCurrentThread) {
+  MOZ_ASSERT(aIsOnCurrentThread);
+  *aIsOnCurrentThread = IsOnCurrentThreadInfallible();
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+ComponentModuleLoader::SyncEventTarget::IsOnCurrentThreadInfallible() {
+  return NS_IsMainThread();
+}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/ComponentModuleLoader.h b/js/xpconnect/loader/ComponentModuleLoader.h
new file mode 100644
index 0000000000..7dd39dd342
--- /dev/null
+++ b/js/xpconnect/loader/ComponentModuleLoader.h
@@ -0,0 +1,119 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_loader_ComponentModuleLoader_h
+#define mozilla_loader_ComponentModuleLoader_h
+
+#include "js/loader/LoadContextBase.h"
+#include "js/loader/ModuleLoaderBase.h"
+
+#include "SkipCheckForBrokenURLOrZeroSized.h"
+
+class mozJSModuleLoader;
+
+namespace mozilla {
+namespace loader {
+
+class ComponentScriptLoader : public JS::loader::ScriptLoaderInterface {
+ public:
+  NS_DECL_ISUPPORTS
+
+ private:
+  ~ComponentScriptLoader() = default;
+
+  nsIURI* GetBaseURI() const override;
+
+  void ReportErrorToConsole(ScriptLoadRequest* aRequest,
+                            nsresult aResult) const override;
+
+  void ReportWarningToConsole(ScriptLoadRequest* aRequest,
+                              const char* aMessageName,
+                              const nsTArray<nsString>& aParams) const override;
+
+  nsresult FillCompileOptionsForRequest(
+      JSContext* cx, ScriptLoadRequest* aRequest, JS::CompileOptions* aOptions,
+      JS::MutableHandle<JSScript*> aIntroductionScript) override;
+};
+
+class ComponentModuleLoader : public JS::loader::ModuleLoaderBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ComponentModuleLoader,
+                                           JS::loader::ModuleLoaderBase)
+
+  ComponentModuleLoader(ComponentScriptLoader* aScriptLoader,
+                        nsIGlobalObject* aGlobalObject);
+
+  [[nodiscard]] nsresult ProcessRequests();
+
+  void MaybeReportLoadError(JSContext* aCx);
+
+ private:
+  // An event target that dispatches runnables by executing them
+  // immediately. This is used to drive mozPromise dispatch for
+  // ComponentModuleLoader.
+  class SyncEventTarget : public nsISerialEventTarget {
+   public:
+    NS_DECL_THREADSAFE_ISUPPORTS
+    NS_DECL_NSIEVENTTARGET_FULL
+   private:
+    virtual ~SyncEventTarget() = default;
+  };
+
+  ~ComponentModuleLoader();
+
+  already_AddRefed<ModuleLoadRequest> CreateStaticImport(
+      nsIURI* aURI, ModuleLoadRequest* aParent) override;
+
+  already_AddRefed<ModuleLoadRequest> CreateDynamicImport(
+      JSContext* aCx, nsIURI* aURI, LoadedScript* aMaybeActiveScript,
+      JS::Handle<JS::Value> aReferencingPrivate,
+      JS::Handle<JSString*> aSpecifier,
+      JS::Handle<JSObject*> aPromise) override;
+
+  bool CanStartLoad(ModuleLoadRequest* aRequest, nsresult* aRvOut) override;
+
+  nsresult StartFetch(ModuleLoadRequest* aRequest) override;
+
+  nsresult CompileFetchedModule(
+      JSContext* aCx, JS::Handle<JSObject*> aGlobal,
+      JS::CompileOptions& aOptions, ModuleLoadRequest* aRequest,
+      JS::MutableHandle<JSObject*> aModuleScript) override;
+
+  void OnModuleLoadComplete(ModuleLoadRequest* aRequest) override;
+
+  JS::loader::ScriptLoadRequestList mLoadRequests;
+
+  // If any of module scripts failed to load, exception is set here until it's
+  // reported by MaybeReportLoadError.
+  JS::PersistentRooted<JS::Value> mLoadException;
+};
+
+// Data specific to ComponentModuleLoader that is associated with each load
+// request.
+class ComponentLoadContext : public JS::loader::LoadContextBase {
+ public:
+  ComponentLoadContext()
+      : LoadContextBase(JS::loader::ContextKind::Component) {}
+
+ public:
+  // The result of compiling a module script. These fields are used temporarily
+  // before being passed to the module loader.
+  nsresult mRv;
+
+  SkipCheckForBrokenURLOrZeroSized mSkipCheck;
+
+  // The exception thrown during compiling a module script. These fields are
+  // used temporarily before being passed to the module loader.
+  JS::PersistentRooted<JS::Value> mExceptionValue;
+
+  JS::PersistentRooted<JSScript*> mScript;
+};
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // mozilla_loader_ComponentModuleLoader_h
diff --git a/js/xpconnect/loader/ComponentUtils.sys.mjs b/js/xpconnect/loader/ComponentUtils.sys.mjs
new file mode 100644
index 0000000000..cfffb96ef7
--- /dev/null
+++ b/js/xpconnect/loader/ComponentUtils.sys.mjs
@@ -0,0 +1,33 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+ * vim: sw=2 ts=2 sts=2 et filetype=javascript
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Deprecated utilities for JavaScript components loaded by the JS component
+ * loader.
+ */
+
+const nsIFactoryQI = ChromeUtils.generateQI(["nsIFactory"]);
+
+export var ComponentUtils = {
+  /**
+   * Generates a singleton nsIFactory implementation that can be used as
+   * an argument to nsIComponentRegistrar.registerFactory.
+   * @param aServiceConstructor
+   *        Constructor function of the component.
+   */
+  generateSingletonFactory(aServiceConstructor) {
+    return {
+      _instance: null,
+      createInstance(aIID) {
+        if (this._instance === null) {
+          this._instance = new aServiceConstructor();
+        }
+        return this._instance.QueryInterface(aIID);
+      },
+      QueryInterface: nsIFactoryQI,
+    };
+  },
+};
diff --git a/js/xpconnect/loader/IOBuffers.h b/js/xpconnect/loader/IOBuffers.h
new file mode 100644
index 0000000000..e26b0c3bca
--- /dev/null
+++ b/js/xpconnect/loader/IOBuffers.h
@@ -0,0 +1,148 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef IOBuffers_h
+#define IOBuffers_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/EndianUtils.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/Range.h"
+#include "mozilla/Span.h"
+#include "nsString.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace loader {
+
+class OutputBuffer {
+ public:
+  OutputBuffer() {}
+
+  uint8_t* write(size_t size) {
+    auto buf = data.AppendElements(size);
+    cursor_ += size;
+    return buf;
+  }
+
+  void codeUint8(const uint8_t& val) { *write(sizeof val) = val; }
+
+  template <typename T>
+  void codeUint8(const EnumSet<T>& val) {
+    // EnumSets are always represented as uint32_t values, so we need to
+    // assert that the value actually fits in a uint8 before writing it.
+    uint32_t value = val.serialize();
+    codeUint8(CheckedUint8(value).value());
+  }
+
+  void codeUint16(const uint16_t& val) {
+    LittleEndian::writeUint16(write(sizeof val), val);
+  }
+
+  void codeUint32(const uint32_t& val) {
+    LittleEndian::writeUint32(write(sizeof val), val);
+  }
+
+  void codeString(const nsCString& str) {
+    auto len = CheckedUint16(str.Length()).value();
+
+    codeUint16(len);
+    memcpy(write(len), str.BeginReading(), len);
+  }
+
+  size_t cursor() const { return cursor_; }
+
+  uint8_t* Get() { return data.Elements(); }
+
+  const uint8_t* Get() const { return data.Elements(); }
+
+ private:
+  nsTArray<uint8_t> data;
+  size_t cursor_ = 0;
+};
+
+class InputBuffer {
+ public:
+  explicit InputBuffer(const Range<uint8_t>& buffer) : data(buffer) {}
+
+  const uint8_t* read(size_t size) {
+    MOZ_ASSERT(checkCapacity(size));
+
+    auto buf = &data[cursor_];
+    cursor_ += size;
+    return buf;
+  }
+
+  bool codeUint8(uint8_t& val) {
+    if (checkCapacity(sizeof val)) {
+      val = *read(sizeof val);
+    }
+    return !error_;
+  }
+
+  template <typename T>
+  bool codeUint8(EnumSet<T>& val) {
+    uint8_t value;
+    if (codeUint8(value)) {
+      val.deserialize(value);
+    }
+    return !error_;
+  }
+
+  bool codeUint16(uint16_t& val) {
+    if (checkCapacity(sizeof val)) {
+      val = LittleEndian::readUint16(read(sizeof val));
+    }
+    return !error_;
+  }
+
+  bool codeUint32(uint32_t& val) {
+    if (checkCapacity(sizeof val)) {
+      val = LittleEndian::readUint32(read(sizeof val));
+    }
+    return !error_;
+  }
+
+  bool codeString(nsCString& str) {
+    uint16_t len;
+    if (codeUint16(len)) {
+      if (checkCapacity(len)) {
+        str.SetLength(len);
+        memcpy(str.BeginWriting(), read(len), len);
+      }
+    }
+    return !error_;
+  }
+
+  bool error() { return error_; }
+
+  bool finished() { return error_ || !remainingCapacity(); }
+
+  size_t remainingCapacity() { return data.length() - cursor_; }
+
+  size_t cursor() const { return cursor_; }
+
+  const uint8_t* Get() const { return data.begin().get(); }
+
+ private:
+  bool checkCapacity(size_t size) {
+    if (size > remainingCapacity()) {
+      error_ = true;
+    }
+    return !error_;
+  }
+
+  bool error_ = false;
+
+ public:
+  const Range<uint8_t>& data;
+  size_t cursor_ = 0;
+};
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // IOBuffers_h
diff --git a/js/xpconnect/loader/JSMEnvironmentProxy.cpp b/js/xpconnect/loader/JSMEnvironmentProxy.cpp
new file mode 100644
index 0000000000..6c4a532d44
--- /dev/null
+++ b/js/xpconnect/loader/JSMEnvironmentProxy.cpp
@@ -0,0 +1,260 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "JSMEnvironmentProxy.h"
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+#include "mozilla/Maybe.h"       // mozilla::Maybe
+
+#include <stddef.h>  // size_t
+
+#include "jsapi.h"  // JS_HasExtensibleLexicalEnvironment, JS_ExtensibleLexicalEnvironment
+#include "js/Class.h"               // JS::ObjectOpResult
+#include "js/ErrorReport.h"         // JS_ReportOutOfMemory
+#include "js/GCVector.h"            // JS::RootedVector
+#include "js/Id.h"                  // JS::PropertyKey
+#include "js/PropertyAndElement.h"  // JS::IdVector, JS_HasPropertyById, JS_HasOwnPropertyById, JS_GetPropertyById, JS_Enumerate
+#include "js/PropertyDescriptor.h"  // JS::PropertyDescriptor, JS_GetOwnPropertyDescriptorById
+#include "js/PropertyDescriptor.h"  // JS::PropertyDescriptor, JS_GetOwnPropertyDescriptorById
+#include "js/Proxy.h"  // js::ProxyOptions, js::NewProxyObject, js::GetProxyPrivate
+#include "js/RootingAPI.h"  // JS::Rooted, JS::Handle, JS::MutableHandle
+#include "js/TypeDecls.h"   // JSContext, JSObject, JS::MutableHandleVector
+#include "js/Value.h"  // JS::Value, JS::UndefinedValue, JS_UNINITIALIZED_LEXICAL
+#include "js/friend/ErrorMessages.h"  // JSMSG_*
+
+namespace mozilla {
+namespace loader {
+
+struct JSMEnvironmentProxyHandler : public js::BaseProxyHandler {
+  JSMEnvironmentProxyHandler() : BaseProxyHandler(&gFamily, false) {}
+
+  bool defineProperty(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                      JS::Handle<JS::PropertyKey> aId,
+                      JS::Handle<JS::PropertyDescriptor> aDesc,
+                      JS::ObjectOpResult& aResult) const override {
+    return aResult.fail(JSMSG_CANT_DEFINE_PROP_OBJECT_NOT_EXTENSIBLE);
+  }
+
+  bool getPrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    JS::MutableHandle<JSObject*> aProtop) const override {
+    aProtop.set(nullptr);
+    return true;
+  }
+
+  bool setPrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    JS::Handle<JSObject*> aProto,
+                    JS::ObjectOpResult& aResult) const override {
+    if (!aProto) {
+      return aResult.succeed();
+    }
+    return aResult.failCantSetProto();
+  }
+
+  bool getPrototypeIfOrdinary(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy, bool* aIsOrdinary,
+      JS::MutableHandle<JSObject*> aProtop) const override {
+    *aIsOrdinary = false;
+    return true;
+  }
+
+  bool setImmutablePrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                             bool* aSucceeded) const override {
+    *aSucceeded = true;
+    return true;
+  }
+
+  bool preventExtensions(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                         JS::ObjectOpResult& aResult) const override {
+    aResult.succeed();
+    return true;
+  }
+
+  bool isExtensible(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    bool* aExtensible) const override {
+    *aExtensible = false;
+    return true;
+  }
+
+  bool set(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, JS::Handle<JS::Value> aValue,
+           JS::Handle<JS::Value> aReceiver,
+           JS::ObjectOpResult& aResult) const override {
+    return aResult.failReadOnly();
+  }
+
+  bool delete_(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+               JS::Handle<JS::PropertyKey> aId,
+               JS::ObjectOpResult& aResult) const override {
+    return aResult.failCantDelete();
+  }
+
+  bool getOwnPropertyDescriptor(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::Handle<JS::PropertyKey> aId,
+      JS::MutableHandle<mozilla::Maybe<JS::PropertyDescriptor>> aDesc)
+      const override;
+  bool has(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, bool* aBp) const override;
+  bool get(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::Value> aReceiver, JS::Handle<JS::PropertyKey> aId,
+           JS::MutableHandle<JS::Value> aVp) const override;
+  bool ownPropertyKeys(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::MutableHandleVector<JS::PropertyKey> aProps) const override;
+
+ private:
+  static JSObject* getGlobal(JSContext* aCx, JS::Handle<JSObject*> aProxy) {
+    JS::Rooted<JSObject*> globalObj(aCx,
+                                    &js::GetProxyPrivate(aProxy).toObject());
+    return globalObj;
+  }
+
+ public:
+  static const char gFamily;
+  static const JSMEnvironmentProxyHandler gHandler;
+};
+
+const JSMEnvironmentProxyHandler JSMEnvironmentProxyHandler::gHandler;
+const char JSMEnvironmentProxyHandler::gFamily = 0;
+
+JSObject* ResolveModuleObjectPropertyById(JSContext* aCx,
+                                          JS::Handle<JSObject*> aModObj,
+                                          JS::Handle<JS::PropertyKey> aId) {
+  if (JS_HasExtensibleLexicalEnvironment(aModObj)) {
+    JS::Rooted<JSObject*> lexical(aCx,
+                                  JS_ExtensibleLexicalEnvironment(aModObj));
+    bool found;
+    if (!JS_HasOwnPropertyById(aCx, lexical, aId, &found)) {
+      return nullptr;
+    }
+    if (found) {
+      return lexical;
+    }
+  }
+  return aModObj;
+}
+
+JSObject* ResolveModuleObjectProperty(JSContext* aCx,
+                                      JS::Handle<JSObject*> aModObj,
+                                      const char* aName) {
+  if (JS_HasExtensibleLexicalEnvironment(aModObj)) {
+    JS::RootedObject lexical(aCx, JS_ExtensibleLexicalEnvironment(aModObj));
+    bool found;
+    if (!JS_HasOwnProperty(aCx, lexical, aName, &found)) {
+      return nullptr;
+    }
+    if (found) {
+      return lexical;
+    }
+  }
+  return aModObj;
+}
+
+bool JSMEnvironmentProxyHandler::getOwnPropertyDescriptor(
+    JSContext* aCx, JS::Handle<JSObject*> aProxy,
+    JS::Handle<JS::PropertyKey> aId,
+    JS::MutableHandle<mozilla::Maybe<JS::PropertyDescriptor>> aDesc) const {
+  JS::Rooted<JSObject*> globalObj(aCx, getGlobal(aCx, aProxy));
+  JS::Rooted<JSObject*> holder(
+      aCx, ResolveModuleObjectPropertyById(aCx, globalObj, aId));
+  if (!JS_GetOwnPropertyDescriptorById(aCx, holder, aId, aDesc)) {
+    return false;
+  }
+
+  if (aDesc.get().isNothing()) {
+    return true;
+  }
+
+  JS::PropertyDescriptor& desc = *aDesc.get();
+
+  if (desc.hasValue()) {
+    if (desc.value().isMagic(JS_UNINITIALIZED_LEXICAL)) {
+      desc.setValue(JS::UndefinedValue());
+    }
+  }
+
+  desc.setConfigurable(false);
+  desc.setEnumerable(true);
+  if (!desc.isAccessorDescriptor()) {
+    desc.setWritable(false);
+  }
+
+  return true;
+}
+
+bool JSMEnvironmentProxyHandler::has(JSContext* aCx,
+                                     JS::Handle<JSObject*> aProxy,
+                                     JS::Handle<JS::PropertyKey> aId,
+                                     bool* aBp) const {
+  JS::Rooted<JSObject*> globalObj(aCx, getGlobal(aCx, aProxy));
+  JS::Rooted<JSObject*> holder(
+      aCx, ResolveModuleObjectPropertyById(aCx, globalObj, aId));
+  return JS_HasPropertyById(aCx, holder, aId, aBp);
+}
+
+bool JSMEnvironmentProxyHandler::get(JSContext* aCx,
+                                     JS::Handle<JSObject*> aProxy,
+                                     JS::Handle<JS::Value> aReceiver,
+                                     JS::Handle<JS::PropertyKey> aId,
+                                     JS::MutableHandle<JS::Value> aVp) const {
+  JS::Rooted<JSObject*> globalObj(aCx, getGlobal(aCx, aProxy));
+  JS::Rooted<JSObject*> holder(
+      aCx, ResolveModuleObjectPropertyById(aCx, globalObj, aId));
+  if (!JS_GetPropertyById(aCx, holder, aId, aVp)) {
+    return false;
+  }
+
+  if (aVp.isMagic(JS_UNINITIALIZED_LEXICAL)) {
+    aVp.setUndefined();
+  }
+
+  return true;
+}
+
+bool JSMEnvironmentProxyHandler::ownPropertyKeys(
+    JSContext* aCx, JS::Handle<JSObject*> aProxy,
+    JS::MutableHandleVector<JS::PropertyKey> aProps) const {
+  JS::Rooted<JSObject*> globalObj(aCx, getGlobal(aCx, aProxy));
+  JS::Rooted<JS::IdVector> globalIds(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, globalObj, &globalIds)) {
+    return false;
+  }
+
+  for (size_t i = 0; i < globalIds.length(); i++) {
+    if (!aProps.append(globalIds[i])) {
+      JS_ReportOutOfMemory(aCx);
+      return false;
+    }
+  }
+
+  JS::RootedObject lexicalEnv(aCx, JS_ExtensibleLexicalEnvironment(globalObj));
+  JS::Rooted<JS::IdVector> lexicalIds(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, lexicalEnv, &lexicalIds)) {
+    return false;
+  }
+
+  for (size_t i = 0; i < lexicalIds.length(); i++) {
+    if (!aProps.append(lexicalIds[i])) {
+      JS_ReportOutOfMemory(aCx);
+      return false;
+    }
+  }
+
+  return true;
+}
+
+JSObject* CreateJSMEnvironmentProxy(JSContext* aCx,
+                                    JS::Handle<JSObject*> aGlobalObj) {
+  js::ProxyOptions options;
+  options.setLazyProto(true);
+
+  JS::Rooted<JS::Value> globalVal(aCx, JS::ObjectValue(*aGlobalObj));
+  return NewProxyObject(aCx, &JSMEnvironmentProxyHandler::gHandler, globalVal,
+                        nullptr, options);
+}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/JSMEnvironmentProxy.h b/js/xpconnect/loader/JSMEnvironmentProxy.h
new file mode 100644
index 0000000000..f45d7e3801
--- /dev/null
+++ b/js/xpconnect/loader/JSMEnvironmentProxy.h
@@ -0,0 +1,31 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_loader_JSMEnvironmentProxy_h
+#define mozilla_loader_JSMEnvironmentProxy_h
+
+#include "js/Id.h"          // JS::PropertyKey
+#include "js/TypeDecls.h"   // JSContext, JSObject
+#include "js/RootingAPI.h"  // JS::Handle
+
+namespace mozilla {
+namespace loader {
+
+JSObject* ResolveModuleObjectPropertyById(JSContext* aCx,
+                                          JS::Handle<JSObject*> aModObj,
+                                          JS::Handle<JS::PropertyKey> aId);
+
+JSObject* ResolveModuleObjectProperty(JSContext* aCx,
+                                      JS::Handle<JSObject*> aModObj,
+                                      const char* aName);
+
+JSObject* CreateJSMEnvironmentProxy(JSContext* aCx,
+                                    JS::Handle<JSObject*> aGlobalObj);
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // mozilla_loader_JSMEnvironmentProxy_h
diff --git a/js/xpconnect/loader/ModuleEnvironmentProxy.cpp b/js/xpconnect/loader/ModuleEnvironmentProxy.cpp
new file mode 100644
index 0000000000..d513bc6194
--- /dev/null
+++ b/js/xpconnect/loader/ModuleEnvironmentProxy.cpp
@@ -0,0 +1,238 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ModuleEnvironmentProxy.h"
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+#include "mozilla/Maybe.h"       // mozilla::Maybe
+
+#include <stddef.h>  // size_t
+
+#include "js/Class.h"        // JS::ObjectOpResult
+#include "js/ErrorReport.h"  // JS_ReportOutOfMemory
+#include "js/GCVector.h"     // JS::RootedVector
+#include "js/Id.h"           // JS::PropertyKey
+#include "js/PropertyAndElement.h"  // JS::IdVector, JS_HasPropertyById, JS_GetPropertyById, JS_Enumerate
+#include "js/PropertyDescriptor.h"  // JS::PropertyDescriptor, JS_GetOwnPropertyDescriptorById
+#include "js/PropertyDescriptor.h"  // JS::PropertyDescriptor, JS_GetOwnPropertyDescriptorById
+#include "js/Proxy.h"  // js::ProxyOptions, js::NewProxyObject, js::GetProxyPrivate
+#include "js/RootingAPI.h"  // JS::Rooted, JS::Handle, JS::MutableHandle
+#include "js/TypeDecls.h"   // JSContext, JSObject, JS::MutableHandleVector
+#include "js/Value.h"       // JS::Value
+#include "js/friend/ErrorMessages.h"  // JSMSG_*
+#include "js/String.h"
+#include "js/Modules.h"
+
+namespace mozilla {
+namespace loader {
+
+struct ModuleEnvironmentProxyHandler : public js::BaseProxyHandler {
+  ModuleEnvironmentProxyHandler() : js::BaseProxyHandler(&gFamily, false) {}
+
+  bool defineProperty(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                      JS::Handle<JS::PropertyKey> aId,
+                      JS::Handle<JS::PropertyDescriptor> aDesc,
+                      JS::ObjectOpResult& aResult) const override {
+    return aResult.fail(JSMSG_CANT_DEFINE_PROP_OBJECT_NOT_EXTENSIBLE);
+  }
+
+  bool getPrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    JS::MutableHandle<JSObject*> aProtop) const override {
+    aProtop.set(nullptr);
+    return true;
+  }
+
+  bool setPrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    JS::Handle<JSObject*> aProto,
+                    JS::ObjectOpResult& aResult) const override {
+    if (!aProto) {
+      return aResult.succeed();
+    }
+    return aResult.failCantSetProto();
+  }
+
+  bool getPrototypeIfOrdinary(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy, bool* aIsOrdinary,
+      JS::MutableHandle<JSObject*> aProtop) const override {
+    *aIsOrdinary = false;
+    return true;
+  }
+
+  bool setImmutablePrototype(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                             bool* aSucceeded) const override {
+    *aSucceeded = true;
+    return true;
+  }
+
+  bool preventExtensions(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                         JS::ObjectOpResult& aResult) const override {
+    aResult.succeed();
+    return true;
+  }
+
+  bool isExtensible(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                    bool* aExtensible) const override {
+    *aExtensible = false;
+    return true;
+  }
+
+  bool set(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, JS::Handle<JS::Value> aValue,
+           JS::Handle<JS::Value> aReceiver,
+           JS::ObjectOpResult& aResult) const override {
+    return aResult.failReadOnly();
+  }
+
+  bool delete_(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+               JS::Handle<JS::PropertyKey> aId,
+               JS::ObjectOpResult& aResult) const override {
+    return aResult.failCantDelete();
+  }
+
+  bool getOwnPropertyDescriptor(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::Handle<JS::PropertyKey> aId,
+      JS::MutableHandle<mozilla::Maybe<JS::PropertyDescriptor>> aDesc)
+      const override;
+  bool has(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, bool* aBp) const override;
+  bool get(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::Value> receiver, JS::Handle<JS::PropertyKey> aId,
+           JS::MutableHandle<JS::Value> aVp) const override;
+  bool ownPropertyKeys(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::MutableHandleVector<JS::PropertyKey> aProps) const override;
+
+ private:
+  static JSObject* getEnvironment(JS::Handle<JSObject*> aProxy) {
+    return &js::GetProxyPrivate(aProxy).toObject();
+  }
+
+  static bool equalsNamespace(JSContext* aCx, JS::Handle<JS::PropertyKey> aId,
+                              bool* aMatch) {
+    if (!aId.isString()) {
+      *aMatch = false;
+      return true;
+    }
+    return JS_StringEqualsLiteral(aCx, aId.toString(), "*namespace*", aMatch);
+  }
+
+ public:
+  static const char gFamily;
+  static const ModuleEnvironmentProxyHandler gHandler;
+};
+
+const ModuleEnvironmentProxyHandler ModuleEnvironmentProxyHandler::gHandler;
+const char ModuleEnvironmentProxyHandler::gFamily = 0;
+
+bool ModuleEnvironmentProxyHandler::getOwnPropertyDescriptor(
+    JSContext* aCx, JS::Handle<JSObject*> aProxy,
+    JS::Handle<JS::PropertyKey> aId,
+    JS::MutableHandle<mozilla::Maybe<JS::PropertyDescriptor>> aDesc) const {
+  bool isNamespace;
+  if (!equalsNamespace(aCx, aId, &isNamespace)) {
+    return false;
+  }
+  if (isNamespace) {
+    aDesc.reset();
+    return true;
+  }
+
+  JS::Rooted<JSObject*> envObj(aCx, getEnvironment(aProxy));
+  if (!JS_GetOwnPropertyDescriptorById(aCx, envObj, aId, aDesc)) {
+    return false;
+  }
+
+  if (aDesc.get().isNothing()) {
+    return true;
+  }
+
+  JS::PropertyDescriptor& desc = *aDesc.get();
+
+  desc.setConfigurable(false);
+  desc.setWritable(false);
+  desc.setEnumerable(true);
+
+  return true;
+}
+
+bool ModuleEnvironmentProxyHandler::has(JSContext* aCx,
+                                        JS::Handle<JSObject*> aProxy,
+                                        JS::Handle<JS::PropertyKey> aId,
+                                        bool* aBp) const {
+  bool isNamespace;
+  if (!equalsNamespace(aCx, aId, &isNamespace)) {
+    return false;
+  }
+  if (isNamespace) {
+    *aBp = false;
+    return true;
+  }
+
+  JS::Rooted<JSObject*> envObj(aCx, getEnvironment(aProxy));
+  return JS_HasOwnPropertyById(aCx, envObj, aId, aBp);
+}
+
+bool ModuleEnvironmentProxyHandler::get(
+    JSContext* aCx, JS::Handle<JSObject*> aProxy,
+    JS::Handle<JS::Value> aReceiver, JS::Handle<JS::PropertyKey> aId,
+    JS::MutableHandle<JS::Value> aVp) const {
+  bool isNamespace;
+  if (!equalsNamespace(aCx, aId, &isNamespace)) {
+    return false;
+  }
+  if (isNamespace) {
+    aVp.setUndefined();
+    return true;
+  }
+
+  JS::Rooted<JSObject*> envObj(aCx, getEnvironment(aProxy));
+  return JS_GetPropertyById(aCx, envObj, aId, aVp);
+}
+
+bool ModuleEnvironmentProxyHandler::ownPropertyKeys(
+    JSContext* aCx, JS::Handle<JSObject*> aProxy,
+    JS::MutableHandleVector<JS::PropertyKey> aProps) const {
+  JS::Rooted<JSObject*> envObj(aCx, getEnvironment(aProxy));
+  JS::Rooted<JS::IdVector> ids(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, envObj, &ids)) {
+    return false;
+  }
+
+  for (size_t i = 0; i < ids.length(); i++) {
+    bool isNamespace;
+    if (!equalsNamespace(aCx, ids[i], &isNamespace)) {
+      return false;
+    }
+    if (isNamespace) {
+      continue;
+    }
+    if (!aProps.append(ids[i])) {
+      JS_ReportOutOfMemory(aCx);
+      return false;
+    }
+  }
+
+  return true;
+}
+
+JSObject* CreateModuleEnvironmentProxy(JSContext* aCx,
+                                       JS::Handle<JSObject*> aModuleObj) {
+  js::ProxyOptions options;
+  options.setLazyProto(true);
+
+  JS::Rooted<JSObject*> envObj(aCx, JS::GetModuleEnvironment(aCx, aModuleObj));
+  if (!envObj) {
+    return nullptr;
+  }
+
+  JS::Rooted<JS::Value> envVal(aCx, JS::ObjectValue(*envObj));
+  return NewProxyObject(aCx, &ModuleEnvironmentProxyHandler::gHandler, envVal,
+                        nullptr, options);
+}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/ModuleEnvironmentProxy.h b/js/xpconnect/loader/ModuleEnvironmentProxy.h
new file mode 100644
index 0000000000..d59f6de5b3
--- /dev/null
+++ b/js/xpconnect/loader/ModuleEnvironmentProxy.h
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_loader_ModuleEnvironmentProxy_h
+#define mozilla_loader_ModuleEnvironmentProxy_h
+
+#include "js/TypeDecls.h"   // JSContext, JSObject
+#include "js/RootingAPI.h"  // JS::Handle
+
+namespace mozilla {
+namespace loader {
+
+// Create an object that works in the same way as global object returned by
+// `Cu.import`. This proxy exposes all global variables, including lexical
+// variables.
+//
+// This is a temporary workaround to support not-in-tree code that depends on
+// `Cu.import` return value.
+//
+// This will eventually be removed once ESM-ification finishes.
+JSObject* CreateModuleEnvironmentProxy(JSContext* aCx,
+                                       JS::Handle<JSObject*> aModuleObj);
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // mozilla_loader_ModuleEnvironmentProxy_h
diff --git a/js/xpconnect/loader/PScriptCache.ipdl b/js/xpconnect/loader/PScriptCache.ipdl
new file mode 100644
index 0000000000..e22166c16d
--- /dev/null
+++ b/js/xpconnect/loader/PScriptCache.ipdl
@@ -0,0 +1,36 @@
+/* -*- Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 8 -*- */
+/* vim: set sw=4 ts=8 et tw=80 ft=cpp : */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PContent;
+
+include "mozilla/loader/ScriptCacheActors.h";
+
+using class mozilla::TimeStamp from "mozilla/TimeStamp.h";
+using struct mozilla::void_t from "mozilla/ipc/IPCCore.h";
+
+namespace mozilla {
+namespace loader {
+
+struct ScriptData {
+    nsCString url;
+    nsCString cachePath;
+    TimeStamp loadTime;
+    // This will be an empty array if script data is present in the previous
+    // session's cache.
+    uint8_t[] xdrData;
+};
+
+[ManualDealloc, ChildImpl="ScriptCacheChild", ParentImpl="ScriptCacheParent"]
+protocol PScriptCache
+{
+    manager PContent;
+
+parent:
+    async __delete__(ScriptData[] scripts);
+};
+
+} // namespace loader
+} // namespace mozilla
diff --git a/js/xpconnect/loader/PrecompiledScript.h b/js/xpconnect/loader/PrecompiledScript.h
new file mode 100644
index 0000000000..2b49c05373
--- /dev/null
+++ b/js/xpconnect/loader/PrecompiledScript.h
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_PrecompiledScript_h
+#define mozilla_dom_PrecompiledScript_h
+
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/PrecompiledScriptBinding.h"
+#include "mozilla/RefPtr.h"
+
+#include "js/experimental/JSStencil.h"
+#include "js/TypeDecls.h"
+
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+
+namespace JS {
+class ReadOnlyCompileOptions;
+}
+
+namespace mozilla {
+namespace dom {
+class PrecompiledScript : public nsISupports, public nsWrapperCache {
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_WRAPPERCACHE_CLASS(PrecompiledScript)
+
+  explicit PrecompiledScript(nsISupports* aParent, RefPtr<JS::Stencil> aStencil,
+                             JS::ReadOnlyCompileOptions& aOptions);
+
+  void ExecuteInGlobal(JSContext* aCx, JS::Handle<JSObject*> aGlobal,
+                       const ExecuteInGlobalOptions& aOptions,
+                       JS::MutableHandle<JS::Value> aRval, ErrorResult& aRv);
+
+  void GetUrl(nsAString& aUrl);
+
+  bool HasReturnValue();
+
+  nsISupports* GetParentObject() const { return mParent; }
+
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override;
+
+ protected:
+  virtual ~PrecompiledScript() = default;
+
+ private:
+  bool IsBlackForCC(bool aTracingNeeded);
+
+  nsCOMPtr<nsISupports> mParent;
+
+  RefPtr<JS::Stencil> mStencil;
+  nsCString mURL;
+  const bool mHasReturnValue;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_PrecompiledScript_h
diff --git a/js/xpconnect/loader/ScriptCacheActors.cpp b/js/xpconnect/loader/ScriptCacheActors.cpp
new file mode 100644
index 0000000000..9b44f0ffe6
--- /dev/null
+++ b/js/xpconnect/loader/ScriptCacheActors.cpp
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/ScriptPreloader.h"
+#include "ScriptPreloader-inl.h"
+#include "mozilla/loader/ScriptCacheActors.h"
+
+#include "mozilla/dom/ContentParent.h"
+
+namespace mozilla {
+namespace loader {
+
+void ScriptCacheChild::Init(const Maybe<FileDescriptor>& cacheFile,
+                            bool wantCacheData) {
+  mWantCacheData = wantCacheData;
+
+  auto& cache = ScriptPreloader::GetChildSingleton();
+  Unused << cache.InitCache(cacheFile, this);
+
+  if (!wantCacheData) {
+    // If the parent process isn't expecting any cache data from us, we're
+    // done.
+    Send__delete__(this, AutoTArray<ScriptData, 0>());
+  }
+}
+
+// Finalize the script cache for the content process, and send back data about
+// any scripts executed up to this point.
+void ScriptCacheChild::SendScriptsAndFinalize(
+    ScriptPreloader::ScriptHash& scripts) {
+  MOZ_ASSERT(mWantCacheData);
+
+  AutoSafeJSAPI jsapi;
+
+  auto matcher = ScriptPreloader::Match<ScriptPreloader::ScriptStatus::Saved>();
+
+  nsTArray<ScriptData> dataArray;
+  for (auto& script : IterHash(scripts, matcher)) {
+    if (!script->mSize && !script->XDREncode(jsapi.cx())) {
+      continue;
+    }
+
+    auto data = dataArray.AppendElement();
+
+    data->url() = script->mURL;
+    data->cachePath() = script->mCachePath;
+    data->loadTime() = script->mLoadTime;
+
+    if (script->HasBuffer()) {
+      auto& xdrData = script->Buffer();
+      data->xdrData().AppendElements(xdrData.begin(), xdrData.length());
+      script->FreeData();
+    }
+  }
+
+  Send__delete__(this, dataArray);
+}
+
+void ScriptCacheChild::ActorDestroy(ActorDestroyReason aWhy) {
+  auto& cache = ScriptPreloader::GetChildSingleton();
+  cache.mChildActor = nullptr;
+}
+
+IPCResult ScriptCacheParent::Recv__delete__(nsTArray<ScriptData>&& scripts) {
+  if (!mWantCacheData && scripts.Length()) {
+    return IPC_FAIL(this, "UnexpectedScriptData");
+  }
+
+  // We don't want any more data from the process at this point.
+  mWantCacheData = false;
+
+  // Merge the child's script data with the parent's.
+  auto parent = static_cast<dom::ContentParent*>(Manager());
+  auto processType =
+      ScriptPreloader::GetChildProcessType(parent->GetRemoteType());
+
+  auto& cache = ScriptPreloader::GetChildSingleton();
+  for (auto& script : scripts) {
+    cache.NoteStencil(script.url(), script.cachePath(), processType,
+                      std::move(script.xdrData()), script.loadTime());
+  }
+
+  return IPC_OK();
+}
+
+void ScriptCacheParent::ActorDestroy(ActorDestroyReason aWhy) {}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/ScriptCacheActors.h b/js/xpconnect/loader/ScriptCacheActors.h
new file mode 100644
index 0000000000..92148464ea
--- /dev/null
+++ b/js/xpconnect/loader/ScriptCacheActors.h
@@ -0,0 +1,59 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ScriptCache_h
+#define ScriptCache_h
+
+#include "mozilla/ScriptPreloader.h"
+#include "mozilla/loader/PScriptCacheChild.h"
+#include "mozilla/loader/PScriptCacheParent.h"
+
+namespace mozilla {
+namespace ipc {
+class FileDescriptor;
+}
+
+namespace loader {
+
+using mozilla::ipc::FileDescriptor;
+using mozilla::ipc::IPCResult;
+
+class ScriptCacheParent final : public PScriptCacheParent {
+  friend class PScriptCacheParent;
+
+ public:
+  explicit ScriptCacheParent(bool wantCacheData)
+      : mWantCacheData(wantCacheData) {}
+
+ protected:
+  IPCResult Recv__delete__(nsTArray<ScriptData>&& scripts);
+
+  virtual void ActorDestroy(ActorDestroyReason aWhy) override;
+
+ private:
+  bool mWantCacheData;
+};
+
+class ScriptCacheChild final : public PScriptCacheChild {
+  friend class mozilla::ScriptPreloader;
+
+ public:
+  ScriptCacheChild() = default;
+
+  void Init(const Maybe<FileDescriptor>& cacheFile, bool wantCacheData);
+
+ protected:
+  virtual void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  void SendScriptsAndFinalize(ScriptPreloader::ScriptHash& scripts);
+
+ private:
+  bool mWantCacheData = false;
+};
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // ScriptCache_h
diff --git a/js/xpconnect/loader/ScriptPreloader-inl.h b/js/xpconnect/loader/ScriptPreloader-inl.h
new file mode 100644
index 0000000000..5908600616
--- /dev/null
+++ b/js/xpconnect/loader/ScriptPreloader-inl.h
@@ -0,0 +1,167 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ScriptPreloader_inl_h
+#define ScriptPreloader_inl_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/EndianUtils.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/Range.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "nsString.h"
+#include "nsTArray.h"
+
+#include <prio.h>
+
+namespace mozilla {
+
+namespace loader {
+
+using mozilla::dom::AutoJSAPI;
+
+static inline Result<Ok, nsresult> Write(PRFileDesc* fd, const void* data,
+                                         int32_t len) {
+  if (PR_Write(fd, data, len) != len) {
+    return Err(NS_ERROR_FAILURE);
+  }
+  return Ok();
+}
+
+static inline Result<Ok, nsresult> WritePadding(PRFileDesc* fd,
+                                                uint8_t padding) {
+  static const char paddingBytes[8] = "PADBYTE";
+  MOZ_DIAGNOSTIC_ASSERT(padding <= sizeof(paddingBytes));
+
+  if (padding == 0) {
+    return Ok();
+  }
+
+  if (PR_Write(fd, static_cast<const void*>(paddingBytes), padding) !=
+      padding) {
+    return Err(NS_ERROR_FAILURE);
+  }
+  return Ok();
+}
+
+struct MOZ_RAII AutoSafeJSAPI : public AutoJSAPI {
+  AutoSafeJSAPI() { Init(); }
+};
+
+template <typename T>
+struct Matcher;
+
+// Wraps the iterator for a nsTHashTable so that it may be used as a range
+// iterator. Each iterator result acts as a smart pointer to the hash element,
+// and has a Remove() method which will remove the element from the hash.
+//
+// It also accepts an optional Matcher instance against which to filter the
+// elements which should be iterated over.
+//
+// Example:
+//
+//    for (auto& elem : HashElemIter<HashType>(hash)) {
+//        if (elem->IsDead()) {
+//            elem.Remove();
+//        }
+//    }
+template <typename T>
+class HashElemIter {
+  using Iterator = typename T::Iterator;
+  using ElemType = typename T::UserDataType;
+
+  T& hash_;
+  Matcher<ElemType>* matcher_;
+  Iterator iter_;
+
+ public:
+  explicit HashElemIter(T& hash, Matcher<ElemType>* matcher = nullptr)
+      : hash_(hash), matcher_(matcher), iter_(hash.Iter()) {}
+
+  class Elem {
+    friend class HashElemIter<T>;
+
+    HashElemIter<T>& iter_;
+    bool done_;
+
+    Elem(HashElemIter& iter, bool done) : iter_(iter), done_(done) {
+      skipNonMatching();
+    }
+
+    Iterator& iter() { return iter_.iter_; }
+
+    void skipNonMatching() {
+      if (iter_.matcher_) {
+        while (!done_ && !iter_.matcher_->Matches(get())) {
+          iter().Next();
+          done_ = iter().Done();
+        }
+      }
+    }
+
+   public:
+    Elem& operator*() { return *this; }
+
+    ElemType get() {
+      if (done_) {
+        return nullptr;
+      }
+      return iter().UserData();
+    }
+
+    const ElemType get() const { return const_cast<Elem*>(this)->get(); }
+
+    ElemType operator->() { return get(); }
+
+    const ElemType operator->() const { return get(); }
+
+    operator ElemType() { return get(); }
+
+    void Remove() { iter().Remove(); }
+
+    Elem& operator++() {
+      MOZ_ASSERT(!done_);
+
+      iter().Next();
+      done_ = iter().Done();
+
+      skipNonMatching();
+      return *this;
+    }
+
+    bool operator!=(Elem& other) const {
+      return done_ != other.done_ || this->get() != other.get();
+    }
+  };
+
+  Elem begin() { return Elem(*this, iter_.Done()); }
+
+  Elem end() { return Elem(*this, true); }
+};
+
+template <typename T>
+HashElemIter<T> IterHash(T& hash,
+                         Matcher<typename T::UserDataType>* matcher = nullptr) {
+  return HashElemIter<T>(hash, matcher);
+}
+
+template <typename T, typename F>
+bool Find(T&& iter, F&& match) {
+  for (auto& elem : iter) {
+    if (match(elem)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+};  // namespace loader
+};  // namespace mozilla
+
+#endif  // ScriptPreloader_inl_h
diff --git a/js/xpconnect/loader/ScriptPreloader.cpp b/js/xpconnect/loader/ScriptPreloader.cpp
new file mode 100644
index 0000000000..22b788127c
--- /dev/null
+++ b/js/xpconnect/loader/ScriptPreloader.cpp
@@ -0,0 +1,1319 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptPreloader-inl.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Monitor.h"
+
+#include "mozilla/ScriptPreloader.h"
+#include "mozilla/loader/ScriptCacheActors.h"
+
+#include "mozilla/URLPreloader.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Components.h"
+#include "mozilla/FileUtils.h"
+#include "mozilla/IOBuffers.h"
+#include "mozilla/Logging.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Services.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/ContentChild.h"
+#include "mozilla/dom/ContentParent.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/scache/StartupCache.h"
+
+#include "crc32c.h"
+#include "js/CompileOptions.h"  // JS::ReadOnlyCompileOptions
+#include "js/experimental/JSStencil.h"
+#include "js/Transcoding.h"
+#include "MainThreadUtils.h"
+#include "nsDebug.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIFile.h"
+#include "nsIObserverService.h"
+#include "nsJSUtils.h"
+#include "nsMemoryReporterManager.h"
+#include "nsNetUtil.h"
+#include "nsProxyRelease.h"
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+#include "xpcpublic.h"
+
+#define STARTUP_COMPLETE_TOPIC "browser-delayed-startup-finished"
+#define DOC_ELEM_INSERTED_TOPIC "document-element-inserted"
+#define CONTENT_DOCUMENT_LOADED_TOPIC "content-document-loaded"
+#define CACHE_WRITE_TOPIC "browser-idle-startup-tasks-finished"
+#define XPCOM_SHUTDOWN_TOPIC "xpcom-shutdown"
+#define CACHE_INVALIDATE_TOPIC "startupcache-invalidate"
+
+// The maximum time we'll wait for a child process to finish starting up before
+// we send its script data back to the parent.
+constexpr uint32_t CHILD_STARTUP_TIMEOUT_MS = 8000;
+
+namespace mozilla {
+namespace {
+static LazyLogModule gLog("ScriptPreloader");
+
+#define LOG(level, ...) MOZ_LOG(gLog, LogLevel::level, (__VA_ARGS__))
+}  // namespace
+
+using mozilla::dom::AutoJSAPI;
+using mozilla::dom::ContentChild;
+using mozilla::dom::ContentParent;
+using namespace mozilla::loader;
+using mozilla::scache::StartupCache;
+
+using namespace JS;
+
+ProcessType ScriptPreloader::sProcessType;
+
+nsresult ScriptPreloader::CollectReports(nsIHandleReportCallback* aHandleReport,
+                                         nsISupports* aData, bool aAnonymize) {
+  MOZ_COLLECT_REPORT(
+      "explicit/script-preloader/heap/saved-scripts", KIND_HEAP, UNITS_BYTES,
+      SizeOfHashEntries<ScriptStatus::Saved>(mScripts, MallocSizeOf),
+      "Memory used to hold the scripts which have been executed in this "
+      "session, and will be written to the startup script cache file.");
+
+  MOZ_COLLECT_REPORT(
+      "explicit/script-preloader/heap/restored-scripts", KIND_HEAP, UNITS_BYTES,
+      SizeOfHashEntries<ScriptStatus::Restored>(mScripts, MallocSizeOf),
+      "Memory used to hold the scripts which have been restored from the "
+      "startup script cache file, but have not been executed in this session.");
+
+  MOZ_COLLECT_REPORT("explicit/script-preloader/heap/other", KIND_HEAP,
+                     UNITS_BYTES, ShallowHeapSizeOfIncludingThis(MallocSizeOf),
+                     "Memory used by the script cache service itself.");
+
+  // Since the mem-mapped cache file is mapped into memory, we want to report
+  // it as explicit memory somewhere. But since the child cache is shared
+  // between all processes, we don't want to report it as explicit memory for
+  // all of them. So we report it as explicit only in the parent process, and
+  // non-explicit everywhere else.
+  if (XRE_IsParentProcess()) {
+    MOZ_COLLECT_REPORT("explicit/script-preloader/non-heap/memmapped-cache",
+                       KIND_NONHEAP, UNITS_BYTES,
+                       mCacheData->nonHeapSizeOfExcludingThis(),
+                       "The memory-mapped startup script cache file.");
+  } else {
+    MOZ_COLLECT_REPORT("script-preloader-memmapped-cache", KIND_NONHEAP,
+                       UNITS_BYTES, mCacheData->nonHeapSizeOfExcludingThis(),
+                       "The memory-mapped startup script cache file.");
+  }
+
+  return NS_OK;
+}
+
+StaticRefPtr<ScriptPreloader> ScriptPreloader::gScriptPreloader;
+StaticRefPtr<ScriptPreloader> ScriptPreloader::gChildScriptPreloader;
+UniquePtr<AutoMemMap> ScriptPreloader::gCacheData;
+UniquePtr<AutoMemMap> ScriptPreloader::gChildCacheData;
+
+ScriptPreloader& ScriptPreloader::GetSingleton() {
+  if (!gScriptPreloader) {
+    if (XRE_IsParentProcess()) {
+      gCacheData = MakeUnique<AutoMemMap>();
+      gScriptPreloader = new ScriptPreloader(gCacheData.get());
+      gScriptPreloader->mChildCache = &GetChildSingleton();
+      Unused << gScriptPreloader->InitCache();
+    } else {
+      gScriptPreloader = &GetChildSingleton();
+    }
+  }
+
+  return *gScriptPreloader;
+}
+
+// The child singleton is available in all processes, including the parent, and
+// is used for scripts which are expected to be loaded into child processes
+// (such as process and frame scripts), or scripts that have already been loaded
+// into a child. The child caches are managed as follows:
+//
+// - Every startup, we open the cache file from the last session, move it to a
+//  new location, and begin pre-loading the scripts that are stored in it. There
+//  is a separate cache file for parent and content processes, but the parent
+//  process opens both the parent and content cache files.
+//
+// - Once startup is complete, we write a new cache file for the next session,
+//   containing only the scripts that were used during early startup, so we
+//   don't waste pre-loading scripts that may not be needed.
+//
+// - For content processes, opening and writing the cache file is handled in the
+//  parent process. The first content process of each type sends back the data
+//  for scripts that were loaded in early startup, and the parent merges them
+//  and writes them to a cache file.
+//
+// - Currently, content processes only benefit from the cache data written
+//  during the *previous* session. Ideally, new content processes should
+//  probably use the cache data written during this session if there was no
+//  previous cache file, but I'd rather do that as a follow-up.
+ScriptPreloader& ScriptPreloader::GetChildSingleton() {
+  if (!gChildScriptPreloader) {
+    gChildCacheData = MakeUnique<AutoMemMap>();
+    gChildScriptPreloader = new ScriptPreloader(gChildCacheData.get());
+    if (XRE_IsParentProcess()) {
+      Unused << gChildScriptPreloader->InitCache(u"scriptCache-child"_ns);
+    }
+  }
+
+  return *gChildScriptPreloader;
+}
+
+/* static */
+void ScriptPreloader::DeleteSingleton() {
+  gScriptPreloader = nullptr;
+  gChildScriptPreloader = nullptr;
+}
+
+/* static */
+void ScriptPreloader::DeleteCacheDataSingleton() {
+  MOZ_ASSERT(!gScriptPreloader);
+  MOZ_ASSERT(!gChildScriptPreloader);
+
+  gCacheData = nullptr;
+  gChildCacheData = nullptr;
+}
+
+void ScriptPreloader::InitContentChild(ContentParent& parent) {
+  auto& cache = GetChildSingleton();
+  cache.mSaveMonitor.AssertOnWritingThread();
+
+  // We want startup script data from the first process of a given type.
+  // That process sends back its script data before it executes any
+  // untrusted code, and then we never accept further script data for that
+  // type of process for the rest of the session.
+  //
+  // The script data from each process type is merged with the data from the
+  // parent process's frame and process scripts, and shared between all
+  // content process types in the next session.
+  //
+  // Note that if the first process of a given type crashes or shuts down
+  // before sending us its script data, we silently ignore it, and data for
+  // that process type is not included in the next session's cache. This
+  // should be a sufficiently rare occurrence that it's not worth trying to
+  // handle specially.
+  auto processType = GetChildProcessType(parent.GetRemoteType());
+  bool wantScriptData = !cache.mInitializedProcesses.contains(processType);
+  cache.mInitializedProcesses += processType;
+
+  auto fd = cache.mCacheData->cloneFileDescriptor();
+  // Don't send original cache data to new processes if the cache has been
+  // invalidated.
+  if (fd.IsValid() && !cache.mCacheInvalidated) {
+    Unused << parent.SendPScriptCacheConstructor(fd, wantScriptData);
+  } else {
+    Unused << parent.SendPScriptCacheConstructor(NS_ERROR_FILE_NOT_FOUND,
+                                                 wantScriptData);
+  }
+}
+
+ProcessType ScriptPreloader::GetChildProcessType(const nsACString& remoteType) {
+  if (remoteType == EXTENSION_REMOTE_TYPE) {
+    return ProcessType::Extension;
+  }
+  if (remoteType == PRIVILEGEDABOUT_REMOTE_TYPE) {
+    return ProcessType::PrivilegedAbout;
+  }
+  return ProcessType::Web;
+}
+
+ScriptPreloader::ScriptPreloader(AutoMemMap* cacheData)
+    : mCacheData(cacheData),
+      mMonitor("[ScriptPreloader.mMonitor]"),
+      mSaveMonitor("[ScriptPreloader.mSaveMonitor]", this) {
+  // We do not set the process type for child processes here because the
+  // remoteType in ContentChild is not ready yet.
+  if (XRE_IsParentProcess()) {
+    sProcessType = ProcessType::Parent;
+  }
+
+  nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+  MOZ_RELEASE_ASSERT(obs);
+
+  if (XRE_IsParentProcess()) {
+    // In the parent process, we want to freeze the script cache as soon
+    // as idle tasks for the first browser window have completed.
+    obs->AddObserver(this, STARTUP_COMPLETE_TOPIC, false);
+    obs->AddObserver(this, CACHE_WRITE_TOPIC, false);
+  }
+
+  obs->AddObserver(this, XPCOM_SHUTDOWN_TOPIC, false);
+  obs->AddObserver(this, CACHE_INVALIDATE_TOPIC, false);
+}
+
+ScriptPreloader::~ScriptPreloader() { Cleanup(); }
+
+void ScriptPreloader::Cleanup() {
+  mScripts.Clear();
+  UnregisterWeakMemoryReporter(this);
+}
+
+void ScriptPreloader::StartCacheWrite() {
+  MOZ_DIAGNOSTIC_ASSERT(!mSaveThread);
+
+  Unused << NS_NewNamedThread("SaveScripts", getter_AddRefs(mSaveThread), this);
+
+  nsCOMPtr<nsIAsyncShutdownClient> barrier = GetShutdownBarrier();
+  barrier->AddBlocker(this, NS_LITERAL_STRING_FROM_CSTRING(__FILE__), __LINE__,
+                      u""_ns);
+}
+
+void ScriptPreloader::InvalidateCache() {
+  {
+    mMonitor.AssertNotCurrentThreadOwns();
+    MonitorAutoLock mal(mMonitor);
+
+    // Wait for pending off-thread parses to finish, since they depend on the
+    // memory allocated by our CachedScripts, and can't be canceled
+    // asynchronously.
+    FinishPendingParses(mal);
+
+    // Pending scripts should have been cleared by the above, and new parses
+    // should not have been queued.
+    MOZ_ASSERT(mParsingScripts.empty());
+    MOZ_ASSERT(mParsingSources.empty());
+    MOZ_ASSERT(mPendingScripts.isEmpty());
+
+    mScripts.Clear();
+
+    // If we've already finished saving the cache at this point, start a new
+    // delayed save operation. This will write out an empty cache file in place
+    // of any cache file we've already written out this session, which will
+    // prevent us from falling back to the current session's cache file on the
+    // next startup.
+    if (mSaveComplete && !mSaveThread && mChildCache) {
+      mSaveComplete = false;
+
+      StartCacheWrite();
+    }
+  }
+
+  {
+    MonitorSingleWriterAutoLock saveMonitorAutoLock(mSaveMonitor);
+
+    mCacheInvalidated = true;
+  }
+
+  // If we're waiting on a timeout to finish saving, interrupt it and just save
+  // immediately.
+  mSaveMonitor.NotifyAll();
+}
+
+nsresult ScriptPreloader::Observe(nsISupports* subject, const char* topic,
+                                  const char16_t* data) {
+  nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+  if (!strcmp(topic, STARTUP_COMPLETE_TOPIC)) {
+    obs->RemoveObserver(this, STARTUP_COMPLETE_TOPIC);
+
+    MOZ_ASSERT(XRE_IsParentProcess());
+
+    mStartupFinished = true;
+    URLPreloader::GetSingleton().SetStartupFinished();
+  } else if (!strcmp(topic, CACHE_WRITE_TOPIC)) {
+    obs->RemoveObserver(this, CACHE_WRITE_TOPIC);
+
+    MOZ_ASSERT(mStartupFinished);
+    MOZ_ASSERT(XRE_IsParentProcess());
+
+    if (mChildCache && !mSaveComplete && !mSaveThread) {
+      StartCacheWrite();
+    }
+  } else if (mContentStartupFinishedTopic.Equals(topic)) {
+    // If this is an uninitialized about:blank viewer or a chrome: document
+    // (which should always be an XBL binding document), ignore it. We don't
+    // have to worry about it loading malicious content.
+    if (nsCOMPtr<dom::Document> doc = do_QueryInterface(subject)) {
+      nsCOMPtr<nsIURI> uri = doc->GetDocumentURI();
+
+      if ((NS_IsAboutBlank(uri) &&
+           doc->GetReadyStateEnum() == doc->READYSTATE_UNINITIALIZED) ||
+          uri->SchemeIs("chrome")) {
+        return NS_OK;
+      }
+    }
+    FinishContentStartup();
+  } else if (!strcmp(topic, "timer-callback")) {
+    FinishContentStartup();
+  } else if (!strcmp(topic, XPCOM_SHUTDOWN_TOPIC)) {
+    // Wait for any pending parses to finish at this point, to avoid creating
+    // new stencils during destroying the JS runtime.
+    MonitorAutoLock mal(mMonitor);
+    FinishPendingParses(mal);
+  } else if (!strcmp(topic, CACHE_INVALIDATE_TOPIC)) {
+    InvalidateCache();
+  }
+
+  return NS_OK;
+}
+
+void ScriptPreloader::FinishContentStartup() {
+  MOZ_ASSERT(XRE_IsContentProcess());
+
+#ifdef DEBUG
+  if (mContentStartupFinishedTopic.Equals(CONTENT_DOCUMENT_LOADED_TOPIC)) {
+    MOZ_ASSERT(sProcessType == ProcessType::PrivilegedAbout);
+  } else {
+    MOZ_ASSERT(sProcessType != ProcessType::PrivilegedAbout);
+  }
+#endif /* DEBUG */
+
+  nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+  obs->RemoveObserver(this, mContentStartupFinishedTopic.get());
+
+  mSaveTimer = nullptr;
+
+  mStartupFinished = true;
+
+  if (mChildActor) {
+    mChildActor->SendScriptsAndFinalize(mScripts);
+  }
+
+#ifdef XP_WIN
+  // Record the amount of USS at startup. This is Windows-only for now,
+  // we could turn it on for Linux relatively cheaply. On macOS it can have
+  // a perf impact. Only record this for non-privileged processes because
+  // privileged processes record this value at a different time, leading to
+  // a higher value which skews the telemetry.
+  if (sProcessType != ProcessType::PrivilegedAbout) {
+    mozilla::Telemetry::Accumulate(
+        mozilla::Telemetry::MEMORY_UNIQUE_CONTENT_STARTUP,
+        nsMemoryReporterManager::ResidentUnique() / 1024);
+  }
+#endif
+}
+
+bool ScriptPreloader::WillWriteScripts() {
+  return !mDataPrepared && (XRE_IsParentProcess() || mChildActor);
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> ScriptPreloader::GetCacheFile(
+    const nsAString& suffix) {
+  NS_ENSURE_TRUE(mProfD, Err(NS_ERROR_NOT_INITIALIZED));
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY(mProfD->Clone(getter_AddRefs(cacheFile)));
+
+  MOZ_TRY(cacheFile->AppendNative("startupCache"_ns));
+  Unused << cacheFile->Create(nsIFile::DIRECTORY_TYPE, 0777);
+
+  MOZ_TRY(cacheFile->Append(mBaseName + suffix));
+
+  return std::move(cacheFile);
+}
+
+static const uint8_t MAGIC[] = "mozXDRcachev003";
+
+Result<Ok, nsresult> ScriptPreloader::OpenCache() {
+  if (StartupCache::GetIgnoreDiskCache()) {
+    return Err(NS_ERROR_ABORT);
+  }
+
+  MOZ_TRY(NS_GetSpecialDirectory("ProfLDS", getter_AddRefs(mProfD)));
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY_VAR(cacheFile, GetCacheFile(u".bin"_ns));
+
+  bool exists;
+  MOZ_TRY(cacheFile->Exists(&exists));
+  if (exists) {
+    MOZ_TRY(cacheFile->MoveTo(nullptr, mBaseName + u"-current.bin"_ns));
+  } else {
+    MOZ_TRY(cacheFile->SetLeafName(mBaseName + u"-current.bin"_ns));
+    MOZ_TRY(cacheFile->Exists(&exists));
+    if (!exists) {
+      return Err(NS_ERROR_FILE_NOT_FOUND);
+    }
+  }
+
+  MOZ_TRY(mCacheData->init(cacheFile));
+
+  return Ok();
+}
+
+// Opens the script cache file for this session, and initializes the script
+// cache based on its contents. See WriteCache for details of the cache file.
+Result<Ok, nsresult> ScriptPreloader::InitCache(const nsAString& basePath) {
+  mSaveMonitor.AssertOnWritingThread();
+  mCacheInitialized = true;
+  mBaseName = basePath;
+
+  RegisterWeakMemoryReporter(this);
+
+  if (!XRE_IsParentProcess()) {
+    return Ok();
+  }
+
+  // Grab the compilation scope before initializing the URLPreloader, since
+  // it's not safe to run component loader code during its critical section.
+  AutoSafeJSAPI jsapi;
+  JS::RootedObject scope(jsapi.cx(), xpc::CompilationScope());
+
+  // Note: Code on the main thread *must not access Omnijar in any way* until
+  // this AutoBeginReading guard is destroyed.
+  URLPreloader::AutoBeginReading abr;
+
+  MOZ_TRY(OpenCache());
+
+  return InitCacheInternal(scope);
+}
+
+Result<Ok, nsresult> ScriptPreloader::InitCache(
+    const Maybe<ipc::FileDescriptor>& cacheFile, ScriptCacheChild* cacheChild) {
+  mSaveMonitor.AssertOnWritingThread();
+  MOZ_ASSERT(XRE_IsContentProcess());
+
+  mCacheInitialized = true;
+  mChildActor = cacheChild;
+  sProcessType =
+      GetChildProcessType(dom::ContentChild::GetSingleton()->GetRemoteType());
+
+  nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+  MOZ_RELEASE_ASSERT(obs);
+
+  if (sProcessType == ProcessType::PrivilegedAbout) {
+    // Since we control all of the documents loaded in the privileged
+    // content process, we can increase the window of active time for the
+    // ScriptPreloader to include the scripts that are loaded until the
+    // first document finishes loading.
+    mContentStartupFinishedTopic.AssignLiteral(CONTENT_DOCUMENT_LOADED_TOPIC);
+  } else {
+    // In the child process, we need to freeze the script cache before any
+    // untrusted code has been executed. The insertion of the first DOM
+    // document element may sometimes be earlier than is ideal, but at
+    // least it should always be safe.
+    mContentStartupFinishedTopic.AssignLiteral(DOC_ELEM_INSERTED_TOPIC);
+  }
+  obs->AddObserver(this, mContentStartupFinishedTopic.get(), false);
+
+  RegisterWeakMemoryReporter(this);
+
+  auto cleanup = MakeScopeExit([&] {
+    // If the parent is expecting cache data from us, make sure we send it
+    // before it writes out its cache file. For normal proceses, this isn't
+    // a concern, since they begin loading documents quite early. For the
+    // preloaded process, we may end up waiting a long time (or, indeed,
+    // never loading a document), so we need an additional timeout.
+    if (cacheChild) {
+      NS_NewTimerWithObserver(getter_AddRefs(mSaveTimer), this,
+                              CHILD_STARTUP_TIMEOUT_MS,
+                              nsITimer::TYPE_ONE_SHOT);
+    }
+  });
+
+  if (cacheFile.isNothing()) {
+    return Ok();
+  }
+
+  MOZ_TRY(mCacheData->init(cacheFile.ref()));
+
+  return InitCacheInternal();
+}
+
+Result<Ok, nsresult> ScriptPreloader::InitCacheInternal(
+    JS::HandleObject scope) {
+  auto size = mCacheData->size();
+
+  uint32_t headerSize;
+  uint32_t crc;
+  if (size < sizeof(MAGIC) + sizeof(headerSize) + sizeof(crc)) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  auto data = mCacheData->get<uint8_t>();
+  MOZ_RELEASE_ASSERT(JS::IsTranscodingBytecodeAligned(data.get()));
+
+  auto end = data + size;
+
+  if (memcmp(MAGIC, data.get(), sizeof(MAGIC))) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+  data += sizeof(MAGIC);
+
+  headerSize = LittleEndian::readUint32(data.get());
+  data += sizeof(headerSize);
+
+  crc = LittleEndian::readUint32(data.get());
+  data += sizeof(crc);
+
+  if (data + headerSize > end) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  if (crc != ComputeCrc32c(~0, data.get(), headerSize)) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  {
+    auto cleanup = MakeScopeExit([&]() { mScripts.Clear(); });
+
+    LinkedList<CachedStencil> scripts;
+
+    Range<uint8_t> header(data, data + headerSize);
+    data += headerSize;
+
+    // Reconstruct alignment padding if required.
+    size_t currentOffset = data - mCacheData->get<uint8_t>();
+    data += JS::AlignTranscodingBytecodeOffset(currentOffset) - currentOffset;
+
+    InputBuffer buf(header);
+
+    size_t offset = 0;
+    while (!buf.finished()) {
+      auto script = MakeUnique<CachedStencil>(*this, buf);
+      MOZ_RELEASE_ASSERT(script);
+
+      auto scriptData = data + script->mOffset;
+      if (!JS::IsTranscodingBytecodeAligned(scriptData.get())) {
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+
+      if (scriptData + script->mSize > end) {
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+
+      // Make sure offsets match what we'd expect based on script ordering and
+      // size, as a basic sanity check.
+      if (script->mOffset != offset) {
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+      offset += script->mSize;
+
+      script->mXDRRange.emplace(scriptData, scriptData + script->mSize);
+
+      // Don't pre-decode the script unless it was used in this process type
+      // during the previous session.
+      if (script->mOriginalProcessTypes.contains(CurrentProcessType())) {
+        scripts.insertBack(script.get());
+      } else {
+        script->mReadyToExecute = true;
+      }
+
+      const auto& cachePath = script->mCachePath;
+      mScripts.InsertOrUpdate(cachePath, std::move(script));
+    }
+
+    if (buf.error()) {
+      return Err(NS_ERROR_UNEXPECTED);
+    }
+
+    mPendingScripts = std::move(scripts);
+    cleanup.release();
+  }
+
+  DecodeNextBatch(OFF_THREAD_FIRST_CHUNK_SIZE, scope);
+  return Ok();
+}
+
+void ScriptPreloader::PrepareCacheWriteInternal() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  mMonitor.AssertCurrentThreadOwns();
+
+  auto cleanup = MakeScopeExit([&]() {
+    if (mChildCache) {
+      mChildCache->PrepareCacheWrite();
+    }
+  });
+
+  if (mDataPrepared) {
+    return;
+  }
+
+  AutoSafeJSAPI jsapi;
+  JSAutoRealm ar(jsapi.cx(), xpc::PrivilegedJunkScope());
+  bool found = false;
+  for (auto& script : IterHash(mScripts, Match<ScriptStatus::Saved>())) {
+    // Don't write any scripts that are also in the child cache. They'll be
+    // loaded from the child cache in that case, so there's no need to write
+    // them twice.
+    CachedStencil* childScript =
+        mChildCache ? mChildCache->mScripts.Get(script->mCachePath) : nullptr;
+    if (childScript && !childScript->mProcessTypes.isEmpty()) {
+      childScript->UpdateLoadTime(script->mLoadTime);
+      childScript->mProcessTypes += script->mProcessTypes;
+      script.Remove();
+      continue;
+    }
+
+    if (!(script->mProcessTypes == script->mOriginalProcessTypes)) {
+      // Note: EnumSet doesn't support operator!=, hence the weird form above.
+      found = true;
+    }
+
+    if (!script->mSize && !script->XDREncode(jsapi.cx())) {
+      script.Remove();
+    }
+  }
+
+  if (!found) {
+    mSaveComplete = true;
+    return;
+  }
+
+  mDataPrepared = true;
+}
+
+void ScriptPreloader::PrepareCacheWrite() {
+  MonitorAutoLock mal(mMonitor);
+
+  PrepareCacheWriteInternal();
+}
+
+// Writes out a script cache file for the scripts accessed during early
+// startup in this session. The cache file is a little-endian binary file with
+// the following format:
+//
+// - A uint32 containing the size of the header block.
+//
+// - A header entry for each file stored in the cache containing:
+//   - The URL that the script was originally read from.
+//   - Its cache key.
+//   - The offset of its XDR data within the XDR data block.
+//   - The size of its XDR data in the XDR data block.
+//   - A bit field describing which process types the script is used in.
+//
+// - A block of XDR data for the encoded scripts, with each script's data at
+//   an offset from the start of the block, as specified above.
+Result<Ok, nsresult> ScriptPreloader::WriteCache() {
+  MOZ_ASSERT(!NS_IsMainThread());
+  mSaveMonitor.AssertCurrentThreadOwns();
+
+  if (!mDataPrepared && !mSaveComplete) {
+    MonitorSingleWriterAutoUnlock mau(mSaveMonitor);
+
+    NS_DispatchAndSpinEventLoopUntilComplete(
+        "ScriptPreloader::PrepareCacheWrite"_ns,
+        GetMainThreadSerialEventTarget(),
+        NewRunnableMethod("ScriptPreloader::PrepareCacheWrite", this,
+                          &ScriptPreloader::PrepareCacheWrite));
+  }
+
+  if (mSaveComplete) {
+    // If we don't have anything we need to save, we're done.
+    return Ok();
+  }
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY_VAR(cacheFile, GetCacheFile(u"-new.bin"_ns));
+
+  bool exists;
+  MOZ_TRY(cacheFile->Exists(&exists));
+  if (exists) {
+    MOZ_TRY(cacheFile->Remove(false));
+  }
+
+  {
+    AutoFDClose fd;
+    MOZ_TRY(cacheFile->OpenNSPRFileDesc(PR_WRONLY | PR_CREATE_FILE, 0644,
+                                        &fd.rwget()));
+
+    // We also need to hold mMonitor while we're touching scripts in
+    // mScripts, or they may be freed before we're done with them.
+    mMonitor.AssertNotCurrentThreadOwns();
+    MonitorAutoLock mal(mMonitor);
+
+    nsTArray<CachedStencil*> scripts;
+    for (auto& script : IterHash(mScripts, Match<ScriptStatus::Saved>())) {
+      scripts.AppendElement(script);
+    }
+
+    // Sort scripts by load time, with async loaded scripts before sync scripts.
+    // Since async scripts are always loaded immediately at startup, it helps to
+    // have them stored contiguously.
+    scripts.Sort(CachedStencil::Comparator());
+
+    OutputBuffer buf;
+    size_t offset = 0;
+    for (auto script : scripts) {
+      script->mOffset = offset;
+      MOZ_DIAGNOSTIC_ASSERT(
+          JS::IsTranscodingBytecodeOffsetAligned(script->mOffset));
+      script->Code(buf);
+
+      offset += script->mSize;
+      MOZ_DIAGNOSTIC_ASSERT(
+          JS::IsTranscodingBytecodeOffsetAligned(script->mSize));
+    }
+
+    uint8_t headerSize[4];
+    LittleEndian::writeUint32(headerSize, buf.cursor());
+
+    uint8_t crc[4];
+    LittleEndian::writeUint32(crc, ComputeCrc32c(~0, buf.Get(), buf.cursor()));
+
+    MOZ_TRY(Write(fd, MAGIC, sizeof(MAGIC)));
+    MOZ_TRY(Write(fd, headerSize, sizeof(headerSize)));
+    MOZ_TRY(Write(fd, crc, sizeof(crc)));
+    MOZ_TRY(Write(fd, buf.Get(), buf.cursor()));
+
+    // Align the start of the scripts section to the transcode alignment.
+    size_t written = sizeof(MAGIC) + sizeof(headerSize) + buf.cursor();
+    size_t padding = JS::AlignTranscodingBytecodeOffset(written) - written;
+    if (padding) {
+      MOZ_TRY(WritePadding(fd, padding));
+      written += padding;
+    }
+
+    for (auto script : scripts) {
+      MOZ_DIAGNOSTIC_ASSERT(JS::IsTranscodingBytecodeOffsetAligned(written));
+      MOZ_TRY(Write(fd, script->Range().begin().get(), script->mSize));
+
+      written += script->mSize;
+      // We can only free the XDR data if the stencil isn't borrowing data from
+      // it.
+      if (script->mStencil && !JS::StencilIsBorrowed(script->mStencil)) {
+        script->FreeData();
+      }
+    }
+  }
+
+  MOZ_TRY(cacheFile->MoveTo(nullptr, mBaseName + u".bin"_ns));
+
+  return Ok();
+}
+
+nsresult ScriptPreloader::GetName(nsACString& aName) {
+  aName.AssignLiteral("ScriptPreloader");
+  return NS_OK;
+}
+
+// Runs in the mSaveThread thread, and writes out the cache file for the next
+// session after a reasonable delay.
+nsresult ScriptPreloader::Run() {
+  MonitorSingleWriterAutoLock mal(mSaveMonitor);
+
+  // Ideally wait about 10 seconds before saving, to avoid unnecessary IO
+  // during early startup. But only if the cache hasn't been invalidated,
+  // since that can trigger a new write during shutdown, and we don't want to
+  // cause shutdown hangs.
+  if (!mCacheInvalidated) {
+    mal.Wait(TimeDuration::FromSeconds(10));
+  }
+
+  auto result = URLPreloader::GetSingleton().WriteCache();
+  Unused << NS_WARN_IF(result.isErr());
+
+  result = WriteCache();
+  Unused << NS_WARN_IF(result.isErr());
+
+  {
+    MonitorSingleWriterAutoLock lock(mChildCache->mSaveMonitor);
+    result = mChildCache->WriteCache();
+  }
+  Unused << NS_WARN_IF(result.isErr());
+
+  NS_DispatchToMainThread(
+      NewRunnableMethod("ScriptPreloader::CacheWriteComplete", this,
+                        &ScriptPreloader::CacheWriteComplete),
+      NS_DISPATCH_NORMAL);
+  return NS_OK;
+}
+
+void ScriptPreloader::CacheWriteComplete() {
+  mSaveThread->AsyncShutdown();
+  mSaveThread = nullptr;
+  mSaveComplete = true;
+
+  nsCOMPtr<nsIAsyncShutdownClient> barrier = GetShutdownBarrier();
+  barrier->RemoveBlocker(this);
+}
+
+void ScriptPreloader::NoteStencil(const nsCString& url,
+                                  const nsCString& cachePath,
+                                  JS::Stencil* stencil, bool isRunOnce) {
+  if (!Active()) {
+    if (isRunOnce) {
+      if (auto script = mScripts.Get(cachePath)) {
+        script->mIsRunOnce = true;
+        script->MaybeDropStencil();
+      }
+    }
+    return;
+  }
+
+  // Don't bother trying to cache any URLs with cache-busting query
+  // parameters.
+  if (cachePath.FindChar('?') >= 0) {
+    return;
+  }
+
+  // Don't bother caching files that belong to the mochitest harness.
+  constexpr auto mochikitPrefix = "chrome://mochikit/"_ns;
+  if (StringHead(url, mochikitPrefix.Length()) == mochikitPrefix) {
+    return;
+  }
+
+  auto* script =
+      mScripts.GetOrInsertNew(cachePath, *this, url, cachePath, stencil);
+  if (isRunOnce) {
+    script->mIsRunOnce = true;
+  }
+
+  if (!script->MaybeDropStencil() && !script->mStencil) {
+    MOZ_ASSERT(stencil);
+    script->mStencil = stencil;
+    script->mReadyToExecute = true;
+  }
+
+  script->UpdateLoadTime(TimeStamp::Now());
+  script->mProcessTypes += CurrentProcessType();
+}
+
+void ScriptPreloader::NoteStencil(const nsCString& url,
+                                  const nsCString& cachePath,
+                                  ProcessType processType,
+                                  nsTArray<uint8_t>&& xdrData,
+                                  TimeStamp loadTime) {
+  // After data has been prepared, there's no point in noting further scripts,
+  // since the cache either has already been written, or is about to be
+  // written. Any time prior to the data being prepared, we can safely mutate
+  // mScripts without locking. After that point, the save thread is free to
+  // access it, and we can't alter it without locking.
+  if (mDataPrepared) {
+    return;
+  }
+
+  auto* script =
+      mScripts.GetOrInsertNew(cachePath, *this, url, cachePath, nullptr);
+
+  if (!script->HasRange()) {
+    MOZ_ASSERT(!script->HasArray());
+
+    script->mSize = xdrData.Length();
+    script->mXDRData.construct<nsTArray<uint8_t>>(
+        std::forward<nsTArray<uint8_t>>(xdrData));
+
+    auto& data = script->Array();
+    script->mXDRRange.emplace(data.Elements(), data.Length());
+  }
+
+  if (!script->mSize && !script->mStencil) {
+    // If the content process is sending us an entry for a stencil
+    // which was in the cache at startup, it expects us to already have this
+    // script data, so it doesn't send it.
+    //
+    // However, the cache may have been invalidated at this point (usually
+    // due to the add-on manager installing or uninstalling a legacy
+    // extension during very early startup), which means we may no longer
+    // have an entry for this script. Since that means we have no data to
+    // write to the new cache, and no JSScript to generate it from, we need
+    // to discard this entry.
+    mScripts.Remove(cachePath);
+    return;
+  }
+
+  script->UpdateLoadTime(loadTime);
+  script->mProcessTypes += processType;
+}
+
+/* static */
+void ScriptPreloader::FillCompileOptionsForCachedStencil(
+    JS::CompileOptions& options) {
+  // Users of the cache do not require return values, so inform the JS parser in
+  // order for it to generate simpler bytecode.
+  options.setNoScriptRval(true);
+
+  // The ScriptPreloader trades off having bytecode available but not source
+  // text. This means the JS syntax-only parser is not used. If `toString` is
+  // called on functions in these scripts, the source-hook will fetch it over,
+  // so using `toString` of functions should be avoided in chrome js.
+  options.setSourceIsLazy(true);
+}
+
+/* static */
+void ScriptPreloader::FillDecodeOptionsForCachedStencil(
+    JS::DecodeOptions& options) {
+  // ScriptPreloader's XDR buffer is alive during the Stencil is alive.
+  // The decoded stencil can borrow from it.
+  //
+  // NOTE: The XDR buffer is alive during the entire browser lifetime only
+  //       when it's mmapped.
+  options.borrowBuffer = true;
+}
+
+already_AddRefed<JS::Stencil> ScriptPreloader::GetCachedStencil(
+    JSContext* cx, const JS::DecodeOptions& options, const nsCString& path) {
+  MOZ_RELEASE_ASSERT(
+      !(XRE_IsContentProcess() && !mCacheInitialized),
+      "ScriptPreloader must be initialized before getting cached "
+      "scripts in the content process.");
+
+  // If a script is used by both the parent and the child, it's stored only
+  // in the child cache.
+  if (mChildCache) {
+    RefPtr<JS::Stencil> stencil =
+        mChildCache->GetCachedStencilInternal(cx, options, path);
+    if (stencil) {
+      Telemetry::AccumulateCategorical(
+          Telemetry::LABELS_SCRIPT_PRELOADER_REQUESTS::HitChild);
+      return stencil.forget();
+    }
+  }
+
+  RefPtr<JS::Stencil> stencil = GetCachedStencilInternal(cx, options, path);
+  Telemetry::AccumulateCategorical(
+      stencil ? Telemetry::LABELS_SCRIPT_PRELOADER_REQUESTS::Hit
+              : Telemetry::LABELS_SCRIPT_PRELOADER_REQUESTS::Miss);
+  return stencil.forget();
+}
+
+already_AddRefed<JS::Stencil> ScriptPreloader::GetCachedStencilInternal(
+    JSContext* cx, const JS::DecodeOptions& options, const nsCString& path) {
+  auto* cachedScript = mScripts.Get(path);
+  if (cachedScript) {
+    return WaitForCachedStencil(cx, options, cachedScript);
+  }
+  return nullptr;
+}
+
+already_AddRefed<JS::Stencil> ScriptPreloader::WaitForCachedStencil(
+    JSContext* cx, const JS::DecodeOptions& options, CachedStencil* script) {
+  // Always check for finished operations so that we can move on to decoding the
+  // next batch as soon as possible after the pending batch is ready. If we wait
+  // until we hit an unfinished script, we wind up having at most one batch of
+  // buffered scripts, and occasionally under-running that buffer.
+  if (JS::OffThreadToken* token = mToken.exchange(nullptr)) {
+    FinishOffThreadDecode(token);
+  }
+
+  if (!script->mReadyToExecute) {
+    LOG(Info, "Must wait for async script load: %s\n", script->mURL.get());
+    auto start = TimeStamp::Now();
+
+    // If script is small enough, we'd rather recompile on main-thread than wait
+    // for a decode task to complete.
+    if (script->mSize < MAX_MAINTHREAD_DECODE_SIZE) {
+      LOG(Info, "Script is small enough to recompile on main thread\n");
+
+      script->mReadyToExecute = true;
+      Telemetry::ScalarAdd(
+          Telemetry::ScalarID::SCRIPT_PRELOADER_MAINTHREAD_RECOMPILE, 1);
+    } else {
+      MonitorAutoLock mal(mMonitor);
+
+      // Process script batches until our target is found.
+      while (!script->mReadyToExecute) {
+        if (JS::OffThreadToken* token = mToken.exchange(nullptr)) {
+          MonitorAutoUnlock mau(mMonitor);
+          FinishOffThreadDecode(token);
+        } else {
+          MOZ_ASSERT(!mParsingScripts.empty());
+          mWaitingForDecode = true;
+          mal.Wait();
+          mWaitingForDecode = false;
+        }
+      }
+    }
+
+    double waitedMS = (TimeStamp::Now() - start).ToMilliseconds();
+    Telemetry::Accumulate(Telemetry::SCRIPT_PRELOADER_WAIT_TIME, int(waitedMS));
+    LOG(Debug, "Waited %fms\n", waitedMS);
+  }
+
+  return script->GetStencil(cx, options);
+}
+
+/* static */
+void ScriptPreloader::OffThreadDecodeCallback(JS::OffThreadToken* token,
+                                              void* context) {
+  auto cache = static_cast<ScriptPreloader*>(context);
+
+  // Make the token available to main-thread asynchronously. The lock below is
+  // used for Wait/Notify machinery and isn't needed to update the token itself.
+  MOZ_ALWAYS_FALSE(cache->mToken.exchange(token));
+
+  cache->mMonitor.AssertNotCurrentThreadOwns();
+  MonitorAutoLock mal(cache->mMonitor);
+
+  if (cache->mWaitingForDecode) {
+    // Wake up the blocked main thread.
+    mal.Notify();
+  } else if (!cache->mFinishDecodeRunnablePending) {
+    // Issue a Runnable to ensure batches continue to decode even if the next
+    // WaitForCachedScript call has not happened yet.
+    cache->mFinishDecodeRunnablePending = true;
+    NS_DispatchToMainThread(
+        NewRunnableMethod("ScriptPreloader::DoFinishOffThreadDecode", cache,
+                          &ScriptPreloader::DoFinishOffThreadDecode));
+  }
+}
+
+void ScriptPreloader::FinishPendingParses(MonitorAutoLock& aMal) {
+  mMonitor.AssertCurrentThreadOwns();
+
+  // Clear out scripts that we have not issued batch for yet.
+  mPendingScripts.clear();
+
+  // Process any pending decodes that are in flight.
+  while (!mParsingScripts.empty()) {
+    if (JS::OffThreadToken* token = mToken.exchange(nullptr)) {
+      MonitorAutoUnlock mau(mMonitor);
+      FinishOffThreadDecode(token);
+    } else {
+      mWaitingForDecode = true;
+      aMal.Wait();
+      mWaitingForDecode = false;
+    }
+  }
+}
+
+void ScriptPreloader::DoFinishOffThreadDecode() {
+  {
+    MonitorAutoLock mal(mMonitor);
+    mFinishDecodeRunnablePending = false;
+  }
+
+  if (JS::OffThreadToken* token = mToken.exchange(nullptr)) {
+    FinishOffThreadDecode(token);
+  }
+}
+
+void ScriptPreloader::FinishOffThreadDecode(JS::OffThreadToken* token) {
+  mMonitor.AssertNotCurrentThreadOwns();
+  MOZ_ASSERT(token);
+
+  auto cleanup = MakeScopeExit([&]() {
+    mParsingSources.clear();
+    mParsingScripts.clear();
+
+    DecodeNextBatch(OFF_THREAD_CHUNK_SIZE);
+  });
+
+  AutoSafeJSAPI jsapi;
+  JSContext* cx = jsapi.cx();
+
+  JSAutoRealm ar(cx, xpc::CompilationScope());
+  Vector<RefPtr<JS::Stencil>> stencils;
+
+  // If this fails, we still need to mark the scripts as finished. Any that
+  // weren't successfully compiled in this operation (which should never
+  // happen under ordinary circumstances) will be re-decoded on the main
+  // thread, and raise the appropriate errors when they're executed.
+  //
+  // The exception from the off-thread decode operation will be reported when
+  // we pop the AutoJSAPI off the stack.
+  Unused << JS::FinishDecodeMultiStencilsOffThread(cx, token, &stencils);
+
+  unsigned i = 0;
+  for (auto script : mParsingScripts) {
+    LOG(Debug, "Finished off-thread decode of %s\n", script->mURL.get());
+    if (i < stencils.length()) {
+      script->mStencil = stencils[i++].forget();
+    }
+    script->mReadyToExecute = true;
+  }
+}
+
+void ScriptPreloader::DecodeNextBatch(size_t chunkSize,
+                                      JS::HandleObject scope) {
+  MOZ_ASSERT(mParsingSources.length() == 0);
+  MOZ_ASSERT(mParsingScripts.length() == 0);
+
+  auto cleanup = MakeScopeExit([&]() {
+    mParsingScripts.clearAndFree();
+    mParsingSources.clearAndFree();
+  });
+
+  auto start = TimeStamp::Now();
+  LOG(Debug, "Off-thread decoding scripts...\n");
+
+  size_t size = 0;
+  for (CachedStencil* next = mPendingScripts.getFirst(); next;) {
+    auto* script = next;
+    next = script->getNext();
+
+    MOZ_ASSERT(script->IsMemMapped());
+
+    // Skip any scripts that we decoded on the main thread rather than
+    // waiting for an off-thread operation to complete.
+    if (script->mReadyToExecute) {
+      script->remove();
+      continue;
+    }
+    // If we have enough data for one chunk and this script would put us
+    // over our chunk size limit, we're done.
+    if (size > SMALL_SCRIPT_CHUNK_THRESHOLD &&
+        size + script->mSize > chunkSize) {
+      break;
+    }
+    if (!mParsingScripts.append(script) ||
+        !mParsingSources.emplaceBack(script->Range(), script->mURL.get(), 0)) {
+      break;
+    }
+
+    LOG(Debug, "Beginning off-thread decode of script %s (%u bytes)\n",
+        script->mURL.get(), script->mSize);
+
+    script->remove();
+    size += script->mSize;
+  }
+
+  if (size == 0 && mPendingScripts.isEmpty()) {
+    return;
+  }
+
+  AutoSafeJSAPI jsapi;
+  JSContext* cx = jsapi.cx();
+  JSAutoRealm ar(cx, scope ? scope : xpc::CompilationScope());
+
+  JS::CompileOptions options(cx);
+  FillCompileOptionsForCachedStencil(options);
+
+  // All XDR buffers are mmapped and live longer than JS runtime.
+  // The bytecode can be borrowed from the buffer.
+  options.borrowBuffer = true;
+  options.usePinnedBytecode = true;
+
+  JS::DecodeOptions decodeOptions(options);
+
+  if (!JS::CanDecodeOffThread(cx, decodeOptions, size) ||
+      !JS::DecodeMultiStencilsOffThread(cx, decodeOptions, mParsingSources,
+                                        OffThreadDecodeCallback,
+                                        static_cast<void*>(this))) {
+    // If we fail here, we don't move on to process the next batch, so make
+    // sure we don't have any other scripts left to process.
+    MOZ_ASSERT(mPendingScripts.isEmpty());
+    for (auto script : mPendingScripts) {
+      script->mReadyToExecute = true;
+    }
+
+    LOG(Info, "Can't decode %lu bytes of scripts off-thread",
+        (unsigned long)size);
+    for (auto script : mParsingScripts) {
+      script->mReadyToExecute = true;
+    }
+    return;
+  }
+
+  cleanup.release();
+
+  LOG(Debug, "Initialized decoding of %u scripts (%u bytes) in %fms\n",
+      (unsigned)mParsingSources.length(), (unsigned)size,
+      (TimeStamp::Now() - start).ToMilliseconds());
+}
+
+ScriptPreloader::CachedStencil::CachedStencil(ScriptPreloader& cache,
+                                              InputBuffer& buf)
+    : mCache(cache) {
+  Code(buf);
+
+  // Swap the mProcessTypes and mOriginalProcessTypes values, since we want to
+  // start with an empty set of processes loaded into for this session, and
+  // compare against last session's values later.
+  mOriginalProcessTypes = mProcessTypes;
+  mProcessTypes = {};
+}
+
+bool ScriptPreloader::CachedStencil::XDREncode(JSContext* cx) {
+  auto cleanup = MakeScopeExit([&]() { MaybeDropStencil(); });
+
+  mXDRData.construct<JS::TranscodeBuffer>();
+
+  JS::TranscodeResult code = JS::EncodeStencil(cx, mStencil, Buffer());
+  if (code == JS::TranscodeResult::Ok) {
+    mXDRRange.emplace(Buffer().begin(), Buffer().length());
+    mSize = Range().length();
+    return true;
+  }
+  mXDRData.destroy();
+  JS_ClearPendingException(cx);
+  return false;
+}
+
+already_AddRefed<JS::Stencil> ScriptPreloader::CachedStencil::GetStencil(
+    JSContext* cx, const JS::DecodeOptions& options) {
+  MOZ_ASSERT(mReadyToExecute);
+  if (mStencil) {
+    return do_AddRef(mStencil);
+  }
+
+  if (!HasRange()) {
+    // We've already executed the script, and thrown it away. But it wasn't
+    // in the cache at startup, so we don't have any data to decode. Give
+    // up.
+    return nullptr;
+  }
+
+  // If we have no script at this point, the script was too small to decode
+  // off-thread, or it was needed before the off-thread compilation was
+  // finished, and is small enough to decode on the main thread rather than
+  // wait for the off-thread decoding to finish. In either case, we decode
+  // it synchronously the first time it's needed.
+
+  auto start = TimeStamp::Now();
+  LOG(Info, "Decoding stencil %s on main thread...\n", mURL.get());
+
+  RefPtr<JS::Stencil> stencil;
+  if (JS::DecodeStencil(cx, options, Range(), getter_AddRefs(stencil)) ==
+      JS::TranscodeResult::Ok) {
+    // Lock the monitor here to avoid data races on mScript
+    // from other threads like the cache writing thread.
+    //
+    // It is possible that we could end up decoding the same
+    // script twice, because DecodeScript isn't being guarded
+    // by the monitor; however, to encourage off-thread decode
+    // to proceed for other scripts we don't hold the monitor
+    // while doing main thread decode, merely while updating
+    // mScript.
+    mCache.mMonitor.AssertNotCurrentThreadOwns();
+    MonitorAutoLock mal(mCache.mMonitor);
+
+    mStencil = stencil.forget();
+
+    if (mCache.mSaveComplete) {
+      // We can only free XDR data if the stencil isn't borrowing data out of
+      // it.
+      if (!JS::StencilIsBorrowed(mStencil)) {
+        FreeData();
+      }
+    }
+  }
+
+  LOG(Debug, "Finished decoding in %fms",
+      (TimeStamp::Now() - start).ToMilliseconds());
+
+  return do_AddRef(mStencil);
+}
+
+// nsIAsyncShutdownBlocker
+
+nsresult ScriptPreloader::GetName(nsAString& aName) {
+  aName.AssignLiteral(u"ScriptPreloader: Saving bytecode cache");
+  return NS_OK;
+}
+
+nsresult ScriptPreloader::GetState(nsIPropertyBag** aState) {
+  *aState = nullptr;
+  return NS_OK;
+}
+
+nsresult ScriptPreloader::BlockShutdown(
+    nsIAsyncShutdownClient* aBarrierClient) {
+  // If we're waiting on a timeout to finish saving, interrupt it and just save
+  // immediately.
+  mSaveMonitor.NotifyAll();
+  return NS_OK;
+}
+
+already_AddRefed<nsIAsyncShutdownClient> ScriptPreloader::GetShutdownBarrier() {
+  nsCOMPtr<nsIAsyncShutdownService> svc = components::AsyncShutdown::Service();
+  MOZ_RELEASE_ASSERT(svc);
+
+  nsCOMPtr<nsIAsyncShutdownClient> barrier;
+  Unused << svc->GetXpcomWillShutdown(getter_AddRefs(barrier));
+  MOZ_RELEASE_ASSERT(barrier);
+
+  return barrier.forget();
+}
+
+NS_IMPL_ISUPPORTS(ScriptPreloader, nsIObserver, nsIRunnable, nsIMemoryReporter,
+                  nsINamed, nsIAsyncShutdownBlocker)
+
+#undef LOG
+
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/ScriptPreloader.h b/js/xpconnect/loader/ScriptPreloader.h
new file mode 100644
index 0000000000..e1868b6f40
--- /dev/null
+++ b/js/xpconnect/loader/ScriptPreloader.h
@@ -0,0 +1,543 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ScriptPreloader_h
+#define ScriptPreloader_h
+
+#include "mozilla/Atomics.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MaybeOneOf.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Range.h"
+#include "mozilla/Vector.h"
+#include "mozilla/Result.h"
+#include "mozilla/loader/AutoMemMap.h"
+#include "MainThreadUtils.h"
+#include "nsClassHashtable.h"
+#include "nsIAsyncShutdown.h"
+#include "nsIFile.h"
+#include "nsIMemoryReporter.h"
+#include "nsIObserver.h"
+#include "nsIThread.h"
+#include "nsITimer.h"
+
+#include "js/CompileOptions.h"  // JS::DecodeOptions
+#include "js/experimental/JSStencil.h"
+#include "js/GCAnnotations.h"  // for JS_HAZ_NON_GC_POINTER
+#include "js/RootingAPI.h"     // for Handle, Heap
+#include "js/Transcoding.h"  // for TranscodeBuffer, TranscodeRange, TranscodeSources
+#include "js/TypeDecls.h"  // for HandleObject, HandleScript
+
+#include <prio.h>
+
+namespace JS {
+class CompileOptions;
+class OffThreadToken;
+}  // namespace JS
+
+namespace mozilla {
+namespace dom {
+class ContentParent;
+}
+namespace ipc {
+class FileDescriptor;
+}
+namespace loader {
+class InputBuffer;
+class ScriptCacheChild;
+
+enum class ProcessType : uint8_t {
+  Uninitialized,
+  Parent,
+  Web,
+  Extension,
+  PrivilegedAbout,
+};
+
+template <typename T>
+struct Matcher {
+  virtual bool Matches(T) = 0;
+};
+}  // namespace loader
+
+using namespace mozilla::loader;
+
+class ScriptPreloader : public nsIObserver,
+                        public nsIMemoryReporter,
+                        public nsIRunnable,
+                        public nsINamed,
+                        public nsIAsyncShutdownBlocker,
+                        public SingleWriterLockOwner {
+  MOZ_DEFINE_MALLOC_SIZE_OF(MallocSizeOf)
+
+  friend class mozilla::loader::ScriptCacheChild;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+  NS_DECL_NSIMEMORYREPORTER
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSINAMED
+  NS_DECL_NSIASYNCSHUTDOWNBLOCKER
+
+ private:
+  static StaticRefPtr<ScriptPreloader> gScriptPreloader;
+  static StaticRefPtr<ScriptPreloader> gChildScriptPreloader;
+  static UniquePtr<AutoMemMap> gCacheData;
+  static UniquePtr<AutoMemMap> gChildCacheData;
+
+ public:
+  static ScriptPreloader& GetSingleton();
+  static ScriptPreloader& GetChildSingleton();
+
+  static void DeleteSingleton();
+  static void DeleteCacheDataSingleton();
+
+  static ProcessType GetChildProcessType(const nsACString& remoteType);
+
+  // Fill some options that should be consistent across all scripts stored
+  // into preloader cache.
+  static void FillCompileOptionsForCachedStencil(JS::CompileOptions& options);
+  static void FillDecodeOptionsForCachedStencil(JS::DecodeOptions& options);
+
+  bool OnWritingThread() const override { return NS_IsMainThread(); }
+
+  // Retrieves the stencil with the given cache key from the cache.
+  // Returns null if the stencil is not cached.
+  already_AddRefed<JS::Stencil> GetCachedStencil(
+      JSContext* cx, const JS::DecodeOptions& options, const nsCString& path);
+
+  // Notes the execution of a script with the given URL and cache key.
+  // Depending on the stage of startup, the script may be serialized and
+  // stored to the startup script cache.
+  //
+  // If isRunOnce is true, this script is expected to run only once per
+  // process per browser session. A cached instance will not be kept alive
+  // for repeated execution.
+  void NoteStencil(const nsCString& url, const nsCString& cachePath,
+                   JS::Stencil* stencil, bool isRunOnce = false);
+
+  // Notes the IPC arrival of the XDR data of a stencil compiled by some
+  // child process. See ScriptCacheChild::SendScriptsAndFinalize.
+  void NoteStencil(const nsCString& url, const nsCString& cachePath,
+                   ProcessType processType, nsTArray<uint8_t>&& xdrData,
+                   TimeStamp loadTime);
+
+  // Initializes the script cache from the startup script cache file.
+  Result<Ok, nsresult> InitCache(const nsAString& = u"scriptCache"_ns);
+
+  Result<Ok, nsresult> InitCache(const Maybe<ipc::FileDescriptor>& cacheFile,
+                                 ScriptCacheChild* cacheChild);
+
+  bool Active() const { return mCacheInitialized && !mStartupFinished; }
+
+ private:
+  Result<Ok, nsresult> InitCacheInternal(JS::Handle<JSObject*> scope = nullptr);
+  already_AddRefed<JS::Stencil> GetCachedStencilInternal(
+      JSContext* cx, const JS::DecodeOptions& options, const nsCString& path);
+
+ public:
+  static ProcessType CurrentProcessType() {
+    MOZ_ASSERT(sProcessType != ProcessType::Uninitialized);
+    return sProcessType;
+  }
+
+  static void InitContentChild(dom::ContentParent& parent);
+
+ protected:
+  virtual ~ScriptPreloader();
+
+ private:
+  enum class ScriptStatus {
+    Restored,
+    Saved,
+  };
+
+  // Represents a cached script stencil, either initially read from the
+  // cache file, to be added to the next session's stencil cache file, or
+  // both.
+  //
+  //  - Read from the cache, and being decoded off thread. In this case,
+  //    mReadyToExecute is false, and mToken is null.
+  //  - Off-thread decode has finished, but the stencil has not yet been
+  //    executed. In this case, mReadyToExecute is true, and mToken has a
+  //    non-null value.
+  //  - Read from the cache, but too small or needed to immediately to be
+  //    compiled off-thread. In this case, mReadyToExecute is true, and both
+  //    mToken and mStencil are null.
+  //  - Fully decoded, and ready to be added to the next session's cache
+  //    file. In this case, mReadyToExecute is true, and mStencil is non-null.
+  //
+  // A stencil to be added to the next session's cache file always has a
+  // non-null mStencil value. If it was read from the last session's cache
+  // file, it also has a non-empty mXDRRange range, which will be stored in
+  // the next session's cache file. If it was compiled in this session, its
+  // mXDRRange will initially be empty, and its mXDRData buffer will be
+  // populated just before it is written to the cache file.
+  class CachedStencil : public LinkedListElement<CachedStencil> {
+   public:
+    CachedStencil(CachedStencil&&) = delete;
+
+    CachedStencil(ScriptPreloader& cache, const nsCString& url,
+                  const nsCString& cachePath, JS::Stencil* stencil)
+        : mCache(cache),
+          mURL(url),
+          mCachePath(cachePath),
+          mStencil(stencil),
+          mReadyToExecute(true),
+          mIsRunOnce(false) {}
+
+    inline CachedStencil(ScriptPreloader& cache, InputBuffer& buf);
+
+    ~CachedStencil() = default;
+
+    ScriptStatus Status() const {
+      return mProcessTypes.isEmpty() ? ScriptStatus::Restored
+                                     : ScriptStatus::Saved;
+    }
+
+    // For use with nsTArray::Sort.
+    //
+    // Orders scripts by script load time, so that scripts which are needed
+    // earlier are stored earlier, and scripts needed at approximately the
+    // same time are stored approximately contiguously.
+    struct Comparator {
+      bool Equals(const CachedStencil* a, const CachedStencil* b) const {
+        return a->mLoadTime == b->mLoadTime;
+      }
+
+      bool LessThan(const CachedStencil* a, const CachedStencil* b) const {
+        return a->mLoadTime < b->mLoadTime;
+      }
+    };
+
+    struct StatusMatcher final : public Matcher<CachedStencil*> {
+      explicit StatusMatcher(ScriptStatus status) : mStatus(status) {}
+
+      virtual bool Matches(CachedStencil* script) override {
+        return script->Status() == mStatus;
+      }
+
+      const ScriptStatus mStatus;
+    };
+
+    void FreeData() {
+      // If the script data isn't mmapped, we need to release both it
+      // and the Range that points to it at the same time.
+      if (!IsMemMapped()) {
+        mXDRRange.reset();
+        mXDRData.destroy();
+      }
+    }
+
+    void UpdateLoadTime(const TimeStamp& loadTime) {
+      if (mLoadTime.IsNull() || loadTime < mLoadTime) {
+        mLoadTime = loadTime;
+      }
+    }
+
+    // Checks whether the cached JSScript for this entry will be needed
+    // again and, if not, drops it and returns true. This is the case for
+    // run-once scripts that do not still need to be encoded into the
+    // cache.
+    //
+    // If this method returns false, callers may set mScript to a cached
+    // JSScript instance for this entry. If it returns true, they should
+    // not.
+    bool MaybeDropStencil() {
+      if (mIsRunOnce && (HasRange() || !mCache.WillWriteScripts())) {
+        mStencil = nullptr;
+        return true;
+      }
+      return false;
+    }
+
+    // Encodes this script into XDR data, and stores the result in mXDRData.
+    // Returns true on success, false on failure.
+    bool XDREncode(JSContext* cx);
+
+    // Encodes or decodes this script, in the storage format required by the
+    // script cache file.
+    template <typename Buffer>
+    void Code(Buffer& buffer) {
+      buffer.codeString(mURL);
+      buffer.codeString(mCachePath);
+      buffer.codeUint32(mOffset);
+      buffer.codeUint32(mSize);
+      buffer.codeUint8(mProcessTypes);
+    }
+
+    // Returns the XDR data generated for this script during this session. See
+    // mXDRData.
+    JS::TranscodeBuffer& Buffer() {
+      MOZ_ASSERT(HasBuffer());
+      return mXDRData.ref<JS::TranscodeBuffer>();
+    }
+
+    bool HasBuffer() { return mXDRData.constructed<JS::TranscodeBuffer>(); }
+
+    // Returns the read-only XDR data for this script. See mXDRRange.
+    const JS::TranscodeRange& Range() {
+      MOZ_ASSERT(HasRange());
+      return mXDRRange.ref();
+    }
+
+    bool HasRange() { return mXDRRange.isSome(); }
+
+    bool IsMemMapped() const { return mXDRData.empty(); }
+
+    nsTArray<uint8_t>& Array() {
+      MOZ_ASSERT(HasArray());
+      return mXDRData.ref<nsTArray<uint8_t>>();
+    }
+
+    bool HasArray() { return mXDRData.constructed<nsTArray<uint8_t>>(); }
+
+    already_AddRefed<JS::Stencil> GetStencil(JSContext* cx,
+                                             const JS::DecodeOptions& options);
+
+    size_t HeapSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+      auto size = mallocSizeOf(this);
+
+      if (HasArray()) {
+        size += Array().ShallowSizeOfExcludingThis(mallocSizeOf);
+      } else if (HasBuffer()) {
+        size += Buffer().sizeOfExcludingThis(mallocSizeOf);
+      }
+
+      if (mStencil) {
+        size += JS::SizeOfStencil(mStencil, mallocSizeOf);
+      }
+
+      // Note: mURL and mCachePath use the same string for scripts loaded
+      // by the message manager. The following statement avoids
+      // double-measuring in that case.
+      size += (mURL.SizeOfExcludingThisIfUnshared(mallocSizeOf) +
+               mCachePath.SizeOfExcludingThisEvenIfShared(mallocSizeOf));
+
+      return size;
+    }
+
+    ScriptPreloader& mCache;
+
+    // The URL from which this script was initially read and compiled.
+    nsCString mURL;
+    // A unique identifier for this script's filesystem location, used as a
+    // primary cache lookup value.
+    nsCString mCachePath;
+
+    // The offset of this script in the cache file, from the start of the XDR
+    // data block.
+    uint32_t mOffset = 0;
+    // The size of this script's encoded XDR data.
+    uint32_t mSize = 0;
+
+    TimeStamp mLoadTime{};
+
+    RefPtr<JS::Stencil> mStencil;
+
+    // True if this script is ready to be executed. This means that either the
+    // off-thread portion of an off-thread decode has finished, or the script
+    // is too small to be decoded off-thread, and may be immediately decoded
+    // whenever it is first executed.
+    bool mReadyToExecute = false;
+
+    // True if this script is expected to run once per process. If so, its
+    // JSScript instance will be dropped as soon as the script has
+    // executed and been encoded into the cache.
+    bool mIsRunOnce = false;
+
+    // The set of processes in which this script has been used.
+    EnumSet<ProcessType> mProcessTypes{};
+
+    // The set of processes which the script was loaded into during the
+    // last session, as read from the cache file.
+    EnumSet<ProcessType> mOriginalProcessTypes{};
+
+    // The read-only XDR data for this script, which was either read from an
+    // existing cache file, or generated by encoding a script which was
+    // compiled during this session.
+    Maybe<JS::TranscodeRange> mXDRRange;
+
+    // XDR data which was generated from a script compiled during this
+    // session, and will be written to the cache file.
+    //
+    // The format is JS::TranscodeBuffer if the script was XDR'd as part
+    // of this process, or nsTArray<> if the script was transfered by IPC
+    // from a child process.
+    MaybeOneOf<JS::TranscodeBuffer, nsTArray<uint8_t>> mXDRData;
+  } JS_HAZ_NON_GC_POINTER;
+
+  template <ScriptStatus status>
+  static Matcher<CachedStencil*>* Match() {
+    static CachedStencil::StatusMatcher matcher{status};
+    return &matcher;
+  }
+
+  // There's a significant setup cost for each off-thread decode operation,
+  // so scripts are decoded in chunks to minimize the overhead. There's a
+  // careful balancing act in choosing the size of chunks, to minimize the
+  // number of decode operations, while also minimizing the number of buffer
+  // underruns that require the main thread to wait for a script to finish
+  // decoding.
+  //
+  // For the first chunk, we don't have much time between the start of the
+  // decode operation and the time the first script is needed, so that chunk
+  // needs to be fairly small. After the first chunk is finished, we have
+  // some buffered scripts to fall back on, and a lot more breathing room,
+  // so the chunks can be a bit bigger, but still not too big.
+  static constexpr int OFF_THREAD_FIRST_CHUNK_SIZE = 128 * 1024;
+  static constexpr int OFF_THREAD_CHUNK_SIZE = 512 * 1024;
+
+  // Ideally, we want every chunk to be smaller than the chunk sizes
+  // specified above. However, if we have some number of small scripts
+  // followed by a huge script that would put us over the normal chunk size,
+  // we're better off processing them as a single chunk.
+  //
+  // In order to guarantee that the JS engine will process a chunk
+  // off-thread, it needs to be at least 100K (which is an implementation
+  // detail that can change at any time), so make sure that we always hit at
+  // least that size, with a bit of breathing room to be safe.
+  static constexpr int SMALL_SCRIPT_CHUNK_THRESHOLD = 128 * 1024;
+
+  // The maximum size of scripts to re-decode on the main thread if off-thread
+  // decoding hasn't finished yet. In practice, we don't hit this very often,
+  // but when we do, re-decoding some smaller scripts on the main thread gives
+  // the background decoding a chance to catch up without blocking the main
+  // thread for quite as long.
+  static constexpr int MAX_MAINTHREAD_DECODE_SIZE = 50 * 1024;
+
+  explicit ScriptPreloader(AutoMemMap* cacheData);
+
+  void Cleanup();
+
+  void FinishPendingParses(MonitorAutoLock& aMal);
+  void InvalidateCache();
+
+  // Opens the cache file for reading.
+  Result<Ok, nsresult> OpenCache();
+
+  // Writes a new cache file to disk. Must not be called on the main thread.
+  Result<Ok, nsresult> WriteCache() MOZ_REQUIRES(mSaveMonitor);
+
+  void StartCacheWrite();
+
+  // Prepares scripts for writing to the cache, serializing new scripts to
+  // XDR, and calculating their size-based offsets.
+  void PrepareCacheWrite();
+
+  void PrepareCacheWriteInternal();
+
+  void CacheWriteComplete();
+
+  void FinishContentStartup();
+
+  // Returns true if scripts added to the cache now will be encoded and
+  // written to the cache. If we've already encoded scripts for the cache
+  // write, or this is a content process which hasn't been asked to return
+  // script bytecode, this will return false.
+  bool WillWriteScripts();
+
+  // Returns a file pointer for the cache file with the given name in the
+  // current profile.
+  Result<nsCOMPtr<nsIFile>, nsresult> GetCacheFile(const nsAString& suffix);
+
+  // Waits for the given cached script to finish compiling off-thread, or
+  // decodes it synchronously on the main thread, as appropriate.
+  already_AddRefed<JS::Stencil> WaitForCachedStencil(
+      JSContext* cx, const JS::DecodeOptions& options, CachedStencil* script);
+
+  void DecodeNextBatch(size_t chunkSize, JS::Handle<JSObject*> scope = nullptr);
+
+  static void OffThreadDecodeCallback(JS::OffThreadToken* token, void* context);
+  void FinishOffThreadDecode(JS::OffThreadToken* token);
+  void DoFinishOffThreadDecode();
+
+  already_AddRefed<nsIAsyncShutdownClient> GetShutdownBarrier();
+
+  size_t ShallowHeapSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+    return (mallocSizeOf(this) +
+            mScripts.ShallowSizeOfExcludingThis(mallocSizeOf) +
+            mallocSizeOf(mSaveThread.get()) + mallocSizeOf(mProfD.get()));
+  }
+
+  using ScriptHash = nsClassHashtable<nsCStringHashKey, CachedStencil>;
+
+  template <ScriptStatus status>
+  static size_t SizeOfHashEntries(ScriptHash& scripts,
+                                  mozilla::MallocSizeOf mallocSizeOf) {
+    size_t size = 0;
+    for (auto elem : IterHash(scripts, Match<status>())) {
+      size += elem->HeapSizeOfIncludingThis(mallocSizeOf);
+    }
+    return size;
+  }
+
+  ScriptHash mScripts;
+
+  // True after we've shown the first window, and are no longer adding new
+  // scripts to the cache.
+  bool mStartupFinished = false;
+
+  bool mCacheInitialized = false;
+  bool mSaveComplete = false;
+  bool mDataPrepared = false;
+  // May only be changed on the main thread, while `mSaveMonitor` is held.
+  bool mCacheInvalidated MOZ_GUARDED_BY(mSaveMonitor) = false;
+
+  // The list of scripts that we read from the initial startup cache file,
+  // but have yet to initiate a decode task for.
+  LinkedList<CachedStencil> mPendingScripts;
+
+  // The lists of scripts and their sources that make up the chunk currently
+  // being decoded in a background thread.
+  JS::TranscodeSources mParsingSources;
+  Vector<CachedStencil*> mParsingScripts;
+
+  // The token for the completed off-thread decode task.
+  Atomic<JS::OffThreadToken*, ReleaseAcquire> mToken{nullptr};
+
+  // True if a runnable has been dispatched to the main thread to finish an
+  // off-thread decode operation. Access only while 'mMonitor' is held.
+  bool mFinishDecodeRunnablePending MOZ_GUARDED_BY(mMonitor) = false;
+
+  // True is main-thread is blocked and we should notify with Monitor. Access
+  // only while `mMonitor` is held.
+  bool mWaitingForDecode MOZ_GUARDED_BY(mMonitor) = false;
+
+  // The process type of the current process.
+  static ProcessType sProcessType;
+
+  // The process types for which remote processes have been initialized, and
+  // are expected to send back script data.
+  EnumSet<ProcessType> mInitializedProcesses{};
+
+  RefPtr<ScriptPreloader> mChildCache;
+  ScriptCacheChild* mChildActor = nullptr;
+
+  nsString mBaseName;
+  nsCString mContentStartupFinishedTopic;
+
+  nsCOMPtr<nsIFile> mProfD;
+  nsCOMPtr<nsIThread> mSaveThread;
+  nsCOMPtr<nsITimer> mSaveTimer;
+
+  // The mmapped cache data from this session's cache file.
+  // The instance is held by either `gCacheData` or `gChildCacheData` static
+  // fields, and its lifetime is guaranteed to be longer than ScriptPreloader
+  // instance.
+  AutoMemMap* mCacheData;
+
+  Monitor mMonitor;
+  MonitorSingleWriter mSaveMonitor MOZ_ACQUIRED_BEFORE(mMonitor);
+};
+
+}  // namespace mozilla
+
+#endif  // ScriptPreloader_h
diff --git a/js/xpconnect/loader/SkipCheckForBrokenURLOrZeroSized.h b/js/xpconnect/loader/SkipCheckForBrokenURLOrZeroSized.h
new file mode 100644
index 0000000000..3a413b898f
--- /dev/null
+++ b/js/xpconnect/loader/SkipCheckForBrokenURLOrZeroSized.h
@@ -0,0 +1,22 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_loader_SkipCheckForBrokenURLOrZeroSized_h
+#define mozilla_loader_SkipCheckForBrokenURLOrZeroSized_h
+
+#include <stdint.h>  // uint8_t
+
+namespace mozilla {
+namespace loader {
+
+// Represents the `aSkipCheckForBrokenURLOrZeroSized` parameter for
+// `NS_NewChannel` function.
+enum class SkipCheckForBrokenURLOrZeroSized : uint8_t { No, Yes };
+
+}  // namespace loader
+}  // namespace mozilla
+
+#endif  // mozilla_loader_SkipCheckForBrokenURLOrZeroSized_h
diff --git a/js/xpconnect/loader/URLPreloader.cpp b/js/xpconnect/loader/URLPreloader.cpp
new file mode 100644
index 0000000000..c5bcbad5bb
--- /dev/null
+++ b/js/xpconnect/loader/URLPreloader.cpp
@@ -0,0 +1,707 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptPreloader-inl.h"
+#include "mozilla/URLPreloader.h"
+#include "mozilla/loader/AutoMemMap.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/FileUtils.h"
+#include "mozilla/IOBuffers.h"
+#include "mozilla/Logging.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Services.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Vector.h"
+#include "mozilla/scache/StartupCache.h"
+
+#include "crc32c.h"
+#include "MainThreadUtils.h"
+#include "nsPrintfCString.h"
+#include "nsDebug.h"
+#include "nsIFile.h"
+#include "nsIFileURL.h"
+#include "nsNetUtil.h"
+#include "nsPromiseFlatString.h"
+#include "nsProxyRelease.h"
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+#include "nsZipArchive.h"
+#include "xpcpublic.h"
+
+namespace mozilla {
+namespace {
+static LazyLogModule gURLLog("URLPreloader");
+
+#define LOG(level, ...) MOZ_LOG(gURLLog, LogLevel::level, (__VA_ARGS__))
+
+template <typename T>
+bool StartsWith(const T& haystack, const T& needle) {
+  return StringHead(haystack, needle.Length()) == needle;
+}
+}  // anonymous namespace
+
+using namespace mozilla::loader;
+using mozilla::scache::StartupCache;
+
+nsresult URLPreloader::CollectReports(nsIHandleReportCallback* aHandleReport,
+                                      nsISupports* aData, bool aAnonymize) {
+  MOZ_COLLECT_REPORT("explicit/url-preloader/other", KIND_HEAP, UNITS_BYTES,
+                     ShallowSizeOfIncludingThis(MallocSizeOf),
+                     "Memory used by the URL preloader service itself.");
+
+  for (const auto& elem : mCachedURLs.Values()) {
+    nsAutoCString pathName;
+    pathName.Append(elem->mPath);
+    // The backslashes will automatically be replaced with slashes in
+    // about:memory, without splitting each path component into a separate
+    // branch in the memory report tree.
+    pathName.ReplaceChar('/', '\\');
+
+    nsPrintfCString path("explicit/url-preloader/cached-urls/%s/[%s]",
+                         elem->TypeString(), pathName.get());
+
+    aHandleReport->Callback(
+        ""_ns, path, KIND_HEAP, UNITS_BYTES,
+        elem->SizeOfIncludingThis(MallocSizeOf),
+        nsLiteralCString("Memory used to hold cache data for files which "
+                         "have been read or pre-loaded during this session."),
+        aData);
+  }
+
+  return NS_OK;
+}
+
+// static
+already_AddRefed<URLPreloader> URLPreloader::Create(bool* aInitialized) {
+  // The static APIs like URLPreloader::Read work in the child process because
+  // they fall back to a synchronous read. The actual preloader must be
+  // explicitly initialized, and this should only be done in the parent.
+  MOZ_RELEASE_ASSERT(XRE_IsParentProcess());
+
+  RefPtr<URLPreloader> preloader = new URLPreloader();
+  if (preloader->InitInternal().isOk()) {
+    *aInitialized = true;
+    RegisterWeakMemoryReporter(preloader);
+  } else {
+    *aInitialized = false;
+  }
+
+  return preloader.forget();
+}
+
+URLPreloader& URLPreloader::GetSingleton() {
+  if (!sSingleton) {
+    sSingleton = Create(&sInitialized);
+    ClearOnShutdown(&sSingleton);
+  }
+
+  return *sSingleton;
+}
+
+bool URLPreloader::sInitialized = false;
+
+StaticRefPtr<URLPreloader> URLPreloader::sSingleton;
+
+URLPreloader::~URLPreloader() {
+  if (sInitialized) {
+    UnregisterWeakMemoryReporter(this);
+    sInitialized = false;
+  }
+}
+
+Result<Ok, nsresult> URLPreloader::InitInternal() {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+
+  if (Omnijar::HasOmnijar(Omnijar::GRE)) {
+    MOZ_TRY(Omnijar::GetURIString(Omnijar::GRE, mGREPrefix));
+  }
+  if (Omnijar::HasOmnijar(Omnijar::APP)) {
+    MOZ_TRY(Omnijar::GetURIString(Omnijar::APP, mAppPrefix));
+  }
+
+  nsresult rv;
+  nsCOMPtr<nsIIOService> ios = do_GetIOService(&rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIProtocolHandler> ph;
+  MOZ_TRY(ios->GetProtocolHandler("resource", getter_AddRefs(ph)));
+
+  mResProto = do_QueryInterface(ph, &rv);
+  MOZ_TRY(rv);
+
+  mChromeReg = services::GetChromeRegistry();
+  if (!mChromeReg) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  MOZ_TRY(NS_GetSpecialDirectory("ProfLDS", getter_AddRefs(mProfD)));
+
+  return Ok();
+}
+
+URLPreloader& URLPreloader::ReInitialize() {
+  MOZ_ASSERT(sSingleton);
+  sSingleton = nullptr;
+  sSingleton = Create(&sInitialized);
+  return *sSingleton;
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> URLPreloader::GetCacheFile(
+    const nsAString& suffix) {
+  if (!mProfD) {
+    return Err(NS_ERROR_NOT_INITIALIZED);
+  }
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY(mProfD->Clone(getter_AddRefs(cacheFile)));
+
+  MOZ_TRY(cacheFile->AppendNative("startupCache"_ns));
+  Unused << cacheFile->Create(nsIFile::DIRECTORY_TYPE, 0777);
+
+  MOZ_TRY(cacheFile->Append(u"urlCache"_ns + suffix));
+
+  return std::move(cacheFile);
+}
+
+static const uint8_t URL_MAGIC[] = "mozURLcachev003";
+
+Result<nsCOMPtr<nsIFile>, nsresult> URLPreloader::FindCacheFile() {
+  if (StartupCache::GetIgnoreDiskCache()) {
+    return Err(NS_ERROR_ABORT);
+  }
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY_VAR(cacheFile, GetCacheFile(u".bin"_ns));
+
+  bool exists;
+  MOZ_TRY(cacheFile->Exists(&exists));
+  if (exists) {
+    MOZ_TRY(cacheFile->MoveTo(nullptr, u"urlCache-current.bin"_ns));
+  } else {
+    MOZ_TRY(cacheFile->SetLeafName(u"urlCache-current.bin"_ns));
+    MOZ_TRY(cacheFile->Exists(&exists));
+    if (!exists) {
+      return Err(NS_ERROR_FILE_NOT_FOUND);
+    }
+  }
+
+  return std::move(cacheFile);
+}
+
+Result<Ok, nsresult> URLPreloader::WriteCache() {
+  MOZ_ASSERT(!NS_IsMainThread());
+  MOZ_DIAGNOSTIC_ASSERT(mStartupFinished);
+
+  // The script preloader might call us a second time, if it has to re-write
+  // its cache after a cache flush. We don't care about cache flushes, since
+  // our cache doesn't store any file data, only paths. And we currently clear
+  // our cached file list after the first write, which means that a second
+  // write would (aside from breaking the invariant that we never touch
+  // mCachedURLs off-main-thread after the first write, and trigger a data
+  // race) mean we get no pre-loading on the next startup.
+  if (mCacheWritten) {
+    return Ok();
+  }
+  mCacheWritten = true;
+
+  LOG(Debug, "Writing cache...");
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY_VAR(cacheFile, GetCacheFile(u"-new.bin"_ns));
+
+  bool exists;
+  MOZ_TRY(cacheFile->Exists(&exists));
+  if (exists) {
+    MOZ_TRY(cacheFile->Remove(false));
+  }
+
+  {
+    AutoFDClose fd;
+    MOZ_TRY(cacheFile->OpenNSPRFileDesc(PR_WRONLY | PR_CREATE_FILE, 0644,
+                                        &fd.rwget()));
+
+    nsTArray<URLEntry*> entries;
+    for (const auto& entry : mCachedURLs.Values()) {
+      if (entry->mReadTime) {
+        entries.AppendElement(entry.get());
+      }
+    }
+
+    entries.Sort(URLEntry::Comparator());
+
+    OutputBuffer buf;
+    for (auto entry : entries) {
+      entry->Code(buf);
+    }
+
+    uint8_t headerSize[4];
+    LittleEndian::writeUint32(headerSize, buf.cursor());
+
+    uint8_t crc[4];
+    LittleEndian::writeUint32(crc, ComputeCrc32c(~0, buf.Get(), buf.cursor()));
+
+    MOZ_TRY(Write(fd, URL_MAGIC, sizeof(URL_MAGIC)));
+    MOZ_TRY(Write(fd, headerSize, sizeof(headerSize)));
+    MOZ_TRY(Write(fd, crc, sizeof(crc)));
+    MOZ_TRY(Write(fd, buf.Get(), buf.cursor()));
+  }
+
+  MOZ_TRY(cacheFile->MoveTo(nullptr, u"urlCache.bin"_ns));
+
+  NS_DispatchToMainThread(
+      NewRunnableMethod("URLPreloader::Cleanup", this, &URLPreloader::Cleanup));
+
+  return Ok();
+}
+
+void URLPreloader::Cleanup() { mCachedURLs.Clear(); }
+
+Result<Ok, nsresult> URLPreloader::ReadCache(
+    LinkedList<URLEntry>& pendingURLs) {
+  LOG(Debug, "Reading cache...");
+
+  nsCOMPtr<nsIFile> cacheFile;
+  MOZ_TRY_VAR(cacheFile, FindCacheFile());
+
+  AutoMemMap cache;
+  MOZ_TRY(cache.init(cacheFile));
+
+  auto size = cache.size();
+
+  uint32_t headerSize;
+  uint32_t crc;
+  if (size < sizeof(URL_MAGIC) + sizeof(headerSize) + sizeof(crc)) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  auto data = cache.get<uint8_t>();
+  auto end = data + size;
+
+  if (memcmp(URL_MAGIC, data.get(), sizeof(URL_MAGIC))) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+  data += sizeof(URL_MAGIC);
+
+  headerSize = LittleEndian::readUint32(data.get());
+  data += sizeof(headerSize);
+
+  crc = LittleEndian::readUint32(data.get());
+  data += sizeof(crc);
+
+  if (data + headerSize > end) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  if (crc != ComputeCrc32c(~0, data.get(), headerSize)) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  {
+    mMonitor.AssertCurrentThreadOwns();
+
+    auto cleanup = MakeScopeExit([&]() {
+      while (auto* elem = pendingURLs.getFirst()) {
+        elem->remove();
+      }
+      mCachedURLs.Clear();
+    });
+
+    Range<uint8_t> header(data, data + headerSize);
+    data += headerSize;
+
+    InputBuffer buf(header);
+    while (!buf.finished()) {
+      CacheKey key(buf);
+
+      LOG(Debug, "Cached file: %s %s", key.TypeString(), key.mPath.get());
+
+      // Don't bother doing anything else if the key didn't load correctly.
+      // We're going to throw it out right away, and it is possible that this
+      // leads to pendingURLs getting into a weird state.
+      if (buf.error()) {
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+
+      auto entry = mCachedURLs.GetOrInsertNew(key, key);
+      entry->mResultCode = NS_ERROR_NOT_INITIALIZED;
+
+      if (entry->isInList()) {
+#ifdef NIGHTLY_BUILD
+        MOZ_DIAGNOSTIC_ASSERT(pendingURLs.contains(entry),
+                              "Entry should be in pendingURLs");
+        MOZ_DIAGNOSTIC_ASSERT(key.mPath.Length() > 0,
+                              "Path should be non-empty");
+        MOZ_DIAGNOSTIC_ASSERT(false, "Entry should be new and not in any list");
+#endif
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+
+      pendingURLs.insertBack(entry);
+    }
+
+    MOZ_RELEASE_ASSERT(!buf.error(),
+                       "We should have already bailed on an error");
+
+    cleanup.release();
+  }
+
+  return Ok();
+}
+
+void URLPreloader::BackgroundReadFiles() {
+  auto cleanup = MakeScopeExit([&]() {
+    auto lock = mReaderThread.Lock();
+    auto& readerThread = lock.ref();
+    NS_DispatchToMainThread(NewRunnableMethod(
+        "nsIThread::AsyncShutdown", readerThread, &nsIThread::AsyncShutdown));
+
+    readerThread = nullptr;
+  });
+
+  Vector<nsZipCursor> cursors;
+  LinkedList<URLEntry> pendingURLs;
+  {
+    MonitorAutoLock mal(mMonitor);
+
+    if (ReadCache(pendingURLs).isErr()) {
+      mReaderInitialized = true;
+      mal.NotifyAll();
+      return;
+    }
+
+    int numZipEntries = 0;
+    for (auto entry : pendingURLs) {
+      if (entry->mType != entry->TypeFile) {
+        numZipEntries++;
+      }
+    }
+    MOZ_RELEASE_ASSERT(cursors.reserve(numZipEntries));
+
+    // Initialize the zip cursors for all files in Omnijar while the monitor
+    // is locked. Omnijar is not threadsafe, so the caller of
+    // AutoBeginReading guard must ensure that no code accesses Omnijar
+    // until this segment is done. Once the cursors have been initialized,
+    // the actual reading and decompression can safely be done off-thread,
+    // as is the case for thread-retargeted jar: channels.
+    for (auto entry : pendingURLs) {
+      if (entry->mType == entry->TypeFile) {
+        continue;
+      }
+
+      RefPtr<nsZipArchive> zip = entry->Archive();
+      if (!zip) {
+        MOZ_CRASH_UNSAFE_PRINTF(
+            "Failed to get Omnijar %s archive for entry (path: \"%s\")",
+            entry->TypeString(), entry->mPath.get());
+      }
+
+      auto item = zip->GetItem(entry->mPath.get());
+      if (!item) {
+        entry->mResultCode = NS_ERROR_FILE_NOT_FOUND;
+        continue;
+      }
+
+      size_t size = item->RealSize();
+
+      entry->mData.SetLength(size);
+      auto data = entry->mData.BeginWriting();
+
+      cursors.infallibleEmplaceBack(item, zip, reinterpret_cast<uint8_t*>(data),
+                                    size, true);
+    }
+
+    mReaderInitialized = true;
+    mal.NotifyAll();
+  }
+
+  // Loop over the entries, read the file's contents, store them in the
+  // entry's mData pointer, and notify any waiting threads to check for
+  // completion.
+  uint32_t i = 0;
+  for (auto entry : pendingURLs) {
+    // If there is any other error code, the entry has already failed at
+    // this point, so don't bother trying to read it again.
+    if (entry->mResultCode != NS_ERROR_NOT_INITIALIZED) {
+      continue;
+    }
+
+    nsresult rv = NS_OK;
+
+    LOG(Debug, "Background reading %s file %s", entry->TypeString(),
+        entry->mPath.get());
+
+    if (entry->mType == entry->TypeFile) {
+      auto result = entry->Read();
+      if (result.isErr()) {
+        rv = result.unwrapErr();
+      }
+    } else {
+      auto& cursor = cursors[i++];
+
+      uint32_t len;
+      cursor.Copy(&len);
+      if (len != entry->mData.Length()) {
+        entry->mData.Truncate();
+        rv = NS_ERROR_FAILURE;
+      }
+    }
+
+    entry->mResultCode = rv;
+    mMonitor.NotifyAll();
+  }
+
+  // We're done reading pending entries, so clear the list.
+  pendingURLs.clear();
+}
+
+void URLPreloader::BeginBackgroundRead() {
+  auto lock = mReaderThread.Lock();
+  auto& readerThread = lock.ref();
+  if (!readerThread && !mReaderInitialized && sInitialized) {
+    nsresult rv;
+    rv = NS_NewNamedThread("BGReadURLs", getter_AddRefs(readerThread));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return;
+    }
+
+    nsCOMPtr<nsIRunnable> runnable =
+        NewRunnableMethod("URLPreloader::BackgroundReadFiles", this,
+                          &URLPreloader::BackgroundReadFiles);
+    rv = readerThread->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      // If we can't launch the task, just destroy the thread
+      readerThread = nullptr;
+      return;
+    }
+  }
+}
+
+Result<nsCString, nsresult> URLPreloader::ReadInternal(const CacheKey& key,
+                                                       ReadType readType) {
+  if (mStartupFinished || !mReaderInitialized) {
+    URLEntry entry(key);
+
+    return entry.Read();
+  }
+
+  auto entry = mCachedURLs.GetOrInsertNew(key, key);
+
+  entry->UpdateUsedTime();
+
+  return entry->ReadOrWait(readType);
+}
+
+Result<nsCString, nsresult> URLPreloader::ReadURIInternal(nsIURI* uri,
+                                                          ReadType readType) {
+  CacheKey key;
+  MOZ_TRY_VAR(key, ResolveURI(uri));
+
+  return ReadInternal(key, readType);
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::Read(const CacheKey& key,
+                                                            ReadType readType) {
+  // If we're being called before the preloader has been initialized (i.e.,
+  // before the profile has been initialized), just fall back to a synchronous
+  // read. This happens when we're reading .ini and preference files that are
+  // needed to locate and initialize the profile.
+  if (!sInitialized) {
+    return URLEntry(key).Read();
+  }
+
+  return GetSingleton().ReadInternal(key, readType);
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::ReadURI(
+    nsIURI* uri, ReadType readType) {
+  if (!sInitialized) {
+    return Err(NS_ERROR_NOT_INITIALIZED);
+  }
+
+  return GetSingleton().ReadURIInternal(uri, readType);
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::ReadFile(
+    nsIFile* file, ReadType readType) {
+  return Read(CacheKey(file), readType);
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::Read(
+    FileLocation& location, ReadType readType) {
+  if (location.IsZip()) {
+    if (location.GetBaseZip()) {
+      nsCString path;
+      location.GetPath(path);
+      return ReadZip(location.GetBaseZip(), path);
+    }
+    return URLEntry::ReadLocation(location);
+  }
+
+  nsCOMPtr<nsIFile> file = location.GetBaseFile();
+  return ReadFile(file, readType);
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::ReadZip(
+    nsZipArchive* zip, const nsACString& path, ReadType readType) {
+  // If the zip archive belongs to an Omnijar location, map it to a cache
+  // entry, and cache it as normal. Otherwise, simply read the entry
+  // synchronously, since other JAR archives are currently unsupported by the
+  // cache.
+  RefPtr<nsZipArchive> reader = Omnijar::GetReader(Omnijar::GRE);
+  if (zip == reader) {
+    CacheKey key(CacheKey::TypeGREJar, path);
+    return Read(key, readType);
+  }
+
+  reader = Omnijar::GetReader(Omnijar::APP);
+  if (zip == reader) {
+    CacheKey key(CacheKey::TypeAppJar, path);
+    return Read(key, readType);
+  }
+
+  // Not an Omnijar archive, so just read it directly.
+  FileLocation location(zip, PromiseFlatCString(path).BeginReading());
+  return URLEntry::ReadLocation(location);
+}
+
+Result<URLPreloader::CacheKey, nsresult> URLPreloader::ResolveURI(nsIURI* uri) {
+  nsCString spec;
+  nsCString scheme;
+  MOZ_TRY(uri->GetSpec(spec));
+  MOZ_TRY(uri->GetScheme(scheme));
+
+  nsCOMPtr<nsIURI> resolved;
+
+  // If the URI is a resource: or chrome: URI, first resolve it to the
+  // underlying URI that it wraps.
+  if (scheme.EqualsLiteral("resource")) {
+    MOZ_TRY(mResProto->ResolveURI(uri, spec));
+    MOZ_TRY(NS_NewURI(getter_AddRefs(resolved), spec));
+  } else if (scheme.EqualsLiteral("chrome")) {
+    MOZ_TRY(mChromeReg->ConvertChromeURL(uri, getter_AddRefs(resolved)));
+    MOZ_TRY(resolved->GetSpec(spec));
+  } else {
+    resolved = uri;
+  }
+  MOZ_TRY(resolved->GetScheme(scheme));
+
+  // Try the GRE and App Omnijar prefixes.
+  if (mGREPrefix.Length() && StartsWith(spec, mGREPrefix)) {
+    return CacheKey(CacheKey::TypeGREJar, Substring(spec, mGREPrefix.Length()));
+  }
+
+  if (mAppPrefix.Length() && StartsWith(spec, mAppPrefix)) {
+    return CacheKey(CacheKey::TypeAppJar, Substring(spec, mAppPrefix.Length()));
+  }
+
+  // Try for a file URI.
+  if (scheme.EqualsLiteral("file")) {
+    nsCOMPtr<nsIFileURL> fileURL = do_QueryInterface(resolved);
+    MOZ_ASSERT(fileURL);
+
+    nsCOMPtr<nsIFile> file;
+    MOZ_TRY(fileURL->GetFile(getter_AddRefs(file)));
+
+    nsString path;
+    MOZ_TRY(file->GetPath(path));
+
+    return CacheKey(CacheKey::TypeFile, NS_ConvertUTF16toUTF8(path));
+  }
+
+  // Not a file or Omnijar URI, so currently unsupported.
+  return Err(NS_ERROR_INVALID_ARG);
+}
+
+size_t URLPreloader::ShallowSizeOfIncludingThis(
+    mozilla::MallocSizeOf mallocSizeOf) {
+  return (mallocSizeOf(this) +
+          mAppPrefix.SizeOfExcludingThisEvenIfShared(mallocSizeOf) +
+          mGREPrefix.SizeOfExcludingThisEvenIfShared(mallocSizeOf) +
+          mCachedURLs.ShallowSizeOfExcludingThis(mallocSizeOf));
+}
+
+Result<FileLocation, nsresult> URLPreloader::CacheKey::ToFileLocation() {
+  if (mType == TypeFile) {
+    nsCOMPtr<nsIFile> file;
+    MOZ_TRY(NS_NewLocalFile(NS_ConvertUTF8toUTF16(mPath), false,
+                            getter_AddRefs(file)));
+    return FileLocation(file);
+  }
+
+  RefPtr<nsZipArchive> zip = Archive();
+  return FileLocation(zip, mPath.get());
+}
+
+Result<nsCString, nsresult> URLPreloader::URLEntry::Read() {
+  FileLocation location;
+  MOZ_TRY_VAR(location, ToFileLocation());
+
+  MOZ_TRY_VAR(mData, ReadLocation(location));
+  return mData;
+}
+
+/* static */ Result<nsCString, nsresult> URLPreloader::URLEntry::ReadLocation(
+    FileLocation& location) {
+  FileLocation::Data data;
+  MOZ_TRY(location.GetData(data));
+
+  uint32_t size;
+  MOZ_TRY(data.GetSize(&size));
+
+  nsCString result;
+  result.SetLength(size);
+  MOZ_TRY(data.Copy(result.BeginWriting(), size));
+
+  return std::move(result);
+}
+
+Result<nsCString, nsresult> URLPreloader::URLEntry::ReadOrWait(
+    ReadType readType) {
+  auto now = TimeStamp::Now();
+  LOG(Info, "Reading %s\n", mPath.get());
+  auto cleanup = MakeScopeExit([&]() {
+    LOG(Info, "Read in %fms\n", (TimeStamp::Now() - now).ToMilliseconds());
+  });
+
+  if (mResultCode == NS_ERROR_NOT_INITIALIZED) {
+    MonitorAutoLock mal(GetSingleton().mMonitor);
+
+    while (mResultCode == NS_ERROR_NOT_INITIALIZED) {
+      mal.Wait();
+    }
+  }
+
+  if (mResultCode == NS_OK && mData.IsVoid()) {
+    LOG(Info, "Reading synchronously...\n");
+    return Read();
+  }
+
+  if (NS_FAILED(mResultCode)) {
+    return Err(mResultCode);
+  }
+
+  nsCString res = mData;
+
+  if (readType == Forget) {
+    mData.SetIsVoid(true);
+  }
+  return res;
+}
+
+inline URLPreloader::CacheKey::CacheKey(InputBuffer& buffer) {
+  Code(buffer);
+  MOZ_DIAGNOSTIC_ASSERT(
+      mType == TypeAppJar || mType == TypeGREJar || mType == TypeFile,
+      "mType should be valid");
+}
+
+NS_IMPL_ISUPPORTS(URLPreloader, nsIMemoryReporter)
+
+#undef LOG
+
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/URLPreloader.h b/js/xpconnect/loader/URLPreloader.h
new file mode 100644
index 0000000000..2573fc89a2
--- /dev/null
+++ b/js/xpconnect/loader/URLPreloader.h
@@ -0,0 +1,318 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef URLPreloader_h
+#define URLPreloader_h
+
+#include "mozilla/DataMutex.h"
+#include "mozilla/FileLocation.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Omnijar.h"
+#include "mozilla/Range.h"
+#include "mozilla/Vector.h"
+#include "mozilla/Result.h"
+#include "nsClassHashtable.h"
+#include "nsHashKeys.h"
+#include "nsIChromeRegistry.h"
+#include "nsIFile.h"
+#include "nsIURI.h"
+#include "nsIMemoryReporter.h"
+#include "nsIResProtocolHandler.h"
+#include "nsIThread.h"
+#include "nsReadableUtils.h"
+
+class nsZipArchive;
+
+namespace mozilla {
+namespace loader {
+class InputBuffer;
+}
+
+using namespace mozilla::loader;
+
+class ScriptPreloader;
+
+/**
+ * A singleton class to manage loading local URLs during startup, recording
+ * them, and pre-loading them during early startup in the next session. URLs
+ * that are not already loaded (or already being pre-loaded) when required are
+ * read synchronously from disk, and (if startup is not already complete)
+ * added to the pre-load list for the next session.
+ */
+class URLPreloader final : public nsIMemoryReporter {
+  MOZ_DEFINE_MALLOC_SIZE_OF(MallocSizeOf)
+
+  URLPreloader() = default;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  static URLPreloader& GetSingleton();
+
+  // The type of read operation to perform.
+  enum ReadType {
+    // Read the file and then immediately forget its data.
+    Forget,
+    // Read the file and retain its data for the next caller.
+    Retain,
+  };
+
+  // Helpers to read the contents of files or JAR archive entries with various
+  // representations. If the preloader has not yet been initialized, or the
+  // given location is not supported by the cache, the entries will be read
+  // synchronously, and not stored in the cache.
+  static Result<nsCString, nsresult> Read(FileLocation& location,
+                                          ReadType readType = Forget);
+
+  static Result<nsCString, nsresult> ReadURI(nsIURI* uri,
+                                             ReadType readType = Forget);
+
+  static Result<nsCString, nsresult> ReadFile(nsIFile* file,
+                                              ReadType readType = Forget);
+
+  static Result<nsCString, nsresult> ReadZip(nsZipArchive* archive,
+                                             const nsACString& path,
+                                             ReadType readType = Forget);
+
+  void SetStartupFinished() { mStartupFinished = true; }
+
+ private:
+  struct CacheKey;
+
+  Result<nsCString, nsresult> ReadInternal(const CacheKey& key,
+                                           ReadType readType);
+
+  Result<nsCString, nsresult> ReadURIInternal(nsIURI* uri, ReadType readType);
+
+  Result<nsCString, nsresult> ReadFileInternal(nsIFile* file,
+                                               ReadType readType);
+
+  static Result<nsCString, nsresult> Read(const CacheKey& key,
+                                          ReadType readType);
+
+  static bool sInitialized;
+
+  static mozilla::StaticRefPtr<URLPreloader> sSingleton;
+
+ protected:
+  friend class AddonManagerStartup;
+  friend class ScriptPreloader;
+
+  virtual ~URLPreloader();
+
+  Result<Ok, nsresult> WriteCache();
+
+  static URLPreloader& ReInitialize();
+
+  // Clear leftover entries after the cache has been written.
+  void Cleanup();
+
+  // Begins reading files off-thread, and ensures that initialization has
+  // completed before leaving the current scope. The caller *must* ensure that
+  // no code on the main thread access Omnijar, either directly or indirectly,
+  // for the lifetime of this guard object.
+  struct MOZ_RAII AutoBeginReading final {
+    AutoBeginReading() { GetSingleton().BeginBackgroundRead(); }
+
+    ~AutoBeginReading() {
+      auto& reader = GetSingleton();
+
+      MonitorAutoLock mal(reader.mMonitor);
+
+      while (!reader.mReaderInitialized && URLPreloader::sInitialized) {
+        mal.Wait();
+      }
+    }
+  };
+
+ private:
+  // Represents a key for an entry in the URI cache, based on its file or JAR
+  // location.
+  struct CacheKey {
+    // The type of the entry. TypeAppJar and TypeGREJar entries are in the
+    // app-specific or toolkit Omnijar files, and are handled specially.
+    // TypeFile entries are plain files in the filesystem.
+    enum EntryType : uint8_t {
+      TypeAppJar,
+      TypeGREJar,
+      TypeFile,
+    };
+
+    CacheKey() = default;
+    CacheKey(const CacheKey& other) = default;
+
+    CacheKey(EntryType type, const nsACString& path)
+        : mType(type), mPath(path) {}
+
+    explicit CacheKey(nsIFile* file) : mType(TypeFile) {
+      nsString path;
+      MOZ_ALWAYS_SUCCEEDS(file->GetPath(path));
+      MOZ_DIAGNOSTIC_ASSERT(path.Length() > 0);
+      CopyUTF16toUTF8(path, mPath);
+    }
+
+    explicit inline CacheKey(InputBuffer& buffer);
+
+    // Encodes or decodes the cache key for storage in a session cache file.
+    template <typename Buffer>
+    void Code(Buffer& buffer) {
+      buffer.codeUint8(*reinterpret_cast<uint8_t*>(&mType));
+      buffer.codeString(mPath);
+      MOZ_DIAGNOSTIC_ASSERT(mPath.Length() > 0);
+    }
+
+    uint32_t Hash() const { return HashGeneric(mType, HashString(mPath)); }
+
+    bool operator==(const CacheKey& other) const {
+      return mType == other.mType && mPath == other.mPath;
+    }
+
+    // Returns the Omnijar type for this entry. This may *only* be called
+    // for Omnijar entries.
+    Omnijar::Type OmnijarType() {
+      switch (mType) {
+        case TypeAppJar:
+          return Omnijar::APP;
+        case TypeGREJar:
+          return Omnijar::GRE;
+        default:
+          MOZ_CRASH("Unexpected entry type");
+          return Omnijar::GRE;
+      }
+    }
+
+    const char* TypeString() const {
+      switch (mType) {
+        case TypeAppJar:
+          return "AppJar";
+        case TypeGREJar:
+          return "GREJar";
+        case TypeFile:
+          return "File";
+      }
+      MOZ_ASSERT_UNREACHABLE("no such type");
+      return "";
+    }
+
+    already_AddRefed<nsZipArchive> Archive() {
+      return Omnijar::GetReader(OmnijarType());
+    }
+
+    Result<FileLocation, nsresult> ToFileLocation();
+
+    EntryType mType = TypeFile;
+
+    // The path of the entry. For Type*Jar entries, this is the path within
+    // the Omnijar archive. For TypeFile entries, this is the full path to
+    // the file.
+    nsCString mPath{};
+  };
+
+  // Represents an entry in the URI cache.
+  struct URLEntry final : public CacheKey, public LinkedListElement<URLEntry> {
+    MOZ_IMPLICIT URLEntry(const CacheKey& key)
+        : CacheKey(key), mData(VoidCString()) {}
+
+    explicit URLEntry(nsIFile* file) : CacheKey(file) {}
+
+    // For use with nsTArray::Sort.
+    //
+    // Sorts entries by the time they were initially read during this
+    // session.
+    struct Comparator final {
+      bool Equals(const URLEntry* a, const URLEntry* b) const {
+        return a->mReadTime == b->mReadTime;
+      }
+
+      bool LessThan(const URLEntry* a, const URLEntry* b) const {
+        return a->mReadTime < b->mReadTime;
+      }
+    };
+
+    // Sets the first-used time of this file to the earlier of its current
+    // first-use time or the given timestamp.
+    void UpdateUsedTime(const TimeStamp& time = TimeStamp::Now()) {
+      if (!mReadTime || time < mReadTime) {
+        mReadTime = time;
+      }
+    }
+
+    Result<nsCString, nsresult> Read();
+    static Result<nsCString, nsresult> ReadLocation(FileLocation& location);
+
+    size_t SizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+      return (mallocSizeOf(this) +
+              mPath.SizeOfExcludingThisEvenIfShared(mallocSizeOf) +
+              mData.SizeOfExcludingThisEvenIfShared(mallocSizeOf));
+    }
+
+    // Reads the contents of the file referenced by this entry, or wait for
+    // an off-thread read operation to finish if it is currently pending,
+    // and return the file's contents.
+    Result<nsCString, nsresult> ReadOrWait(ReadType readType);
+
+    nsCString mData;
+
+    TimeStamp mReadTime{};
+
+    nsresult mResultCode = NS_OK;
+  };
+
+  // Resolves the given URI to a CacheKey, if the URI is cacheable.
+  Result<CacheKey, nsresult> ResolveURI(nsIURI* uri);
+
+  static already_AddRefed<URLPreloader> Create(bool* aInitialized);
+
+  Result<Ok, nsresult> InitInternal();
+
+  // Returns a file pointer to the (possibly nonexistent) cache file with the
+  // given suffix.
+  Result<nsCOMPtr<nsIFile>, nsresult> GetCacheFile(const nsAString& suffix);
+  // Finds the correct cache file to use for this session.
+  Result<nsCOMPtr<nsIFile>, nsresult> FindCacheFile();
+
+  Result<Ok, nsresult> ReadCache(LinkedList<URLEntry>& pendingURLs);
+
+  void BackgroundReadFiles();
+  void BeginBackgroundRead();
+
+  using HashType = nsClassHashtable<nsGenericHashKey<CacheKey>, URLEntry>;
+
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
+
+  bool mStartupFinished = false;
+  bool mReaderInitialized = false;
+
+  // Only to be accessed from the cache write thread.
+  bool mCacheWritten = false;
+
+  // The prefix URLs for files in the GRE and App omni jar archives.
+  nsCString mGREPrefix;
+  nsCString mAppPrefix;
+
+  nsCOMPtr<nsIResProtocolHandler> mResProto;
+  nsCOMPtr<nsIChromeRegistry> mChromeReg;
+  nsCOMPtr<nsIFile> mProfD;
+
+  // Note: We use a RefPtr rather than an nsCOMPtr here because the
+  // AssertNoQueryNeeded checks done by getter_AddRefs happen at a time that
+  // violate data access invariants. It's wrapped in a mutex because
+  // the reader thread needs to be able to null this out to terminate itself.
+  DataMutex<RefPtr<nsIThread>> mReaderThread{"ReaderThread"};
+
+  // A map of URL entries which have were either read this session, or read
+  // from the last session's cache file.
+  HashType mCachedURLs;
+
+  Monitor mMonitor MOZ_UNANNOTATED{"[URLPreloader::mMutex]"};
+};
+
+}  // namespace mozilla
+
+#endif  // URLPreloader_h
diff --git a/js/xpconnect/loader/XPCOMUtils.sys.mjs b/js/xpconnect/loader/XPCOMUtils.sys.mjs
new file mode 100644
index 0000000000..403b17e2be
--- /dev/null
+++ b/js/xpconnect/loader/XPCOMUtils.sys.mjs
@@ -0,0 +1,580 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+ * vim: sw=2 ts=2 sts=2 et filetype=javascript
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+let global = Cu.getGlobalForObject({});
+
+// Some global imports expose additional symbols; for example,
+// `Cu.importGlobalProperties(["MessageChannel"])` imports `MessageChannel`
+// and `MessagePort`. This table maps those extra symbols to the main
+// import name.
+const EXTRA_GLOBAL_NAME_TO_IMPORT_NAME = {
+  MessagePort: "MessageChannel",
+};
+
+/**
+ * Redefines the given property on the given object with the given
+ * value. This can be used to redefine getter properties which do not
+ * implement setters.
+ */
+function redefine(object, prop, value) {
+  Object.defineProperty(object, prop, {
+    configurable: true,
+    enumerable: true,
+    value,
+    writable: true,
+  });
+  return value;
+}
+
+export var XPCOMUtils = {
+  /**
+   * Defines a getter on a specified object that will be created upon first use.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter to define on aObject.
+   * @param aLambda
+   *        A function that returns what the getter should return.  This will
+   *        only ever be called once.
+   */
+  defineLazyGetter(aObject, aName, aLambda) {
+    ChromeUtils.defineLazyGetter(aObject, aName, aLambda);
+  },
+
+  /**
+   * Defines a getter on a specified object for a script.  The script will not
+   * be loaded until first use.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aNames
+   *        The name of the getter to define on aObject for the script.
+   *        This can be a string if the script exports only one symbol,
+   *        or an array of strings if the script can be first accessed
+   *        from several different symbols.
+   * @param aResource
+   *        The URL used to obtain the script.
+   */
+  defineLazyScriptGetter(aObject, aNames, aResource) {
+    if (!Array.isArray(aNames)) {
+      aNames = [aNames];
+    }
+    for (let name of aNames) {
+      Object.defineProperty(aObject, name, {
+        get() {
+          XPCOMUtils._scriptloader.loadSubScript(aResource, aObject);
+          return aObject[name];
+        },
+        set(value) {
+          redefine(aObject, name, value);
+        },
+        configurable: true,
+        enumerable: true,
+      });
+    }
+  },
+
+  /**
+   * Overrides the scriptloader definition for tests to help with globals
+   * tracking. Should only be used for tests.
+   *
+   * @param {object} aObject
+   *        The alternative script loader object to use.
+   */
+  overrideScriptLoaderForTests(aObject) {
+    Cu.crashIfNotInAutomation();
+    delete this._scriptloader;
+    this._scriptloader = aObject;
+  },
+
+  /**
+   * Defines a getter property on the given object for each of the given
+   * global names as accepted by Cu.importGlobalProperties. These
+   * properties are imported into the shared JSM module global, and then
+   * copied onto the given object, no matter which global the object
+   * belongs to.
+   *
+   * @param {object} aObject
+   *        The object on which to define the properties.
+   * @param {string[]} aNames
+   *        The list of global properties to define.
+   */
+  defineLazyGlobalGetters(aObject, aNames) {
+    for (let name of aNames) {
+      this.defineLazyGetter(aObject, name, () => {
+        if (!(name in global)) {
+          let importName = EXTRA_GLOBAL_NAME_TO_IMPORT_NAME[name] || name;
+          // eslint-disable-next-line mozilla/reject-importGlobalProperties, no-unused-vars
+          Cu.importGlobalProperties([importName]);
+        }
+        return global[name];
+      });
+    }
+  },
+
+  /**
+   * Defines a getter on a specified object for a service.  The service will not
+   * be obtained until first use.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter to define on aObject for the service.
+   * @param aContract
+   *        The contract used to obtain the service.
+   * @param aInterfaceName
+   *        The name of the interface to query the service to.
+   */
+  defineLazyServiceGetter(aObject, aName, aContract, aInterfaceName) {
+    this.defineLazyGetter(aObject, aName, () => {
+      if (aInterfaceName) {
+        return Cc[aContract].getService(Ci[aInterfaceName]);
+      }
+      return Cc[aContract].getService().wrappedJSObject;
+    });
+  },
+
+  /**
+   * Defines a lazy service getter on a specified object for each
+   * property in the given object.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aServices
+   *        An object with a property for each service to be
+   *        imported, where the property name is the name of the
+   *        symbol to define, and the value is a 1 or 2 element array
+   *        containing the contract ID and, optionally, the interface
+   *        name of the service, as passed to defineLazyServiceGetter.
+   */
+  defineLazyServiceGetters(aObject, aServices) {
+    for (let [name, service] of Object.entries(aServices)) {
+      // Note: This is hot code, and cross-compartment array wrappers
+      // are not JIT-friendly to destructuring or spread operators, so
+      // we need to use indexed access instead.
+      this.defineLazyServiceGetter(
+        aObject,
+        name,
+        service[0],
+        service[1] || null
+      );
+    }
+  },
+
+  /**
+   * Defines a getter on a specified object for a module.  The module will not
+   * be imported until first use. The getter allows to execute setup and
+   * teardown code (e.g.  to register/unregister to services) and accepts
+   * a proxy object which acts on behalf of the module until it is imported.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter to define on aObject for the module.
+   * @param aResource
+   *        The URL used to obtain the module.
+   * @param aSymbol
+   *        The name of the symbol exported by the module.
+   *        This parameter is optional and defaults to aName.
+   * @param aPreLambda
+   *        A function that is executed when the proxy is set up.
+   *        This will only ever be called once.
+   * @param aPostLambda
+   *        A function that is executed when the module has been imported to
+   *        run optional teardown procedures on the proxy object.
+   *        This will only ever be called once.
+   * @param aProxy
+   *        An object which acts on behalf of the module to be imported until
+   *        the module has been imported.
+   */
+  defineLazyModuleGetter(
+    aObject,
+    aName,
+    aResource,
+    aSymbol,
+    aPreLambda,
+    aPostLambda,
+    aProxy
+  ) {
+    if (arguments.length == 3) {
+      ChromeUtils.defineModuleGetter(aObject, aName, aResource);
+      return;
+    }
+
+    let proxy = aProxy || {};
+
+    if (typeof aPreLambda === "function") {
+      aPreLambda.apply(proxy);
+    }
+
+    this.defineLazyGetter(aObject, aName, () => {
+      var temp = {};
+      try {
+        temp = ChromeUtils.import(aResource);
+
+        if (typeof aPostLambda === "function") {
+          aPostLambda.apply(proxy);
+        }
+      } catch (ex) {
+        console.error("Failed to load module " + aResource + ".");
+        throw ex;
+      }
+      return temp[aSymbol || aName];
+    });
+  },
+
+  /**
+   * Defines a lazy module getter on a specified object for each
+   * property in the given object.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aModules
+   *        An object with a property for each module property to be
+   *        imported, where the property name is the name of the
+   *        imported symbol and the value is the module URI.
+   */
+  defineLazyModuleGetters(aObject, aModules) {
+    for (let [name, module] of Object.entries(aModules)) {
+      ChromeUtils.defineModuleGetter(aObject, name, module);
+    }
+  },
+
+  /**
+   * Defines a getter on a specified object for preference value. The
+   * preference is read the first time that the property is accessed,
+   * and is thereafter kept up-to-date using a preference observer.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter property to define on aObject.
+   * @param aPreference
+   *        The name of the preference to read.
+   * @param aDefaultPrefValue
+   *        The default value to use, if the preference is not defined.
+   *        This is the default value of the pref, before applying aTransform.
+   * @param aOnUpdate
+   *        A function to call upon update. Receives as arguments
+   *         `(aPreference, previousValue, newValue)`
+   * @param aTransform
+   *        An optional function to transform the value.  If provided,
+   *        this function receives the new preference value as an argument
+   *        and its return value is used by the getter.
+   */
+  defineLazyPreferenceGetter(
+    aObject,
+    aName,
+    aPreference,
+    aDefaultPrefValue = null,
+    aOnUpdate = null,
+    aTransform = val => val
+  ) {
+    if (AppConstants.DEBUG && aDefaultPrefValue !== null) {
+      let prefType = Services.prefs.getPrefType(aPreference);
+      if (prefType != Ci.nsIPrefBranch.PREF_INVALID) {
+        // The pref may get defined after the lazy getter is called
+        // at which point the code here won't know the expected type.
+        let prefTypeForDefaultValue = {
+          boolean: Ci.nsIPrefBranch.PREF_BOOL,
+          number: Ci.nsIPrefBranch.PREF_INT,
+          string: Ci.nsIPrefBranch.PREF_STRING,
+        }[typeof aDefaultPrefValue];
+        if (prefTypeForDefaultValue != prefType) {
+          throw new Error(
+            `Default value does not match preference type (Got ${prefTypeForDefaultValue}, expected ${prefType}) for ${aPreference}`
+          );
+        }
+      }
+    }
+
+    // Note: We need to keep a reference to this observer alive as long
+    // as aObject is alive. This means that all of our getters need to
+    // explicitly close over the variable that holds the object, and we
+    // cannot define a value in place of a getter after we read the
+    // preference.
+    let observer = {
+      QueryInterface: XPCU_lazyPreferenceObserverQI,
+
+      value: undefined,
+
+      observe(subject, topic, data) {
+        if (data == aPreference) {
+          if (aOnUpdate) {
+            let previous = this.value;
+
+            // Fetch and cache value.
+            this.value = undefined;
+            let latest = lazyGetter();
+            aOnUpdate(data, previous, latest);
+          } else {
+            // Empty cache, next call to the getter will cause refetch.
+            this.value = undefined;
+          }
+        }
+      },
+    };
+
+    let defineGetter = get => {
+      Object.defineProperty(aObject, aName, {
+        configurable: true,
+        enumerable: true,
+        get,
+      });
+    };
+
+    function lazyGetter() {
+      if (observer.value === undefined) {
+        let prefValue;
+        switch (Services.prefs.getPrefType(aPreference)) {
+          case Ci.nsIPrefBranch.PREF_STRING:
+            prefValue = Services.prefs.getStringPref(aPreference);
+            break;
+
+          case Ci.nsIPrefBranch.PREF_INT:
+            prefValue = Services.prefs.getIntPref(aPreference);
+            break;
+
+          case Ci.nsIPrefBranch.PREF_BOOL:
+            prefValue = Services.prefs.getBoolPref(aPreference);
+            break;
+
+          case Ci.nsIPrefBranch.PREF_INVALID:
+            prefValue = aDefaultPrefValue;
+            break;
+
+          default:
+            // This should never happen.
+            throw new Error(
+              `Error getting pref ${aPreference}; its value's type is ` +
+                `${Services.prefs.getPrefType(aPreference)}, which I don't ` +
+                `know how to handle.`
+            );
+        }
+
+        observer.value = aTransform(prefValue);
+      }
+      return observer.value;
+    }
+
+    defineGetter(() => {
+      Services.prefs.addObserver(aPreference, observer, true);
+
+      defineGetter(lazyGetter);
+      return lazyGetter();
+    });
+  },
+
+  /**
+   * Defines a non-writable property on an object.
+   */
+  defineConstant(aObj, aName, aValue) {
+    Object.defineProperty(aObj, aName, {
+      value: aValue,
+      enumerable: true,
+      writable: false,
+    });
+  },
+
+  /**
+   * Defines a proxy which acts as a lazy object getter that can be passed
+   * around as a reference, and will only be evaluated when something in
+   * that object gets accessed.
+   *
+   * The evaluation can be triggered by a function call, by getting or
+   * setting a property, calling this as a constructor, or enumerating
+   * the properties of this object (e.g. during an iteration).
+   *
+   * Please note that, even after evaluated, the object given to you
+   * remains being the proxy object (which forwards everything to the
+   * real object). This is important to correctly use these objects
+   * in pairs of add+remove listeners, for example.
+   * If your use case requires access to the direct object, you can
+   * get it through the untrap callback.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   *
+   *        You can pass null to aObject if you just want to get this
+   *        proxy through the return value.
+   *
+   * @param aName
+   *        The name of the getter to define on aObject.
+   *
+   * @param aInitFuncOrResource
+   *        A function or a module that defines what this object actually
+   *        should be when it gets evaluated. This will only ever be called once.
+   *
+   *        Short-hand: If you pass a string to this parameter, it will be treated
+   *        as the URI of a module to be imported, and aName will be used as
+   *        the symbol to retrieve from the module.
+   *
+   * @param aStubProperties
+   *        In this parameter, you can provide an object which contains
+   *        properties from the original object that, when accessed, will still
+   *        prevent the entire object from being evaluated.
+   *
+   *        These can be copies or simplified versions of the original properties.
+   *
+   *        One example is to provide an alternative QueryInterface implementation
+   *        to avoid the entire object from being evaluated when it's added as an
+   *        observer (as addObserver calls object.QueryInterface(Ci.nsIObserver)).
+   *
+   *        Once the object has been evaluated, the properties from the real
+   *        object will be used instead of the ones provided here.
+   *
+   * @param aUntrapCallback
+   *        A function that gets called once when the object has just been evaluated.
+   *        You can use this to do some work (e.g. setting properties) that you need
+   *        to do on this object but that can wait until it gets evaluated.
+   *
+   *        Another use case for this is to use during code development to log when
+   *        this object gets evaluated, to make sure you're not accidentally triggering
+   *        it earlier than expected.
+   */
+  defineLazyProxy(
+    aObject,
+    aName,
+    aInitFuncOrResource,
+    aStubProperties,
+    aUntrapCallback
+  ) {
+    let initFunc = aInitFuncOrResource;
+
+    if (typeof aInitFuncOrResource == "string") {
+      initFunc = () => ChromeUtils.import(aInitFuncOrResource)[aName];
+    }
+
+    let handler = new LazyProxyHandler(
+      aName,
+      initFunc,
+      aStubProperties,
+      aUntrapCallback
+    );
+
+    /*
+     * We cannot simply create a lazy getter for the underlying
+     * object and pass it as the target of the proxy, because
+     * just passing it in `new Proxy` means it would get
+     * evaluated. Becase of this, a full handler needs to be
+     * implemented (the LazyProxyHandler).
+     *
+     * So, an empty object is used as the target, and the handler
+     * replaces it on every call with the real object.
+     */
+    let proxy = new Proxy({}, handler);
+
+    if (aObject) {
+      Object.defineProperty(aObject, aName, {
+        value: proxy,
+        enumerable: true,
+        writable: true,
+      });
+    }
+
+    return proxy;
+  },
+};
+
+XPCOMUtils.defineLazyGetter(XPCOMUtils, "_scriptloader", () => {
+  return Services.scriptloader;
+});
+
+/**
+ * LazyProxyHandler
+ * This class implements the handler used
+ * in the proxy from defineLazyProxy.
+ *
+ * This handler forwards all calls to an underlying object,
+ * stored as `this.realObject`, which is obtained as the returned
+ * value from aInitFunc, which will be called on the first time
+ * time that it needs to be used (with an exception in the get() trap
+ * for the properties provided in the `aStubProperties` parameter).
+ */
+
+class LazyProxyHandler {
+  constructor(aName, aInitFunc, aStubProperties, aUntrapCallback) {
+    this.pending = true;
+    this.name = aName;
+    this.initFuncOrResource = aInitFunc;
+    this.stubProperties = aStubProperties;
+    this.untrapCallback = aUntrapCallback;
+  }
+
+  getObject() {
+    if (this.pending) {
+      this.realObject = this.initFuncOrResource.call(null);
+
+      if (this.untrapCallback) {
+        this.untrapCallback.call(null, this.realObject);
+        this.untrapCallback = null;
+      }
+
+      this.pending = false;
+      this.stubProperties = null;
+    }
+    return this.realObject;
+  }
+
+  getPrototypeOf(target) {
+    return Reflect.getPrototypeOf(this.getObject());
+  }
+
+  setPrototypeOf(target, prototype) {
+    return Reflect.setPrototypeOf(this.getObject(), prototype);
+  }
+
+  isExtensible(target) {
+    return Reflect.isExtensible(this.getObject());
+  }
+
+  preventExtensions(target) {
+    return Reflect.preventExtensions(this.getObject());
+  }
+
+  getOwnPropertyDescriptor(target, prop) {
+    return Reflect.getOwnPropertyDescriptor(this.getObject(), prop);
+  }
+
+  defineProperty(target, prop, descriptor) {
+    return Reflect.defineProperty(this.getObject(), prop, descriptor);
+  }
+
+  has(target, prop) {
+    return Reflect.has(this.getObject(), prop);
+  }
+
+  get(target, prop, receiver) {
+    if (
+      this.pending &&
+      this.stubProperties &&
+      Object.prototype.hasOwnProperty.call(this.stubProperties, prop)
+    ) {
+      return this.stubProperties[prop];
+    }
+    return Reflect.get(this.getObject(), prop, receiver);
+  }
+
+  set(target, prop, value, receiver) {
+    return Reflect.set(this.getObject(), prop, value, receiver);
+  }
+
+  deleteProperty(target, prop) {
+    return Reflect.deleteProperty(this.getObject(), prop);
+  }
+
+  ownKeys(target) {
+    return Reflect.ownKeys(this.getObject());
+  }
+}
+
+var XPCU_lazyPreferenceObserverQI = ChromeUtils.generateQI([
+  "nsIObserver",
+  "nsISupportsWeakReference",
+]);
diff --git a/js/xpconnect/loader/moz.build b/js/xpconnect/loader/moz.build
new file mode 100644
index 0000000000..44ec49c955
--- /dev/null
+++ b/js/xpconnect/loader/moz.build
@@ -0,0 +1,67 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += [
+    "AutoMemMap.cpp",
+    "ChromeScriptLoader.cpp",
+    "ComponentModuleLoader.cpp",
+    "JSMEnvironmentProxy.cpp",
+    "ModuleEnvironmentProxy.cpp",
+    "mozJSLoaderUtils.cpp",
+    "mozJSSubScriptLoader.cpp",
+    "nsImportModule.cpp",
+    "ScriptCacheActors.cpp",
+    "ScriptPreloader.cpp",
+    "URLPreloader.cpp",
+]
+
+# mozJSModuleLoader.cpp cannot be built in unified mode because it uses
+# windows.h
+SOURCES += [
+    "mozJSModuleLoader.cpp",
+]
+
+IPDL_SOURCES += [
+    "PScriptCache.ipdl",
+]
+
+EXPORTS += ["nsImportModule.h"]
+
+EXPORTS.mozilla += [
+    "AutoMemMap.h",
+    "IOBuffers.h",
+    "ScriptPreloader.h",
+    "URLPreloader.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "PrecompiledScript.h",
+]
+
+EXPORTS.mozilla.loader += [
+    "AutoMemMap.h",
+    "ComponentModuleLoader.h",
+    "ScriptCacheActors.h",
+    "SkipCheckForBrokenURLOrZeroSized.h",
+]
+
+EXTRA_JS_MODULES += [
+    "ComponentUtils.sys.mjs",
+    "XPCOMUtils.sys.mjs",
+]
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "../src",
+    "../wrappers",
+    "/dom/base",
+    "/js/loader",
+    "/xpcom/base/",
+    "/xpcom/io",  # crc32c.h
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
diff --git a/js/xpconnect/loader/mozJSLoaderUtils.cpp b/js/xpconnect/loader/mozJSLoaderUtils.cpp
new file mode 100644
index 0000000000..3f54e87d3b
--- /dev/null
+++ b/js/xpconnect/loader/mozJSLoaderUtils.cpp
@@ -0,0 +1,75 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/scache/StartupCache.h"
+
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/CompileOptions.h"
+#include "js/Transcoding.h"
+#include "js/experimental/JSStencil.h"
+
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/Span.h"
+
+using namespace JS;
+using namespace mozilla::scache;
+using mozilla::UniquePtr;
+
+static nsresult HandleTranscodeResult(JSContext* cx,
+                                      JS::TranscodeResult result) {
+  if (result == JS::TranscodeResult::Ok) {
+    return NS_OK;
+  }
+
+  if (result == JS::TranscodeResult::Throw) {
+    JS_ClearPendingException(cx);
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  MOZ_ASSERT(IsTranscodeFailureResult(result));
+  return NS_ERROR_FAILURE;
+}
+
+nsresult ReadCachedStencil(StartupCache* cache, nsACString& cachePath,
+                           JSContext* cx, const JS::DecodeOptions& options,
+                           JS::Stencil** stencilOut) {
+  MOZ_ASSERT(options.borrowBuffer);
+  MOZ_ASSERT(!options.usePinnedBytecode);
+
+  const char* buf;
+  uint32_t len;
+  nsresult rv =
+      cache->GetBuffer(PromiseFlatCString(cachePath).get(), &buf, &len);
+  if (NS_FAILED(rv)) {
+    return rv;  // don't warn since NOT_AVAILABLE is an ok error
+  }
+
+  JS::TranscodeRange range(AsBytes(mozilla::Span(buf, len)));
+  JS::TranscodeResult code = JS::DecodeStencil(cx, options, range, stencilOut);
+  return HandleTranscodeResult(cx, code);
+}
+
+nsresult WriteCachedStencil(StartupCache* cache, nsACString& cachePath,
+                            JSContext* cx, JS::Stencil* stencil) {
+  JS::TranscodeBuffer buffer;
+  JS::TranscodeResult code = JS::EncodeStencil(cx, stencil, buffer);
+  if (code != JS::TranscodeResult::Ok) {
+    return HandleTranscodeResult(cx, code);
+  }
+
+  size_t size = buffer.length();
+  if (size > UINT32_MAX) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Move the vector buffer into a unique pointer buffer.
+  mozilla::UniqueFreePtr<char[]> buf(
+      reinterpret_cast<char*>(buffer.extractOrCopyRawBuffer()));
+  nsresult rv = cache->PutBuffer(PromiseFlatCString(cachePath).get(),
+                                 std::move(buf), size);
+  return rv;
+}
diff --git a/js/xpconnect/loader/mozJSLoaderUtils.h b/js/xpconnect/loader/mozJSLoaderUtils.h
new file mode 100644
index 0000000000..8c996c17be
--- /dev/null
+++ b/js/xpconnect/loader/mozJSLoaderUtils.h
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozJSLoaderUtils_h
+#define mozJSLoaderUtils_h
+
+#include "nsString.h"
+
+#include "js/experimental/JSStencil.h"
+#include "js/CompileOptions.h"  // JS::DecodeOptions
+
+namespace mozilla {
+namespace scache {
+class StartupCache;
+}  // namespace scache
+}  // namespace mozilla
+
+nsresult ReadCachedStencil(mozilla::scache::StartupCache* cache,
+                           nsACString& cachePath, JSContext* cx,
+                           const JS::DecodeOptions& options,
+                           JS::Stencil** stencilOut);
+
+nsresult WriteCachedStencil(mozilla::scache::StartupCache* cache,
+                            nsACString& cachePath, JSContext* cx,
+                            JS::Stencil* stencil);
+
+#endif /* mozJSLoaderUtils_h */
diff --git a/js/xpconnect/loader/mozJSModuleLoader.cpp b/js/xpconnect/loader/mozJSModuleLoader.cpp
new file mode 100644
index 0000000000..50102f8770
--- /dev/null
+++ b/js/xpconnect/loader/mozJSModuleLoader.cpp
@@ -0,0 +1,1936 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Attributes.h"
+#include "mozilla/ArrayUtils.h"  // mozilla::ArrayLength
+#include "mozilla/Utf8.h"        // mozilla::Utf8Unit
+
+#include <cstdarg>
+
+#include "mozilla/Logging.h"
+#ifdef ANDROID
+#  include <android/log.h>
+#endif
+#ifdef XP_WIN
+#  include <windows.h>
+#endif
+
+#include "jsapi.h"
+#include "js/Array.h"  // JS::GetArrayLength, JS::IsArrayObject
+#include "js/CharacterEncoding.h"
+#include "js/CompilationAndEvaluation.h"
+#include "js/CompileOptions.h"         // JS::CompileOptions
+#include "js/ErrorReport.h"            // JS_ReportErrorUTF8, JSErrorReport
+#include "js/Exception.h"              // JS_ErrorFromException
+#include "js/friend/JSMEnvironment.h"  // JS::ExecuteInJSMEnvironment, JS::GetJSMEnvironmentOfScriptedCaller, JS::NewJSMEnvironment
+#include "js/friend/ErrorMessages.h"   // JSMSG_*
+#include "js/loader/ModuleLoadRequest.h"
+#include "js/Object.h"  // JS::GetCompartment
+#include "js/Printf.h"
+#include "js/PropertyAndElement.h"  // JS_DefineFunctions, JS_DefineProperty, JS_Enumerate, JS_GetElement, JS_GetProperty, JS_GetPropertyById, JS_HasOwnProperty, JS_HasOwnPropertyById, JS_SetProperty, JS_SetPropertyById
+#include "js/PropertySpec.h"
+#include "js/SourceText.h"  // JS::SourceText
+#include "nsCOMPtr.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIFile.h"
+#include "mozJSModuleLoader.h"
+#include "mozJSLoaderUtils.h"
+#include "nsIFileURL.h"
+#include "nsIJARURI.h"
+#include "nsIChannel.h"
+#include "nsNetUtil.h"
+#include "nsJSUtils.h"
+#include "xpcprivate.h"
+#include "xpcpublic.h"
+#include "nsContentUtils.h"
+#include "nsXULAppAPI.h"
+#include "WrapperFactory.h"
+#include "JSMEnvironmentProxy.h"
+#include "ModuleEnvironmentProxy.h"
+#include "JSServices.h"
+
+#include "mozilla/scache/StartupCache.h"
+#include "mozilla/scache/StartupCacheUtils.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/ScriptPreloader.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/ReferrerPolicyBinding.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/Unused.h"
+
+using namespace mozilla;
+using namespace mozilla::scache;
+using namespace mozilla::loader;
+using namespace xpc;
+using namespace JS;
+
+#define JS_CACHE_PREFIX(aScopeType, aCompilationTarget) \
+  "jsloader/" aScopeType "/" aCompilationTarget
+
+/**
+ * Buffer sizes for serialization and deserialization of scripts.
+ * FIXME: bug #411579 (tune this macro!) Last updated: Jan 2008
+ */
+#define XPC_SERIALIZATION_BUFFER_SIZE (64 * 1024)
+#define XPC_DESERIALIZATION_BUFFER_SIZE (12 * 8192)
+
+// MOZ_LOG=JSModuleLoader:5
+static LazyLogModule gJSCLLog("JSModuleLoader");
+
+#define LOG(args) MOZ_LOG(gJSCLLog, mozilla::LogLevel::Debug, args)
+
+// Components.utils.import error messages
+#define ERROR_SCOPE_OBJ "%s - Second argument must be an object."
+#define ERROR_NO_TARGET_OBJECT "%s - Couldn't find target object for import."
+#define ERROR_NOT_PRESENT "%s - EXPORTED_SYMBOLS is not present."
+#define ERROR_NOT_AN_ARRAY "%s - EXPORTED_SYMBOLS is not an array."
+#define ERROR_GETTING_ARRAY_LENGTH \
+  "%s - Error getting array length of EXPORTED_SYMBOLS."
+#define ERROR_ARRAY_ELEMENT "%s - EXPORTED_SYMBOLS[%d] is not a string."
+#define ERROR_GETTING_SYMBOL "%s - Could not get symbol '%s'."
+#define ERROR_SETTING_SYMBOL "%s - Could not set symbol '%s' on target object."
+#define ERROR_UNINITIALIZED_SYMBOL \
+  "%s - Symbol '%s' accessed before initialization. Cyclic import?"
+
+static constexpr char JSM_Suffix[] = ".jsm";
+static constexpr size_t JSM_SuffixLength = mozilla::ArrayLength(JSM_Suffix) - 1;
+static constexpr char JSM_JS_Suffix[] = ".jsm.js";
+static constexpr size_t JSM_JS_SuffixLength =
+    mozilla::ArrayLength(JSM_JS_Suffix) - 1;
+static constexpr char JS_Suffix[] = ".js";
+static constexpr size_t JS_SuffixLength = mozilla::ArrayLength(JS_Suffix) - 1;
+static constexpr char MJS_Suffix[] = ".sys.mjs";
+static constexpr size_t MJS_SuffixLength = mozilla::ArrayLength(MJS_Suffix) - 1;
+
+static bool IsJSM(const nsACString& aLocation) {
+  if (aLocation.Length() < JSM_SuffixLength) {
+    return false;
+  }
+  const auto ext = Substring(aLocation, aLocation.Length() - JSM_SuffixLength);
+  return ext == JSM_Suffix;
+}
+
+static bool IsJS(const nsACString& aLocation) {
+  if (aLocation.Length() < JS_SuffixLength) {
+    return false;
+  }
+  const auto ext = Substring(aLocation, aLocation.Length() - JS_SuffixLength);
+  return ext == JS_Suffix;
+}
+
+static bool IsJSM_JS(const nsACString& aLocation) {
+  if (aLocation.Length() < JSM_JS_SuffixLength) {
+    return false;
+  }
+  const auto ext =
+      Substring(aLocation, aLocation.Length() - JSM_JS_SuffixLength);
+  return ext == JSM_JS_Suffix;
+}
+
+static bool IsMJS(const nsACString& aLocation) {
+  if (aLocation.Length() < MJS_SuffixLength) {
+    return false;
+  }
+  const auto ext = Substring(aLocation, aLocation.Length() - MJS_SuffixLength);
+  return ext == MJS_Suffix;
+}
+
+static void MJSToJSM(const nsACString& aLocation, nsAutoCString& aOut) {
+  MOZ_ASSERT(IsMJS(aLocation));
+  aOut = Substring(aLocation, 0, aLocation.Length() - MJS_SuffixLength);
+  aOut += JSM_Suffix;
+}
+
+static bool TryToMJS(const nsACString& aLocation, nsAutoCString& aOut) {
+  if (IsJSM(aLocation)) {
+    aOut = Substring(aLocation, 0, aLocation.Length() - JSM_SuffixLength);
+    aOut += MJS_Suffix;
+    return true;
+  }
+
+  if (IsJSM_JS(aLocation)) {
+    aOut = Substring(aLocation, 0, aLocation.Length() - JSM_JS_SuffixLength);
+    aOut += MJS_Suffix;
+    return true;
+  }
+
+  if (IsJS(aLocation)) {
+    aOut = Substring(aLocation, 0, aLocation.Length() - JS_SuffixLength);
+    aOut += MJS_Suffix;
+    return true;
+  }
+
+  return false;
+}
+
+static bool Dump(JSContext* cx, unsigned argc, Value* vp) {
+  if (!nsJSUtils::DumpEnabled()) {
+    return true;
+  }
+
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  if (args.length() == 0) {
+    return true;
+  }
+
+  RootedString str(cx, JS::ToString(cx, args[0]));
+  if (!str) {
+    return false;
+  }
+
+  JS::UniqueChars utf8str = JS_EncodeStringToUTF8(cx, str);
+  if (!utf8str) {
+    return false;
+  }
+
+  MOZ_LOG(nsContentUtils::DOMDumpLog(), mozilla::LogLevel::Debug,
+          ("[Backstage.Dump] %s", utf8str.get()));
+#ifdef ANDROID
+  __android_log_print(ANDROID_LOG_INFO, "Gecko", "%s", utf8str.get());
+#endif
+#ifdef XP_WIN
+  if (IsDebuggerPresent()) {
+    nsAutoJSString wstr;
+    if (!wstr.init(cx, str)) {
+      return false;
+    }
+    OutputDebugStringW(wstr.get());
+  }
+#endif
+  fputs(utf8str.get(), stdout);
+  fflush(stdout);
+  return true;
+}
+
+static bool Debug(JSContext* cx, unsigned argc, Value* vp) {
+#ifdef DEBUG
+  return Dump(cx, argc, vp);
+#else
+  return true;
+#endif
+}
+
+static const JSFunctionSpec gGlobalFun[] = {
+    JS_FN("dump", Dump, 1, 0), JS_FN("debug", Debug, 1, 0),
+    JS_FN("atob", Atob, 1, 0), JS_FN("btoa", Btoa, 1, 0), JS_FS_END};
+
+class MOZ_STACK_CLASS JSCLContextHelper {
+ public:
+  explicit JSCLContextHelper(JSContext* aCx);
+  ~JSCLContextHelper();
+
+  void reportErrorAfterPop(UniqueChars&& buf);
+
+ private:
+  JSContext* mContext;
+  UniqueChars mBuf;
+
+  // prevent copying and assignment
+  JSCLContextHelper(const JSCLContextHelper&) = delete;
+  const JSCLContextHelper& operator=(const JSCLContextHelper&) = delete;
+};
+
+static nsresult MOZ_FORMAT_PRINTF(2, 3)
+    ReportOnCallerUTF8(JSContext* callerContext, const char* format, ...) {
+  if (!callerContext) {
+    return NS_ERROR_FAILURE;
+  }
+
+  va_list ap;
+  va_start(ap, format);
+
+  UniqueChars buf = JS_vsmprintf(format, ap);
+  if (!buf) {
+    va_end(ap);
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  JS_ReportErrorUTF8(callerContext, "%s", buf.get());
+
+  va_end(ap);
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(mozJSModuleLoader, nsIMemoryReporter)
+
+mozJSModuleLoader::mozJSModuleLoader()
+    : mImports(16),
+      mInProgressImports(16),
+      mFallbackImports(16),
+#ifdef STARTUP_RECORDER_ENABLED
+      mImportStacks(16),
+#endif
+      mLocations(16),
+      mInitialized(false),
+      mLoaderGlobal(dom::RootingCx()),
+      mServicesObj(dom::RootingCx()) {
+}
+
+#define ENSURE_DEP(name)          \
+  {                               \
+    nsresult rv = Ensure##name(); \
+    NS_ENSURE_SUCCESS(rv, rv);    \
+  }
+#define ENSURE_DEPS(...) MOZ_FOR_EACH(ENSURE_DEP, (), (__VA_ARGS__));
+#define BEGIN_ENSURE(self, ...) \
+  {                             \
+    if (m##self) return NS_OK;  \
+    ENSURE_DEPS(__VA_ARGS__);   \
+  }
+
+class MOZ_STACK_CLASS ModuleLoaderInfo {
+ public:
+  explicit ModuleLoaderInfo(const nsACString& aLocation,
+                            SkipCheckForBrokenURLOrZeroSized aSkipCheck =
+                                SkipCheckForBrokenURLOrZeroSized::No)
+      : mLocation(&aLocation), mIsModule(false), mSkipCheck(aSkipCheck) {}
+  explicit ModuleLoaderInfo(JS::loader::ModuleLoadRequest* aRequest)
+      : mLocation(nullptr),
+        mURI(aRequest->mURI),
+        mIsModule(true),
+        mSkipCheck(aRequest->GetComponentLoadContext()->mSkipCheck) {}
+
+  SkipCheckForBrokenURLOrZeroSized getSkipCheckForBrokenURLOrZeroSized() const {
+    return mSkipCheck;
+  }
+
+  void resetChannelWithCheckForBrokenURLOrZeroSized() {
+    MOZ_ASSERT(mSkipCheck == SkipCheckForBrokenURLOrZeroSized::Yes);
+    mSkipCheck = SkipCheckForBrokenURLOrZeroSized::No;
+    mScriptChannel = nullptr;
+  }
+
+  nsIIOService* IOService() {
+    MOZ_ASSERT(mIOService);
+    return mIOService;
+  }
+  nsresult EnsureIOService() {
+    if (mIOService) {
+      return NS_OK;
+    }
+    nsresult rv;
+    mIOService = do_GetIOService(&rv);
+    return rv;
+  }
+
+  nsIURI* URI() {
+    MOZ_ASSERT(mURI);
+    return mURI;
+  }
+  nsresult EnsureURI() {
+    BEGIN_ENSURE(URI, IOService);
+    MOZ_ASSERT(mLocation);
+    return mIOService->NewURI(*mLocation, nullptr, nullptr,
+                              getter_AddRefs(mURI));
+  }
+
+  nsIChannel* ScriptChannel() {
+    MOZ_ASSERT(mScriptChannel);
+    return mScriptChannel;
+  }
+  nsresult EnsureScriptChannel() {
+    BEGIN_ENSURE(ScriptChannel, IOService, URI);
+
+    // Skip check for missing URL when handling JSM-to-ESM fallback.
+    return NS_NewChannel(
+        getter_AddRefs(mScriptChannel), mURI,
+        nsContentUtils::GetSystemPrincipal(),
+        nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+        nsIContentPolicy::TYPE_SCRIPT,
+        /* aCookieJarSettings = */ nullptr,
+        /* aPerformanceStorage = */ nullptr,
+        /* aLoadGroup = */ nullptr, /* aCallbacks = */ nullptr,
+        nsIRequest::LOAD_NORMAL, mIOService, /* aSandboxFlags = */ 0,
+        mSkipCheck == SkipCheckForBrokenURLOrZeroSized::Yes);
+  }
+
+  nsIURI* ResolvedURI() {
+    MOZ_ASSERT(mResolvedURI);
+    return mResolvedURI;
+  }
+  nsresult EnsureResolvedURI() {
+    BEGIN_ENSURE(ResolvedURI, URI);
+    return ResolveURI(mURI, getter_AddRefs(mResolvedURI));
+  }
+
+  const nsACString& Key() {
+    MOZ_ASSERT(mLocation);
+    return *mLocation;
+  }
+
+  [[nodiscard]] nsresult GetLocation(nsCString& aLocation) {
+    nsresult rv = EnsureURI();
+    NS_ENSURE_SUCCESS(rv, rv);
+    return mURI->GetSpec(aLocation);
+  }
+
+  bool IsModule() const { return mIsModule; }
+
+ private:
+  const nsACString* mLocation;
+  nsCOMPtr<nsIIOService> mIOService;
+  nsCOMPtr<nsIURI> mURI;
+  nsCOMPtr<nsIChannel> mScriptChannel;
+  nsCOMPtr<nsIURI> mResolvedURI;
+  const bool mIsModule;
+  SkipCheckForBrokenURLOrZeroSized mSkipCheck;
+};
+
+template <typename... Args>
+static nsresult ReportOnCallerUTF8(JSCLContextHelper& helper,
+                                   const char* format, ModuleLoaderInfo& info,
+                                   Args... args) {
+  nsCString location;
+  MOZ_TRY(info.GetLocation(location));
+
+  UniqueChars buf = JS_smprintf(format, location.get(), args...);
+  if (!buf) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  helper.reportErrorAfterPop(std::move(buf));
+  return NS_ERROR_FAILURE;
+}
+
+#undef BEGIN_ENSURE
+#undef ENSURE_DEPS
+#undef ENSURE_DEP
+
+mozJSModuleLoader::~mozJSModuleLoader() {
+  MOZ_ASSERT(!mInitialized,
+             "UnloadModules() was not explicitly called before cleaning up "
+             "mozJSModuleLoader");
+
+  if (mInitialized) {
+    UnloadModules();
+  }
+}
+
+StaticRefPtr<mozJSModuleLoader> mozJSModuleLoader::sSelf;
+StaticRefPtr<mozJSModuleLoader> mozJSModuleLoader::sDevToolsLoader;
+
+void mozJSModuleLoader::FindTargetObject(JSContext* aCx,
+                                         MutableHandleObject aTargetObject) {
+  aTargetObject.set(JS::GetJSMEnvironmentOfScriptedCaller(aCx));
+
+  // The above could fail if the scripted caller is not a JSM (it could be a DOM
+  // scope, for instance).
+  //
+  // If the target object was not in the JSM shared global, return the global
+  // instead. This is needed when calling the subscript loader within a frame
+  // script, since it the FrameScript NSVO will have been found.
+  if (!aTargetObject ||
+      !IsLoaderGlobal(JS::GetNonCCWObjectGlobal(aTargetObject))) {
+    aTargetObject.set(JS::GetScriptedCallerGlobal(aCx));
+
+    // Return nullptr if the scripted caller is in a different compartment.
+    if (JS::GetCompartment(aTargetObject) != js::GetContextCompartment(aCx)) {
+      aTargetObject.set(nullptr);
+    }
+  }
+}
+
+void mozJSModuleLoader::InitStatics() {
+  MOZ_ASSERT(!sSelf);
+  sSelf = new mozJSModuleLoader();
+  RegisterWeakMemoryReporter(sSelf);
+}
+
+void mozJSModuleLoader::UnloadLoaders() {
+  if (sSelf) {
+    sSelf->Unload();
+  }
+  if (sDevToolsLoader) {
+    sDevToolsLoader->Unload();
+  }
+}
+
+void mozJSModuleLoader::Unload() {
+  UnloadModules();
+
+  if (mModuleLoader) {
+    mModuleLoader->Shutdown();
+    mModuleLoader = nullptr;
+  }
+}
+
+void mozJSModuleLoader::ShutdownLoaders() {
+  MOZ_ASSERT(sSelf);
+  UnregisterWeakMemoryReporter(sSelf);
+  sSelf = nullptr;
+
+  if (sDevToolsLoader) {
+    UnregisterWeakMemoryReporter(sDevToolsLoader);
+    sDevToolsLoader = nullptr;
+  }
+}
+
+mozJSModuleLoader* mozJSModuleLoader::GetOrCreateDevToolsLoader() {
+  if (sDevToolsLoader) {
+    return sDevToolsLoader;
+  }
+  sDevToolsLoader = new mozJSModuleLoader();
+  RegisterWeakMemoryReporter(sDevToolsLoader);
+  return sDevToolsLoader;
+}
+
+// This requires that the keys be strings and the values be pointers.
+template <class Key, class Data, class UserData, class Converter>
+static size_t SizeOfTableExcludingThis(
+    const nsBaseHashtable<Key, Data, UserData, Converter>& aTable,
+    MallocSizeOf aMallocSizeOf) {
+  size_t n = aTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (const auto& entry : aTable) {
+    n += entry.GetKey().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+    n += entry.GetData()->SizeOfIncludingThis(aMallocSizeOf);
+  }
+  return n;
+}
+
+#ifdef STARTUP_RECORDER_ENABLED
+template <class Key, class Data, class UserData, class Converter>
+static size_t SizeOfStringTableExcludingThis(
+    const nsBaseHashtable<Key, Data, UserData, Converter>& aTable,
+    MallocSizeOf aMallocSizeOf) {
+  size_t n = aTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (const auto& entry : aTable) {
+    n += entry.GetKey().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+    n += entry.GetData().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+  }
+  return n;
+}
+#endif
+
+size_t mozJSModuleLoader::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) {
+  size_t n = aMallocSizeOf(this);
+  n += SizeOfTableExcludingThis(mImports, aMallocSizeOf);
+  n += mLocations.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  n += SizeOfTableExcludingThis(mInProgressImports, aMallocSizeOf);
+  n += SizeOfTableExcludingThis(mFallbackImports, aMallocSizeOf);
+#ifdef STARTUP_RECORDER_ENABLED
+  n += SizeOfStringTableExcludingThis(mImportStacks, aMallocSizeOf);
+#endif
+  return n;
+}
+
+// Memory report paths are split on '/', with each module displayed as a
+// separate layer of a visual tree. Any slashes which are meant to belong to a
+// particular path module, rather than be used to build a hierarchy, therefore
+// need to be replaced with backslashes, which are displayed as slashes in the
+// UI.
+//
+// If `aAnonymize` is true, this function also attempts to translate any file:
+// URLs to replace the path of the GRE directory with a placeholder containing
+// no private information, and strips all other file: URIs of everything upto
+// their last `/`.
+static nsAutoCString MangleURL(const char* aURL, bool aAnonymize) {
+  nsAutoCString url(aURL);
+
+  if (aAnonymize) {
+    static nsCString greDirURI;
+    if (greDirURI.IsEmpty()) {
+      nsCOMPtr<nsIFile> file;
+      Unused << NS_GetSpecialDirectory(NS_GRE_DIR, getter_AddRefs(file));
+      if (file) {
+        nsCOMPtr<nsIURI> uri;
+        NS_NewFileURI(getter_AddRefs(uri), file);
+        if (uri) {
+          uri->GetSpec(greDirURI);
+          RunOnShutdown([&]() { greDirURI.Truncate(0); });
+        }
+      }
+    }
+
+    url.ReplaceSubstring(greDirURI, "<GREDir>/"_ns);
+
+    if (FindInReadable("file:"_ns, url)) {
+      if (StringBeginsWith(url, "jar:file:"_ns)) {
+        int32_t idx = url.RFindChar('!');
+        url = "jar:file://<anonymized>!"_ns + Substring(url, idx);
+      } else {
+        int32_t idx = url.RFindChar('/');
+        url = "file://<anonymized>/"_ns + Substring(url, idx);
+      }
+    }
+  }
+
+  url.ReplaceChar('/', '\\');
+  return url;
+}
+
+NS_IMETHODIMP
+mozJSModuleLoader::CollectReports(nsIHandleReportCallback* aHandleReport,
+                                  nsISupports* aData, bool aAnonymize) {
+  for (const auto& entry : mImports.Values()) {
+    nsAutoCString path("js-module-loader/modules/");
+    path.Append(MangleURL(entry->location, aAnonymize));
+
+    aHandleReport->Callback(""_ns, path, KIND_NONHEAP, UNITS_COUNT, 1,
+                            "Loaded JS modules"_ns, aData);
+  }
+
+  return NS_OK;
+}
+
+void mozJSModuleLoader::CreateLoaderGlobal(JSContext* aCx,
+                                           const nsACString& aLocation,
+                                           MutableHandleObject aGlobal) {
+  auto backstagePass = MakeRefPtr<BackstagePass>();
+  RealmOptions options;
+  auto& creationOptions = options.creationOptions();
+
+  creationOptions.setFreezeBuiltins(true).setNewCompartmentInSystemZone();
+  if (IsDevToolsLoader()) {
+    creationOptions.setInvisibleToDebugger(true);
+  }
+  xpc::SetPrefableRealmOptions(options);
+
+  // Defer firing OnNewGlobalObject until after the __URI__ property has
+  // been defined so the JS debugger can tell what module the global is
+  // for
+  RootedObject global(aCx);
+
+#ifdef DEBUG
+  // See mozJSModuleLoader::DefineJSServices.
+  mIsInitializingLoaderGlobal = true;
+#endif
+  nsresult rv = xpc::InitClassesWithNewWrappedGlobal(
+      aCx, static_cast<nsIGlobalObject*>(backstagePass),
+      nsContentUtils::GetSystemPrincipal(), xpc::DONT_FIRE_ONNEWGLOBALHOOK,
+      options, &global);
+#ifdef DEBUG
+  mIsInitializingLoaderGlobal = false;
+#endif
+  NS_ENSURE_SUCCESS_VOID(rv);
+
+  NS_ENSURE_TRUE_VOID(global);
+
+  backstagePass->SetGlobalObject(global);
+
+  JSAutoRealm ar(aCx, global);
+  if (!JS_DefineFunctions(aCx, global, gGlobalFun)) {
+    return;
+  }
+
+  if (!CreateJSServices(aCx)) {
+    return;
+  }
+
+  if (!DefineJSServices(aCx, global)) {
+    return;
+  }
+
+  // Set the location information for the new global, so that tools like
+  // about:memory may use that information
+  xpc::SetLocationForGlobal(global, aLocation);
+
+  MOZ_ASSERT(!mModuleLoader);
+  RefPtr<ComponentScriptLoader> scriptLoader = new ComponentScriptLoader;
+  mModuleLoader = new ComponentModuleLoader(scriptLoader, backstagePass);
+  backstagePass->InitModuleLoader(mModuleLoader);
+
+  aGlobal.set(global);
+}
+
+JSObject* mozJSModuleLoader::GetSharedGlobal(JSContext* aCx) {
+  if (!mLoaderGlobal) {
+    JS::RootedObject globalObj(aCx);
+
+    CreateLoaderGlobal(
+        aCx, IsDevToolsLoader() ? "DevTools global"_ns : "shared JSM global"_ns,
+        &globalObj);
+
+    // If we fail to create a module global this early, we're not going to
+    // get very far, so just bail out now.
+    MOZ_RELEASE_ASSERT(globalObj);
+    mLoaderGlobal = globalObj;
+
+    // AutoEntryScript required to invoke debugger hook, which is a
+    // Gecko-specific concept at present.
+    dom::AutoEntryScript aes(globalObj, "module loader report global");
+    JS_FireOnNewGlobalObject(aes.cx(), globalObj);
+  }
+
+  return mLoaderGlobal;
+}
+
+/* static */
+nsresult mozJSModuleLoader::LoadSingleModuleScript(
+    ComponentModuleLoader* aModuleLoader, JSContext* aCx,
+    JS::loader::ModuleLoadRequest* aRequest, MutableHandleScript aScriptOut) {
+  AUTO_PROFILER_MARKER_TEXT(
+      "ChromeUtils.importESModule static import", JS,
+      MarkerOptions(MarkerStack::Capture(),
+                    MarkerInnerWindowIdFromJSContext(aCx)),
+      nsContentUtils::TruncatedURLForDisplay(aRequest->mURI));
+
+  ModuleLoaderInfo info(aRequest);
+  nsresult rv = info.EnsureResolvedURI();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIFile> sourceFile;
+  rv = GetSourceFile(info.ResolvedURI(), getter_AddRefs(sourceFile));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  bool realFile = LocationIsRealFile(aRequest->mURI);
+
+  RootedScript script(aCx);
+  rv = GetScriptForLocation(aCx, info, sourceFile, realFile, aScriptOut);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+#ifdef STARTUP_RECORDER_ENABLED
+  if (aModuleLoader == sSelf->mModuleLoader) {
+    sSelf->RecordImportStack(aCx, aRequest);
+  } else {
+    MOZ_ASSERT(sDevToolsLoader);
+    MOZ_ASSERT(aModuleLoader == sDevToolsLoader->mModuleLoader);
+    sDevToolsLoader->RecordImportStack(aCx, aRequest);
+  }
+#endif
+
+  return NS_OK;
+}
+
+/* static */
+nsresult mozJSModuleLoader::GetSourceFile(nsIURI* aResolvedURI,
+                                          nsIFile** aSourceFileOut) {
+  // Get the JAR if there is one.
+  nsCOMPtr<nsIJARURI> jarURI;
+  nsresult rv = NS_OK;
+  jarURI = do_QueryInterface(aResolvedURI, &rv);
+  nsCOMPtr<nsIFileURL> baseFileURL;
+  if (NS_SUCCEEDED(rv)) {
+    nsCOMPtr<nsIURI> baseURI;
+    while (jarURI) {
+      jarURI->GetJARFile(getter_AddRefs(baseURI));
+      jarURI = do_QueryInterface(baseURI, &rv);
+    }
+    baseFileURL = do_QueryInterface(baseURI, &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+  } else {
+    baseFileURL = do_QueryInterface(aResolvedURI, &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return baseFileURL->GetFile(aSourceFileOut);
+}
+
+/* static */
+bool mozJSModuleLoader::LocationIsRealFile(nsIURI* aURI) {
+  // We need to be extra careful checking for URIs pointing to files.
+  // EnsureFile may not always get called, especially on resource URIs so we
+  // need to call GetFile to make sure this is a valid file.
+  nsresult rv = NS_OK;
+  nsCOMPtr<nsIFileURL> fileURL = do_QueryInterface(aURI, &rv);
+  nsCOMPtr<nsIFile> testFile;
+  if (NS_SUCCEEDED(rv)) {
+    fileURL->GetFile(getter_AddRefs(testFile));
+  }
+
+  return bool(testFile);
+}
+
+JSObject* mozJSModuleLoader::PrepareObjectForLocation(JSContext* aCx,
+                                                      nsIFile* aModuleFile,
+                                                      nsIURI* aURI,
+                                                      bool aRealFile) {
+  RootedObject globalObj(aCx, GetSharedGlobal(aCx));
+  NS_ENSURE_TRUE(globalObj, nullptr);
+  JSAutoRealm ar(aCx, globalObj);
+
+  // |thisObj| is the object we set properties on for a particular .jsm.
+  RootedObject thisObj(aCx, JS::NewJSMEnvironment(aCx));
+  NS_ENSURE_TRUE(thisObj, nullptr);
+
+  if (aRealFile) {
+    if (XRE_IsParentProcess()) {
+      RootedObject locationObj(aCx);
+
+      nsresult rv = nsXPConnect::XPConnect()->WrapNative(
+          aCx, thisObj, aModuleFile, NS_GET_IID(nsIFile),
+          locationObj.address());
+      NS_ENSURE_SUCCESS(rv, nullptr);
+      NS_ENSURE_TRUE(locationObj, nullptr);
+
+      if (!JS_DefineProperty(aCx, thisObj, "__LOCATION__", locationObj, 0)) {
+        return nullptr;
+      }
+    }
+  }
+
+  // Expose the URI from which the script was imported through a special
+  // variable that we insert into the JSM.
+  nsAutoCString nativePath;
+  NS_ENSURE_SUCCESS(aURI->GetSpec(nativePath), nullptr);
+
+  RootedString exposedUri(
+      aCx, JS_NewStringCopyN(aCx, nativePath.get(), nativePath.Length()));
+  NS_ENSURE_TRUE(exposedUri, nullptr);
+
+  if (!JS_DefineProperty(aCx, thisObj, "__URI__", exposedUri, 0)) {
+    return nullptr;
+  }
+
+  return thisObj;
+}
+
+static mozilla::Result<nsCString, nsresult> ReadScript(
+    ModuleLoaderInfo& aInfo) {
+  MOZ_TRY(aInfo.EnsureScriptChannel());
+
+  nsCOMPtr<nsIInputStream> scriptStream;
+  MOZ_TRY(aInfo.ScriptChannel()->Open(getter_AddRefs(scriptStream)));
+
+  uint64_t len64;
+  uint32_t bytesRead;
+
+  MOZ_TRY(scriptStream->Available(&len64));
+  NS_ENSURE_TRUE(len64 < UINT32_MAX, Err(NS_ERROR_FILE_TOO_BIG));
+  NS_ENSURE_TRUE(len64, Err(NS_ERROR_FAILURE));
+  uint32_t len = (uint32_t)len64;
+
+  /* malloc an internal buf the size of the file */
+  nsCString str;
+  if (!str.SetLength(len, fallible)) {
+    return Err(NS_ERROR_OUT_OF_MEMORY);
+  }
+
+  /* read the file in one swoop */
+  MOZ_TRY(scriptStream->Read(str.BeginWriting(), len, &bytesRead));
+  if (bytesRead != len) {
+    return Err(NS_BASE_STREAM_OSERROR);
+  }
+
+  return std::move(str);
+}
+
+nsresult mozJSModuleLoader::ObjectForLocation(
+    ModuleLoaderInfo& aInfo, nsIFile* aModuleFile, MutableHandleObject aObject,
+    MutableHandleScript aTableScript, char** aLocation,
+    bool aPropagateExceptions, MutableHandleValue aException) {
+  MOZ_ASSERT(NS_IsMainThread(), "Must be on main thread.");
+
+  dom::AutoJSAPI jsapi;
+  jsapi.Init();
+  JSContext* cx = jsapi.cx();
+
+  nsresult rv = aInfo.EnsureURI();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  bool realFile = LocationIsRealFile(aInfo.URI());
+
+  RootedObject obj(
+      cx, PrepareObjectForLocation(cx, aModuleFile, aInfo.URI(), realFile));
+  NS_ENSURE_TRUE(obj, NS_ERROR_FAILURE);
+  MOZ_ASSERT(!JS_IsGlobalObject(obj));
+
+  JSAutoRealm ar(cx, obj);
+
+  RootedScript script(cx);
+  rv = GetScriptForLocation(cx, aInfo, aModuleFile, realFile, &script,
+                            aLocation);
+  if (NS_FAILED(rv)) {
+    // Propagate the exception, if one exists. Also, don't leave the stale
+    // exception on this context.
+    if (aPropagateExceptions && jsapi.HasException()) {
+      if (!jsapi.StealException(aException)) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+    }
+
+    return rv;
+  }
+
+  // Assign aObject here so that it's available to recursive imports.
+  // See bug 384168.
+  aObject.set(obj);
+
+  aTableScript.set(script);
+
+  {  // Scope for AutoEntryScript
+    AutoAllowLegacyScriptExecution exemption;
+
+    // We're going to run script via JS_ExecuteScript, so we need an
+    // AutoEntryScript. This is Gecko-specific and not in any spec.
+    dom::AutoEntryScript aes(CurrentGlobalOrNull(cx),
+                             "module loader load module");
+    JSContext* aescx = aes.cx();
+
+    bool executeOk = false;
+    if (JS_IsGlobalObject(obj)) {
+      JS::RootedValue rval(cx);
+      executeOk = JS_ExecuteScript(aescx, script, &rval);
+    } else {
+      executeOk = JS::ExecuteInJSMEnvironment(aescx, script, obj);
+    }
+
+    if (!executeOk) {
+      if (aPropagateExceptions && aes.HasException()) {
+        // Ignore return value because we're returning an error code
+        // anyway.
+        Unused << aes.StealException(aException);
+      }
+      aObject.set(nullptr);
+      aTableScript.set(nullptr);
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return rv;
+}
+
+/* static */
+nsresult mozJSModuleLoader::GetScriptForLocation(
+    JSContext* aCx, ModuleLoaderInfo& aInfo, nsIFile* aModuleFile,
+    bool aUseMemMap, MutableHandleScript aScriptOut, char** aLocationOut) {
+  // JS compilation errors are returned via an exception on the context.
+  MOZ_ASSERT(!JS_IsExceptionPending(aCx));
+
+  aScriptOut.set(nullptr);
+
+  nsAutoCString nativePath;
+  nsresult rv = aInfo.URI()->GetSpec(nativePath);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Before compiling the script, first check to see if we have it in
+  // the preloader cache or the startupcache.  Note: as a rule, preloader cache
+  // errors and startupcache errors are not fatal to loading the script, since
+  // we can always slow-load.
+
+  bool storeIntoStartupCache = false;
+  StartupCache* cache = StartupCache::GetSingleton();
+
+  aInfo.EnsureResolvedURI();
+
+  nsAutoCString cachePath;
+  if (aInfo.IsModule()) {
+    rv = PathifyURI(JS_CACHE_PREFIX("non-syntactic", "module"),
+                    aInfo.ResolvedURI(), cachePath);
+  } else {
+    rv = PathifyURI(JS_CACHE_PREFIX("non-syntactic", "script"),
+                    aInfo.ResolvedURI(), cachePath);
+  }
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  JS::DecodeOptions decodeOptions;
+  ScriptPreloader::FillDecodeOptionsForCachedStencil(decodeOptions);
+
+  RefPtr<JS::Stencil> stencil =
+      ScriptPreloader::GetSingleton().GetCachedStencil(aCx, decodeOptions,
+                                                       cachePath);
+
+  if (!stencil && cache) {
+    ReadCachedStencil(cache, cachePath, aCx, decodeOptions,
+                      getter_AddRefs(stencil));
+    if (!stencil) {
+      JS_ClearPendingException(aCx);
+
+      storeIntoStartupCache = true;
+    }
+  }
+
+  if (stencil) {
+    LOG(("Successfully loaded %s from cache\n", nativePath.get()));
+  } else {
+    // The script wasn't in the cache , so compile it now.
+    LOG(("Slow loading %s\n", nativePath.get()));
+
+    CompileOptions options(aCx);
+    ScriptPreloader::FillCompileOptionsForCachedStencil(options);
+    options.setFileAndLine(nativePath.get(), 1);
+    if (aInfo.IsModule()) {
+      options.setModule();
+      // Top level await is not supported in synchronously loaded modules.
+      options.topLevelAwait = false;
+
+      // Make all top-level `vars` available in `ModuleEnvironmentObject`.
+      options.deoptimizeModuleGlobalVars = true;
+    } else {
+      options.setForceStrictMode();
+      options.setNonSyntacticScope(true);
+    }
+
+    // If we can no longer write to caches, we should stop using lazy sources
+    // and instead let normal syntax parsing occur. This can occur in content
+    // processes after the ScriptPreloader is flushed where we can read but no
+    // longer write.
+    if (!storeIntoStartupCache && !ScriptPreloader::GetSingleton().Active()) {
+      options.setSourceIsLazy(false);
+    }
+
+    if (aUseMemMap) {
+      AutoMemMap map;
+      MOZ_TRY(map.init(aModuleFile));
+
+      // Note: exceptions will get handled further down;
+      // don't early return for them here.
+      auto buf = map.get<char>();
+
+      JS::SourceText<mozilla::Utf8Unit> srcBuf;
+      if (srcBuf.init(aCx, buf.get(), map.size(),
+                      JS::SourceOwnership::Borrowed)) {
+        stencil = CompileStencil(aCx, options, srcBuf, aInfo.IsModule());
+      }
+    } else {
+      nsCString str;
+      MOZ_TRY_VAR(str, ReadScript(aInfo));
+
+      JS::SourceText<mozilla::Utf8Unit> srcBuf;
+      if (srcBuf.init(aCx, str.get(), str.Length(),
+                      JS::SourceOwnership::Borrowed)) {
+        stencil = CompileStencil(aCx, options, srcBuf, aInfo.IsModule());
+      }
+    }
+
+#ifdef DEBUG
+    // The above shouldn't touch any options for instantiation.
+    JS::InstantiateOptions instantiateOptions(options);
+    instantiateOptions.assertDefault();
+#endif
+
+    if (!stencil) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  aScriptOut.set(InstantiateStencil(aCx, stencil, aInfo.IsModule()));
+  if (!aScriptOut) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // ScriptPreloader::NoteScript needs to be called unconditionally, to
+  // reflect the usage into the next session's cache.
+  ScriptPreloader::GetSingleton().NoteStencil(nativePath, cachePath, stencil);
+
+  // Write to startup cache only when we didn't have any cache for the script
+  // and compiled it.
+  if (storeIntoStartupCache) {
+    MOZ_ASSERT(stencil);
+
+    // We successfully compiled the script, so cache it.
+    rv = WriteCachedStencil(cache, cachePath, aCx, stencil);
+
+    // Don't treat failure to write as fatal, since we might be working
+    // with a read-only cache.
+    if (NS_SUCCEEDED(rv)) {
+      LOG(("Successfully wrote to cache\n"));
+    } else {
+      LOG(("Failed to write to cache\n"));
+    }
+  }
+
+  /* Owned by ModuleEntry. Freed when we remove from the table. */
+  if (aLocationOut) {
+    *aLocationOut = ToNewCString(nativePath, mozilla::fallible);
+    if (!*aLocationOut) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+  }
+
+  return NS_OK;
+}
+
+void mozJSModuleLoader::UnloadModules() {
+  mInitialized = false;
+
+  if (mLoaderGlobal) {
+    MOZ_ASSERT(JS_HasExtensibleLexicalEnvironment(mLoaderGlobal));
+    JS::RootedObject lexicalEnv(dom::RootingCx(),
+                                JS_ExtensibleLexicalEnvironment(mLoaderGlobal));
+    JS_SetAllNonReservedSlotsToUndefined(lexicalEnv);
+    JS_SetAllNonReservedSlotsToUndefined(mLoaderGlobal);
+    mLoaderGlobal = nullptr;
+  }
+  mServicesObj = nullptr;
+
+#ifdef STARTUP_RECORDER_ENABLED
+  mImportStacks.Clear();
+#endif
+  mFallbackImports.Clear();
+  mInProgressImports.Clear();
+  mImports.Clear();
+  mLocations.Clear();
+}
+
+/* static */
+already_AddRefed<Stencil> mozJSModuleLoader::CompileStencil(
+    JSContext* aCx, const JS::CompileOptions& aOptions,
+    JS::SourceText<mozilla::Utf8Unit>& aSource, bool aIsModule) {
+  if (aIsModule) {
+    return CompileModuleScriptToStencil(aCx, aOptions, aSource);
+  }
+
+  return CompileGlobalScriptToStencil(aCx, aOptions, aSource);
+}
+
+/* static */
+JSScript* mozJSModuleLoader::InstantiateStencil(JSContext* aCx,
+                                                JS::Stencil* aStencil,
+                                                bool aIsModule) {
+  JS::InstantiateOptions instantiateOptions;
+
+  if (aIsModule) {
+    RootedObject module(aCx);
+    module = JS::InstantiateModuleStencil(aCx, instantiateOptions, aStencil);
+    if (!module) {
+      return nullptr;
+    }
+
+    return JS::GetModuleScript(module);
+  }
+
+  return JS::InstantiateGlobalStencil(aCx, instantiateOptions, aStencil);
+}
+
+nsresult mozJSModuleLoader::ImportInto(const nsACString& registryLocation,
+                                       HandleValue targetValArg, JSContext* cx,
+                                       uint8_t optionalArgc,
+                                       MutableHandleValue retval) {
+  MOZ_ASSERT(nsContentUtils::IsCallerChrome());
+
+  RootedValue targetVal(cx, targetValArg);
+  RootedObject targetObject(cx, nullptr);
+
+  if (optionalArgc) {
+    // The caller passed in the optional second argument. Get it.
+    if (targetVal.isObject()) {
+      // If we're passing in something like a content DOM window, chances
+      // are the caller expects the properties to end up on the object
+      // proper and not on the Xray holder. This is dubious, but can be used
+      // during testing. Given that dumb callers can already leak JSMs into
+      // content by passing a raw content JS object (where Xrays aren't
+      // possible), we aim for consistency here. Waive xray.
+      if (WrapperFactory::IsXrayWrapper(&targetVal.toObject()) &&
+          !WrapperFactory::WaiveXrayAndWrap(cx, &targetVal)) {
+        return NS_ERROR_FAILURE;
+      }
+      targetObject = &targetVal.toObject();
+    } else if (!targetVal.isNull()) {
+      // If targetVal isNull(), we actually want to leave targetObject null.
+      // Not doing so breaks |make package|.
+      return ReportOnCallerUTF8(cx, ERROR_SCOPE_OBJ,
+                                PromiseFlatCString(registryLocation).get());
+    }
+  } else {
+    FindTargetObject(cx, &targetObject);
+    if (!targetObject) {
+      return ReportOnCallerUTF8(cx, ERROR_NO_TARGET_OBJECT,
+                                PromiseFlatCString(registryLocation).get());
+    }
+  }
+
+  js::AssertSameCompartment(cx, targetObject);
+
+  RootedObject global(cx);
+  nsresult rv = ImportInto(registryLocation, targetObject, cx, &global);
+
+  if (global) {
+    if (!JS_WrapObject(cx, &global)) {
+      NS_ERROR("can't wrap return value");
+      return NS_ERROR_FAILURE;
+    }
+
+    retval.setObject(*global);
+  }
+  return rv;
+}
+
+nsresult mozJSModuleLoader::IsModuleLoaded(const nsACString& aLocation,
+                                           bool* retval) {
+  MOZ_ASSERT(nsContentUtils::IsCallerChrome());
+
+  mInitialized = true;
+  ModuleLoaderInfo info(aLocation);
+  if (mImports.Get(info.Key())) {
+    *retval = true;
+    return NS_OK;
+  }
+
+  if (mModuleLoader) {
+    nsAutoCString mjsLocation;
+    if (!TryToMJS(aLocation, mjsLocation)) {
+      *retval = false;
+      return NS_OK;
+    }
+
+    ModuleLoaderInfo mjsInfo(mjsLocation);
+
+    nsresult rv = mjsInfo.EnsureURI();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (mModuleLoader->IsModuleFetched(mjsInfo.URI())) {
+      *retval = true;
+      return NS_OK;
+    }
+  }
+
+  *retval = false;
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::IsJSModuleLoaded(const nsACString& aLocation,
+                                             bool* retval) {
+  MOZ_ASSERT(nsContentUtils::IsCallerChrome());
+
+  mInitialized = true;
+  ModuleLoaderInfo info(aLocation);
+  if (mImports.Get(info.Key())) {
+    *retval = true;
+    return NS_OK;
+  }
+
+  *retval = false;
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::IsESModuleLoaded(const nsACString& aLocation,
+                                             bool* retval) {
+  MOZ_ASSERT(nsContentUtils::IsCallerChrome());
+
+  mInitialized = true;
+  ModuleLoaderInfo info(aLocation);
+
+  nsresult rv = info.EnsureURI();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (mModuleLoader->IsModuleFetched(info.URI())) {
+    *retval = true;
+    return NS_OK;
+  }
+
+  *retval = false;
+  return NS_OK;
+}
+
+void mozJSModuleLoader::GetLoadedModules(nsTArray<nsCString>& aLoadedModules) {
+  aLoadedModules.SetCapacity(mImports.Count());
+  for (const auto& data : mImports.Values()) {
+    aLoadedModules.AppendElement(data->location);
+  }
+}
+
+nsresult mozJSModuleLoader::GetLoadedESModules(
+    nsTArray<nsCString>& aLoadedModules) {
+  return mModuleLoader->GetFetchedModuleURLs(aLoadedModules);
+}
+
+nsresult mozJSModuleLoader::GetLoadedJSAndESModules(
+    nsTArray<nsCString>& aLoadedModules) {
+  GetLoadedModules(aLoadedModules);
+
+  nsTArray<nsCString> modules;
+  nsresult rv = GetLoadedESModules(modules);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  for (const auto& location : modules) {
+    if (IsMJS(location)) {
+      nsAutoCString jsmLocation;
+      // NOTE: Unconditionally convert to *.jsm.  This doesn't cover *.js case
+      //       but given `Cu.loadedModules` is rarely used for system modules,
+      //       this won't cause much compat issue.
+      MJSToJSM(location, jsmLocation);
+      aLoadedModules.AppendElement(jsmLocation);
+    }
+  }
+
+  return NS_OK;
+}
+
+#ifdef STARTUP_RECORDER_ENABLED
+void mozJSModuleLoader::RecordImportStack(JSContext* aCx,
+                                          const nsACString& aLocation) {
+  if (!Preferences::GetBool("browser.startup.record", false)) {
+    return;
+  }
+
+  mImportStacks.InsertOrUpdate(
+      aLocation, xpc_PrintJSStack(aCx, false, false, false).get());
+}
+
+void mozJSModuleLoader::RecordImportStack(
+    JSContext* aCx, JS::loader::ModuleLoadRequest* aRequest) {
+  if (!Preferences::GetBool("browser.startup.record", false)) {
+    return;
+  }
+
+  nsAutoCString location;
+  nsresult rv = aRequest->mURI->GetSpec(location);
+  if (NS_FAILED(rv)) {
+    return;
+  }
+
+  auto recordJSStackOnly = [&]() {
+    mImportStacks.InsertOrUpdate(
+        location, xpc_PrintJSStack(aCx, false, false, false).get());
+  };
+
+  if (aRequest->IsTopLevel()) {
+    recordJSStackOnly();
+    return;
+  }
+
+  nsAutoCString importerSpec;
+  rv = aRequest->mReferrer->GetSpec(importerSpec);
+  if (NS_FAILED(rv)) {
+    recordJSStackOnly();
+    return;
+  }
+
+  ModuleLoaderInfo importerInfo(importerSpec);
+  auto importerStack = mImportStacks.Lookup(importerInfo.Key());
+  if (!importerStack) {
+    // The importer's stack is not collected, possibly due to OOM.
+    recordJSStackOnly();
+    return;
+  }
+
+  nsAutoCString stack;
+
+  stack += "* import [\"";
+  stack += importerSpec;
+  stack += "\"]\n";
+  stack += *importerStack;
+
+  mImportStacks.InsertOrUpdate(location, stack);
+}
+#endif
+
+nsresult mozJSModuleLoader::GetModuleImportStack(const nsACString& aLocation,
+                                                 nsACString& retval) {
+#ifdef STARTUP_RECORDER_ENABLED
+  MOZ_ASSERT(nsContentUtils::IsCallerChrome());
+
+  // When querying the DevTools loader, it may not be initialized yet
+  if (!mInitialized) {
+    return NS_ERROR_FAILURE;
+  }
+
+  ModuleLoaderInfo info(aLocation);
+  auto str = mImportStacks.Lookup(info.Key());
+  if (!str) {
+    return NS_ERROR_FAILURE;
+  }
+
+  retval = *str;
+  return NS_OK;
+#else
+  return NS_ERROR_NOT_IMPLEMENTED;
+#endif
+}
+
+nsresult mozJSModuleLoader::ImportInto(const nsACString& aLocation,
+                                       HandleObject targetObj, JSContext* cx,
+                                       MutableHandleObject vp) {
+  vp.set(nullptr);
+
+  JS::RootedObject exports(cx);
+  MOZ_TRY(Import(cx, aLocation, vp, &exports, !targetObj));
+
+  if (targetObj) {
+    JS::Rooted<JS::IdVector> ids(cx, JS::IdVector(cx));
+    if (!JS_Enumerate(cx, exports, &ids)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    JS::RootedValue value(cx);
+    JS::RootedId id(cx);
+    for (jsid idVal : ids) {
+      id = idVal;
+      if (!JS_GetPropertyById(cx, exports, id, &value) ||
+          !JS_SetPropertyById(cx, targetObj, id, value)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::ExtractExports(JSContext* aCx,
+                                           ModuleLoaderInfo& aInfo,
+                                           ModuleEntry* aMod,
+                                           JS::MutableHandleObject aExports) {
+  // cxhelper must be created before jsapi, so that jsapi is destroyed and
+  // pops any context it has pushed before we report to the caller context.
+  JSCLContextHelper cxhelper(aCx);
+
+  // Even though we are calling JS_SetPropertyById on targetObj, we want
+  // to ensure that we never run script here, so we use an AutoJSAPI and
+  // not an AutoEntryScript.
+  dom::AutoJSAPI jsapi;
+  jsapi.Init();
+  JSContext* cx = jsapi.cx();
+  JSAutoRealm ar(cx, aMod->obj);
+
+  RootedValue symbols(cx);
+  {
+    RootedObject obj(
+        cx, ResolveModuleObjectProperty(cx, aMod->obj, "EXPORTED_SYMBOLS"));
+    if (!obj || !JS_GetProperty(cx, obj, "EXPORTED_SYMBOLS", &symbols)) {
+      return ReportOnCallerUTF8(cxhelper, ERROR_NOT_PRESENT, aInfo);
+    }
+  }
+
+  bool isArray;
+  if (!JS::IsArrayObject(cx, symbols, &isArray)) {
+    return NS_ERROR_FAILURE;
+  }
+  if (!isArray) {
+    return ReportOnCallerUTF8(cxhelper, ERROR_NOT_AN_ARRAY, aInfo);
+  }
+
+  RootedObject symbolsObj(cx, &symbols.toObject());
+
+  // Iterate over symbols array, installing symbols on targetObj:
+
+  uint32_t symbolCount = 0;
+  if (!JS::GetArrayLength(cx, symbolsObj, &symbolCount)) {
+    return ReportOnCallerUTF8(cxhelper, ERROR_GETTING_ARRAY_LENGTH, aInfo);
+  }
+
+#ifdef DEBUG
+  nsAutoCString logBuffer;
+#endif
+
+  aExports.set(JS_NewPlainObject(cx));
+  if (!aExports) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  bool missing = false;
+
+  RootedValue value(cx);
+  RootedId symbolId(cx);
+  RootedObject symbolHolder(cx);
+  for (uint32_t i = 0; i < symbolCount; ++i) {
+    if (!JS_GetElement(cx, symbolsObj, i, &value) || !value.isString() ||
+        !JS_ValueToId(cx, value, &symbolId)) {
+      return ReportOnCallerUTF8(cxhelper, ERROR_ARRAY_ELEMENT, aInfo, i);
+    }
+
+    symbolHolder = ResolveModuleObjectPropertyById(cx, aMod->obj, symbolId);
+    if (!symbolHolder ||
+        !JS_GetPropertyById(cx, symbolHolder, symbolId, &value)) {
+      RootedString symbolStr(cx, symbolId.toString());
+      JS::UniqueChars bytes = JS_EncodeStringToUTF8(cx, symbolStr);
+      if (!bytes) {
+        return NS_ERROR_FAILURE;
+      }
+      return ReportOnCallerUTF8(cxhelper, ERROR_GETTING_SYMBOL, aInfo,
+                                bytes.get());
+    }
+
+    // It's possible |value| is the uninitialized lexical MagicValue when
+    // there's a cyclic import: const obj = ChromeUtils.import("parent.jsm").
+    if (value.isMagic(JS_UNINITIALIZED_LEXICAL)) {
+      RootedString symbolStr(cx, symbolId.toString());
+      JS::UniqueChars bytes = JS_EncodeStringToUTF8(cx, symbolStr);
+      if (!bytes) {
+        return NS_ERROR_FAILURE;
+      }
+      return ReportOnCallerUTF8(cxhelper, ERROR_UNINITIALIZED_SYMBOL, aInfo,
+                                bytes.get());
+    }
+
+    if (value.isUndefined()) {
+      missing = true;
+    }
+
+    if (!JS_SetPropertyById(cx, aExports, symbolId, value)) {
+      RootedString symbolStr(cx, symbolId.toString());
+      JS::UniqueChars bytes = JS_EncodeStringToUTF8(cx, symbolStr);
+      if (!bytes) {
+        return NS_ERROR_FAILURE;
+      }
+      return ReportOnCallerUTF8(cxhelper, ERROR_GETTING_SYMBOL, aInfo,
+                                bytes.get());
+    }
+#ifdef DEBUG
+    if (i == 0) {
+      logBuffer.AssignLiteral("Installing symbols [ ");
+    }
+    JS::UniqueChars bytes = JS_EncodeStringToLatin1(cx, symbolId.toString());
+    if (!!bytes) {
+      logBuffer.Append(bytes.get());
+    }
+    logBuffer.Append(' ');
+    if (i == symbolCount - 1) {
+      nsCString location;
+      MOZ_TRY(aInfo.GetLocation(location));
+      LOG(("%s] from %s\n", logBuffer.get(), location.get()));
+    }
+#endif
+  }
+
+  // Don't cache the exports object if any of its exported symbols are
+  // missing. If the module hasn't finished loading yet, they may be
+  // defined the next time we try to import it.
+  if (!missing) {
+    aMod->exports = aExports;
+  }
+  return NS_OK;
+}
+
+/* static */
+bool mozJSModuleLoader::IsTrustedScheme(nsIURI* aURI) {
+  return aURI->SchemeIs("resource") || aURI->SchemeIs("chrome");
+}
+
+nsresult mozJSModuleLoader::Import(JSContext* aCx, const nsACString& aLocation,
+                                   JS::MutableHandleObject aModuleGlobal,
+                                   JS::MutableHandleObject aModuleExports,
+                                   bool aIgnoreExports) {
+  mInitialized = true;
+
+  AUTO_PROFILER_MARKER_TEXT(
+      "ChromeUtils.import", JS,
+      MarkerOptions(MarkerStack::Capture(),
+                    MarkerInnerWindowIdFromJSContext(aCx)),
+      Substring(aLocation, 0, std::min(size_t(128), aLocation.Length())));
+
+  // The JSM may already be ESM-ified, and in that case the load is expected
+  // to fail.  Suppress the error message, the crash, and also the telemetry
+  // event for the failure.
+  //
+  // If this load fails, it will be redirected to `.sys.mjs` URL
+  // in TryFallbackToImportESModule, and if the redirect also fails,
+  // the load is performed again below, with the check enabled.
+  ModuleLoaderInfo info(aLocation, SkipCheckForBrokenURLOrZeroSized::Yes);
+
+  nsresult rv;
+  ModuleEntry* mod;
+  UniquePtr<ModuleEntry> newEntry;
+  if (!mImports.Get(info.Key(), &mod) &&
+      !mInProgressImports.Get(info.Key(), &mod)) {
+    // We're trying to import a new JSM, but we're late in shutdown and this
+    // will likely not succeed and might even crash, so fail here.
+    if (PastShutdownPhase(ShutdownPhase::XPCOMShutdownFinal)) {
+      return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+    }
+
+    // If we've hit file-not-found and fallback was successful,
+    // return the cached data.
+    bool aFound;
+    rv = TryCachedFallbackToImportESModule(
+        aCx, aLocation, aModuleGlobal, aModuleExports, aIgnoreExports, &aFound);
+    NS_ENSURE_SUCCESS(rv, rv);
+    if (aFound) {
+      return NS_OK;
+    }
+
+    newEntry = MakeUnique<ModuleEntry>(RootingContext::get(aCx));
+
+    // Note: This implies EnsureURI().
+    MOZ_TRY(info.EnsureResolvedURI());
+
+    // Reject imports from untrusted sources.
+    if (!IsTrustedScheme(info.URI())) {
+      return NS_ERROR_DOM_SECURITY_ERR;
+    }
+
+    nsCOMPtr<nsIFile> sourceFile;
+    rv = GetSourceFile(info.ResolvedURI(), getter_AddRefs(sourceFile));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = info.ResolvedURI()->GetSpec(newEntry->resolvedURL);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    nsCString* existingPath;
+    if (mLocations.Get(newEntry->resolvedURL, &existingPath) &&
+        *existingPath != info.Key()) {
+      return NS_ERROR_UNEXPECTED;
+    }
+
+    mLocations.InsertOrUpdate(newEntry->resolvedURL,
+                              MakeUnique<nsCString>(info.Key()));
+
+    RootedValue exception(aCx);
+    {
+      mInProgressImports.InsertOrUpdate(info.Key(), newEntry.get());
+      auto cleanup =
+          MakeScopeExit([&]() { mInProgressImports.Remove(info.Key()); });
+
+      rv = ObjectForLocation(info, sourceFile, &newEntry->obj,
+                             &newEntry->thisObjectKey, &newEntry->location,
+                             true, &exception);
+    }
+
+    if (NS_FAILED(rv)) {
+      mLocations.Remove(newEntry->resolvedURL);
+      if (!exception.isUndefined()) {
+        // An exception was thrown during compilation. Propagate it
+        // out to our caller so they can report it.
+        bool isModuleSyntaxError = false;
+
+        if (exception.isObject()) {
+          JS::Rooted<JSObject*> exceptionObj(aCx, &exception.toObject());
+          JSAutoRealm ar(aCx, exceptionObj);
+          JSErrorReport* report = JS_ErrorFromException(aCx, exceptionObj);
+          if (report) {
+            switch (report->errorNumber) {
+              case JSMSG_IMPORT_DECL_AT_TOP_LEVEL:
+              case JSMSG_EXPORT_DECL_AT_TOP_LEVEL:
+                // If the exception is related to module syntax, it's most
+                // likely because of misuse of API.
+                // Provide better error message.
+                isModuleSyntaxError = true;
+
+                JS_ReportErrorUTF8(aCx,
+                                   "ChromeUtils.import is called against "
+                                   "an ES module script (%s).  Please use "
+                                   "ChromeUtils.importESModule instead "
+                                   "(SyntaxError: %s)",
+                                   aLocation.BeginReading(),
+                                   report->message().c_str());
+                break;
+              default:
+                break;
+            }
+          }
+        }
+
+        if (!isModuleSyntaxError) {
+          if (!JS_WrapValue(aCx, &exception)) {
+            return NS_ERROR_OUT_OF_MEMORY;
+          }
+          JS_SetPendingException(aCx, exception);
+        }
+
+        return NS_ERROR_FAILURE;
+      }
+
+      if (rv == NS_ERROR_FILE_NOT_FOUND || rv == NS_ERROR_FILE_ACCESS_DENIED) {
+        // NS_ERROR_FILE_ACCESS_DENIED happens if the access is blocked by
+        // sandbox.
+        rv = TryFallbackToImportESModule(aCx, aLocation, aModuleGlobal,
+                                         aModuleExports, aIgnoreExports);
+
+        if (rv == NS_ERROR_FILE_NOT_FOUND ||
+            rv == NS_ERROR_FILE_ACCESS_DENIED) {
+          // Both JSM and ESM are not found, with the check inside necko
+          // skipped (See EnsureScriptChannel and mSkipCheck).
+          //
+          // Perform the load again with the check enabled, so that
+          // logging, crash-on-autonation, and telemetry event happen.
+          if (NS_SUCCEEDED(info.EnsureURI()) &&
+              !LocationIsRealFile(info.URI())) {
+            info.resetChannelWithCheckForBrokenURLOrZeroSized();
+            (void)ReadScript(info);
+          }
+        }
+
+        return rv;
+      }
+
+      // Something failed, but we don't know what it is, guess.
+      return NS_ERROR_FILE_NOT_FOUND;
+    }
+
+#ifdef STARTUP_RECORDER_ENABLED
+    RecordImportStack(aCx, aLocation);
+#endif
+
+    mod = newEntry.get();
+  }
+
+  MOZ_ASSERT(mod->obj, "Import table contains entry with no object");
+  JS::RootedObject globalProxy(aCx);
+  {
+    JSAutoRealm ar(aCx, mod->obj);
+
+    globalProxy = CreateJSMEnvironmentProxy(aCx, mod->obj);
+    if (!globalProxy) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+  if (!JS_WrapObject(aCx, &globalProxy)) {
+    return NS_ERROR_FAILURE;
+  }
+  aModuleGlobal.set(globalProxy);
+
+  JS::RootedObject exports(aCx, mod->exports);
+  if (!exports && !aIgnoreExports) {
+    MOZ_TRY(ExtractExports(aCx, info, mod, &exports));
+  }
+
+  if (exports && !JS_WrapObject(aCx, &exports)) {
+    mLocations.Remove(newEntry->resolvedURL);
+    return NS_ERROR_FAILURE;
+  }
+  aModuleExports.set(exports);
+
+  // Cache this module for later
+  if (newEntry) {
+    mImports.InsertOrUpdate(info.Key(), std::move(newEntry));
+  }
+
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::TryFallbackToImportESModule(
+    JSContext* aCx, const nsACString& aLocation,
+    JS::MutableHandleObject aModuleGlobal,
+    JS::MutableHandleObject aModuleExports, bool aIgnoreExports) {
+  nsAutoCString mjsLocation;
+  if (!TryToMJS(aLocation, mjsLocation)) {
+    return NS_ERROR_FILE_NOT_FOUND;
+  }
+
+  JS::RootedObject moduleNamespace(aCx);
+  // The fallback can fail if the URL was not for ESMified JSM.  Suppress the
+  // error message, the crash, and also the telemetry event for the failure.
+  nsresult rv = ImportESModule(aCx, mjsLocation, &moduleNamespace,
+                               SkipCheckForBrokenURLOrZeroSized::Yes);
+  if (rv == NS_ERROR_FILE_NOT_FOUND || rv == NS_ERROR_FILE_ACCESS_DENIED) {
+    // The error for ESModule shouldn't be exposed if the file does not exist,
+    // or the access is blocked by sandbox.
+    if (JS_IsExceptionPending(aCx)) {
+      JS_ClearPendingException(aCx);
+    }
+  }
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  JS::RootedObject globalProxy(aCx);
+  {
+    JSAutoRealm ar(aCx, moduleNamespace);
+
+    JS::RootedObject moduleObject(
+        aCx, JS::GetModuleForNamespace(aCx, moduleNamespace));
+    if (!moduleObject) {
+      return NS_ERROR_FAILURE;
+    }
+
+    globalProxy = CreateModuleEnvironmentProxy(aCx, moduleObject);
+    if (!globalProxy) {
+      return NS_ERROR_FAILURE;
+    }
+
+    // Cache the redirect to use in subsequent imports.
+    ModuleLoaderInfo info(aLocation);
+    auto newEntry = MakeUnique<FallbackModuleEntry>(RootingContext::get(aCx));
+    newEntry->globalProxy = globalProxy;
+    newEntry->moduleNamespace = moduleNamespace;
+    mFallbackImports.InsertOrUpdate(info.Key(), std::move(newEntry));
+  }
+
+  if (!JS_WrapObject(aCx, &globalProxy)) {
+    return NS_ERROR_FAILURE;
+  }
+  aModuleGlobal.set(globalProxy);
+
+  if (!aIgnoreExports) {
+    JS::RootedObject exports(aCx, moduleNamespace);
+    if (!JS_WrapObject(aCx, &exports)) {
+      return NS_ERROR_FAILURE;
+    }
+    aModuleExports.set(exports);
+  }
+
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::TryCachedFallbackToImportESModule(
+    JSContext* aCx, const nsACString& aLocation,
+    JS::MutableHandleObject aModuleGlobal,
+    JS::MutableHandleObject aModuleExports, bool aIgnoreExports, bool* aFound) {
+  ModuleLoaderInfo info(aLocation);
+  FallbackModuleEntry* fallbackMod;
+  if (!mFallbackImports.Get(info.Key(), &fallbackMod)) {
+    *aFound = false;
+    return NS_OK;
+  }
+
+  JS::RootedObject globalProxy(aCx, fallbackMod->globalProxy);
+  if (!JS_WrapObject(aCx, &globalProxy)) {
+    return NS_ERROR_FAILURE;
+  }
+  aModuleGlobal.set(globalProxy);
+
+  if (!aIgnoreExports) {
+    JS::RootedObject exports(aCx, fallbackMod->moduleNamespace);
+    if (!JS_WrapObject(aCx, &exports)) {
+      return NS_ERROR_FAILURE;
+    }
+    aModuleExports.set(exports);
+  }
+
+  *aFound = true;
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::ImportESModule(
+    JSContext* aCx, const nsACString& aLocation,
+    JS::MutableHandleObject aModuleNamespace,
+    SkipCheckForBrokenURLOrZeroSized
+        aSkipCheck /* = SkipCheckForBrokenURLOrZeroSized::No */) {
+  using namespace JS::loader;
+
+  mInitialized = true;
+
+  // Called from ChromeUtils::ImportESModule.
+  nsCString str(aLocation);
+
+  AUTO_PROFILER_MARKER_TEXT(
+      "ChromeUtils.importESModule", JS,
+      MarkerOptions(MarkerStack::Capture(),
+                    MarkerInnerWindowIdFromJSContext(aCx)),
+      Substring(aLocation, 0, std::min(size_t(128), aLocation.Length())));
+
+  RootedObject globalObj(aCx, GetSharedGlobal(aCx));
+  NS_ENSURE_TRUE(globalObj, NS_ERROR_FAILURE);
+  MOZ_ASSERT(xpc::Scriptability::Get(globalObj).Allowed());
+
+  // The module loader should be instantiated when fetching the shared global
+  MOZ_ASSERT(mModuleLoader);
+
+  JSAutoRealm ar(aCx, globalObj);
+
+  nsCOMPtr<nsIURI> uri;
+  nsresult rv = NS_NewURI(getter_AddRefs(uri), aLocation);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIPrincipal> principal =
+      mModuleLoader->GetGlobalObject()->PrincipalOrNull();
+  MOZ_ASSERT(principal);
+
+  RefPtr<ScriptFetchOptions> options = new ScriptFetchOptions(
+      CORS_NONE, dom::ReferrerPolicy::No_referrer, principal);
+
+  RefPtr<ComponentLoadContext> context = new ComponentLoadContext();
+  context->mSkipCheck = aSkipCheck;
+
+  RefPtr<VisitedURLSet> visitedSet =
+      ModuleLoadRequest::NewVisitedSetForTopLevelImport(uri);
+
+  RefPtr<ModuleLoadRequest> request = new ModuleLoadRequest(
+      uri, options, dom::SRIMetadata(),
+      /* aReferrer = */ nullptr, context,
+      /* aIsTopLevel = */ true,
+      /* aIsDynamicImport = */ false, mModuleLoader, visitedSet, nullptr);
+
+  rv = request->StartModuleLoad();
+  if (NS_FAILED(rv)) {
+    mModuleLoader->MaybeReportLoadError(aCx);
+    return rv;
+  }
+
+  rv = mModuleLoader->ProcessRequests();
+  if (NS_FAILED(rv)) {
+    mModuleLoader->MaybeReportLoadError(aCx);
+    return rv;
+  }
+
+  MOZ_ASSERT(request->IsReadyToRun());
+  if (!request->mModuleScript) {
+    mModuleLoader->MaybeReportLoadError(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  // All modules are loaded. MaybeReportLoadError isn't necessary from here.
+
+  if (!request->InstantiateModuleGraph()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  rv = mModuleLoader->EvaluateModuleInContext(aCx, request,
+                                              JS::ThrowModuleErrorsSync);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (JS_IsExceptionPending(aCx)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  RefPtr<ModuleScript> moduleScript = request->mModuleScript;
+  JS::Rooted<JSObject*> module(aCx, moduleScript->ModuleRecord());
+  aModuleNamespace.set(JS::GetModuleNamespace(aCx, module));
+
+  return NS_OK;
+}
+
+nsresult mozJSModuleLoader::Unload(const nsACString& aLocation) {
+  if (!mInitialized) {
+    return NS_OK;
+  }
+
+  ModuleLoaderInfo info(aLocation);
+
+  ModuleEntry* mod;
+  if (mImports.Get(info.Key(), &mod)) {
+    mLocations.Remove(mod->resolvedURL);
+    mImports.Remove(info.Key());
+  }
+
+  // If this is the last module to be unloaded, we will leak mLoaderGlobal
+  // until UnloadModules is called. So be it.
+
+  return NS_OK;
+}
+
+bool mozJSModuleLoader::CreateJSServices(JSContext* aCx) {
+  JSObject* services = NewJSServices(aCx);
+  if (!services) {
+    return false;
+  }
+
+  mServicesObj = services;
+  return true;
+}
+
+bool mozJSModuleLoader::DefineJSServices(JSContext* aCx,
+                                         JS::Handle<JSObject*> aGlobal) {
+  if (!mServicesObj) {
+    // This function is called whenever creating a new global that needs
+    // `Services`, including the loader's shared global.
+    //
+    // This function is no-op if it's called during creating the loader's
+    // shared global.
+    //
+    // See also  CreateAndDefineJSServices.
+    MOZ_ASSERT(!mLoaderGlobal);
+    MOZ_ASSERT(mIsInitializingLoaderGlobal);
+    return true;
+  }
+
+  JS::Rooted<JS::Value> services(aCx, ObjectValue(*mServicesObj));
+  if (!JS_WrapValue(aCx, &services)) {
+    return false;
+  }
+
+  JS::Rooted<JS::PropertyKey> servicesId(
+      aCx, XPCJSContext::Get()->GetStringID(XPCJSContext::IDX_SERVICES));
+  return JS_DefinePropertyById(aCx, aGlobal, servicesId, services, 0);
+}
+
+size_t mozJSModuleLoader::ModuleEntry::SizeOfIncludingThis(
+    MallocSizeOf aMallocSizeOf) const {
+  size_t n = aMallocSizeOf(this);
+  n += aMallocSizeOf(location);
+
+  return n;
+}
+
+//----------------------------------------------------------------------
+
+JSCLContextHelper::JSCLContextHelper(JSContext* aCx)
+    : mContext(aCx), mBuf(nullptr) {}
+
+JSCLContextHelper::~JSCLContextHelper() {
+  if (mBuf) {
+    JS_ReportErrorUTF8(mContext, "%s", mBuf.get());
+  }
+}
+
+void JSCLContextHelper::reportErrorAfterPop(UniqueChars&& buf) {
+  MOZ_ASSERT(!mBuf, "Already called reportErrorAfterPop");
+  mBuf = std::move(buf);
+}
diff --git a/js/xpconnect/loader/mozJSModuleLoader.h b/js/xpconnect/loader/mozJSModuleLoader.h
new file mode 100644
index 0000000000..e5a114a193
--- /dev/null
+++ b/js/xpconnect/loader/mozJSModuleLoader.h
@@ -0,0 +1,264 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozJSModuleLoader_h
+#define mozJSModuleLoader_h
+
+#include "ComponentModuleLoader.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/FileLocation.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/StaticPtr.h"
+#include "nsIMemoryReporter.h"
+#include "nsISupports.h"
+#include "nsIURI.h"
+#include "nsClassHashtable.h"
+#include "jsapi.h"
+#include "js/experimental/JSStencil.h"
+#include "SkipCheckForBrokenURLOrZeroSized.h"
+
+#include "xpcpublic.h"
+
+class nsIFile;
+class ModuleLoaderInfo;
+
+namespace mozilla {
+class ScriptPreloader;
+}  // namespace mozilla
+
+namespace JS::loader {
+class ModuleLoadRequest;
+}  // namespace JS::loader
+
+#if defined(NIGHTLY_BUILD) || defined(MOZ_DEV_EDITION) || defined(DEBUG)
+#  define STARTUP_RECORDER_ENABLED
+#endif
+
+class mozJSModuleLoader final : public nsIMemoryReporter {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  // Returns the list of all JSMs.
+  void GetLoadedModules(nsTArray<nsCString>& aLoadedModules);
+
+  // Returns the list of all ESMs.
+  nsresult GetLoadedESModules(nsTArray<nsCString>& aLoadedModules);
+
+  // Returns the list of all JSMs and ESMs.
+  nsresult GetLoadedJSAndESModules(nsTArray<nsCString>& aLoadedModules);
+
+  nsresult GetModuleImportStack(const nsACString& aLocation,
+                                nsACString& aRetval);
+
+  void FindTargetObject(JSContext* aCx, JS::MutableHandleObject aTargetObject);
+
+  static void InitStatics();
+  static void UnloadLoaders();
+  static void ShutdownLoaders();
+
+  static mozJSModuleLoader* Get() {
+    MOZ_ASSERT(sSelf, "Should have already created the module loader");
+    return sSelf;
+  }
+
+  static mozJSModuleLoader* GetDevToolsLoader() { return sDevToolsLoader; }
+  static mozJSModuleLoader* GetOrCreateDevToolsLoader();
+
+  nsresult ImportInto(const nsACString& aResourceURI,
+                      JS::HandleValue aTargetObj, JSContext* aCx, uint8_t aArgc,
+                      JS::MutableHandleValue aRetval);
+
+  // Load a JSM.
+  nsresult Import(JSContext* aCx, const nsACString& aResourceURI,
+                  JS::MutableHandleObject aModuleGlobal,
+                  JS::MutableHandleObject aModuleExports,
+                  bool aIgnoreExports = false);
+
+  // Load an ES6 module and all its dependencies.
+  nsresult ImportESModule(
+      JSContext* aCx, const nsACString& aResourceURI,
+      JS::MutableHandleObject aModuleNamespace,
+      mozilla::loader::SkipCheckForBrokenURLOrZeroSized aSkipCheck =
+          mozilla::loader::SkipCheckForBrokenURLOrZeroSized::No);
+
+  // Fallback from Import to ImportESModule.
+  nsresult TryFallbackToImportESModule(JSContext* aCx,
+                                       const nsACString& aResourceURI,
+                                       JS::MutableHandleObject aModuleGlobal,
+                                       JS::MutableHandleObject aModuleExports,
+                                       bool aIgnoreExports);
+
+  // If the request was handled by fallback before, fills the output and
+  // sets *aFound to true and returns NS_OK.
+  // If the request wasn't yet handled by fallback, sets *Found to false
+  // and returns NS_OK.
+  nsresult TryCachedFallbackToImportESModule(
+      JSContext* aCx, const nsACString& aResourceURI,
+      JS::MutableHandleObject aModuleGlobal,
+      JS::MutableHandleObject aModuleExports, bool aIgnoreExports,
+      bool* aFound);
+
+#ifdef STARTUP_RECORDER_ENABLED
+  void RecordImportStack(JSContext* aCx, const nsACString& aLocation);
+  void RecordImportStack(JSContext* aCx,
+                         JS::loader::ModuleLoadRequest* aRequest);
+#endif
+
+  nsresult Unload(const nsACString& aResourceURI);
+  nsresult IsModuleLoaded(const nsACString& aResourceURI, bool* aRetval);
+  nsresult IsJSModuleLoaded(const nsACString& aResourceURI, bool* aRetval);
+  nsresult IsESModuleLoaded(const nsACString& aResourceURI, bool* aRetval);
+  bool IsLoaderGlobal(JSObject* aObj) { return mLoaderGlobal == aObj; }
+  bool IsDevToolsLoader() const { return this == sDevToolsLoader; }
+
+  // Public methods for use from ComponentModuleLoader.
+  static bool IsTrustedScheme(nsIURI* aURI);
+  static nsresult LoadSingleModuleScript(
+      mozilla::loader::ComponentModuleLoader* aModuleLoader, JSContext* aCx,
+      JS::loader::ModuleLoadRequest* aRequest,
+      JS::MutableHandleScript aScriptOut);
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
+
+  bool DefineJSServices(JSContext* aCx, JS::Handle<JSObject*> aGlobal);
+
+ protected:
+  mozJSModuleLoader();
+  ~mozJSModuleLoader();
+
+  friend class XPCJSRuntime;
+
+ private:
+  static mozilla::StaticRefPtr<mozJSModuleLoader> sSelf;
+  static mozilla::StaticRefPtr<mozJSModuleLoader> sDevToolsLoader;
+
+  void Unload();
+  void UnloadModules();
+
+  void CreateLoaderGlobal(JSContext* aCx, const nsACString& aLocation,
+                          JS::MutableHandleObject aGlobal);
+  void CreateDevToolsLoaderGlobal(JSContext* aCx, const nsACString& aLocation,
+                                  JS::MutableHandleObject aGlobal);
+
+  bool CreateJSServices(JSContext* aCx);
+
+  JSObject* GetSharedGlobal(JSContext* aCx);
+
+  static nsresult GetSourceFile(nsIURI* aResolvedURI, nsIFile** aSourceFileOut);
+
+  static bool LocationIsRealFile(nsIURI* aURI);
+
+  JSObject* PrepareObjectForLocation(JSContext* aCx, nsIFile* aModuleFile,
+                                     nsIURI* aURI, bool aRealFile);
+
+  nsresult ObjectForLocation(ModuleLoaderInfo& aInfo, nsIFile* aModuleFile,
+                             JS::MutableHandleObject aObject,
+                             JS::MutableHandleScript aTableScript,
+                             char** aLocation, bool aCatchException,
+                             JS::MutableHandleValue aException);
+
+  // Get the script for a given location, either from a cached stencil or by
+  // compiling it from source.
+  static nsresult GetScriptForLocation(JSContext* aCx, ModuleLoaderInfo& aInfo,
+                                       nsIFile* aModuleFile, bool aUseMemMap,
+                                       JS::MutableHandleScript aScriptOut,
+                                       char** aLocationOut = nullptr);
+
+  static already_AddRefed<JS::Stencil> CompileStencil(
+      JSContext* aCx, const JS::CompileOptions& aOptions,
+      JS::SourceText<mozilla::Utf8Unit>& aSource, bool aIsModule);
+  static JSScript* InstantiateStencil(JSContext* aCx, JS::Stencil* aStencil,
+                                      bool aIsModule);
+
+  nsresult ImportInto(const nsACString& aLocation, JS::HandleObject targetObj,
+                      JSContext* callercx, JS::MutableHandleObject vp);
+
+  class ModuleEntry {
+   public:
+    explicit ModuleEntry(JS::RootingContext* aRootingCx)
+        : obj(aRootingCx), exports(aRootingCx), thisObjectKey(aRootingCx) {
+      location = nullptr;
+    }
+
+    ~ModuleEntry() { Clear(); }
+
+    void Clear() {
+      if (obj) {
+        if (JS_HasExtensibleLexicalEnvironment(obj)) {
+          JS::RootedObject lexicalEnv(mozilla::dom::RootingCx(),
+                                      JS_ExtensibleLexicalEnvironment(obj));
+          JS_SetAllNonReservedSlotsToUndefined(lexicalEnv);
+        }
+        JS_SetAllNonReservedSlotsToUndefined(obj);
+        obj = nullptr;
+        thisObjectKey = nullptr;
+      }
+
+      if (location) {
+        free(location);
+      }
+
+      obj = nullptr;
+      thisObjectKey = nullptr;
+      location = nullptr;
+    }
+
+    size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+    JS::PersistentRootedObject obj;
+    JS::PersistentRootedObject exports;
+    JS::PersistentRootedScript thisObjectKey;
+    char* location;
+    nsCString resolvedURL;
+  };
+
+  class FallbackModuleEntry {
+   public:
+    explicit FallbackModuleEntry(JS::RootingContext* aRootingCx)
+        : globalProxy(aRootingCx), moduleNamespace(aRootingCx) {}
+
+    ~FallbackModuleEntry() { Clear(); }
+
+    void Clear() {
+      globalProxy = nullptr;
+      moduleNamespace = nullptr;
+    }
+
+    size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+      return aMallocSizeOf(this);
+    }
+
+    JS::PersistentRootedObject globalProxy;
+    JS::PersistentRootedObject moduleNamespace;
+  };
+
+  nsresult ExtractExports(JSContext* aCx, ModuleLoaderInfo& aInfo,
+                          ModuleEntry* aMod, JS::MutableHandleObject aExports);
+
+  nsClassHashtable<nsCStringHashKey, ModuleEntry> mImports;
+  nsTHashMap<nsCStringHashKey, ModuleEntry*> mInProgressImports;
+  nsClassHashtable<nsCStringHashKey, FallbackModuleEntry> mFallbackImports;
+#ifdef STARTUP_RECORDER_ENABLED
+  nsTHashMap<nsCStringHashKey, nsCString> mImportStacks;
+#endif
+
+  // A map of on-disk file locations which are loaded as modules to the
+  // pre-resolved URIs they were loaded from. Used to prevent the same file
+  // from being loaded separately, from multiple URLs.
+  nsClassHashtable<nsCStringHashKey, nsCString> mLocations;
+
+  bool mInitialized;
+#ifdef DEBUG
+  bool mIsInitializingLoaderGlobal = false;
+#endif
+  JS::PersistentRooted<JSObject*> mLoaderGlobal;
+  JS::PersistentRooted<JSObject*> mServicesObj;
+
+  RefPtr<mozilla::loader::ComponentModuleLoader> mModuleLoader;
+};
+
+#endif
diff --git a/js/xpconnect/loader/mozJSSubScriptLoader.cpp b/js/xpconnect/loader/mozJSSubScriptLoader.cpp
new file mode 100644
index 0000000000..33192bff29
--- /dev/null
+++ b/js/xpconnect/loader/mozJSSubScriptLoader.cpp
@@ -0,0 +1,476 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozJSSubScriptLoader.h"
+#include "js/experimental/JSStencil.h"
+#include "mozJSModuleLoader.h"
+#include "mozJSLoaderUtils.h"
+
+#include "nsIURI.h"
+#include "nsIIOService.h"
+#include "nsIChannel.h"
+#include "nsIInputStream.h"
+#include "nsNetCID.h"
+#include "nsNetUtil.h"
+
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "xpcprivate.h"                   // xpc::OptionsBase
+#include "js/CompilationAndEvaluation.h"  // JS::Compile
+#include "js/CompileOptions.h"  // JS::ReadOnlyCompileOptions, JS::DecodeOptions
+#include "js/friend/JSMEnvironment.h"  // JS::ExecuteInJSMEnvironment, JS::IsJSMEnvironment
+#include "js/SourceText.h"             // JS::Source{Ownership,Text}
+#include "js/Wrapper.h"
+
+#include "mozilla/ContentPrincipal.h"
+#include "mozilla/dom/ScriptLoader.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/ScriptPreloader.h"
+#include "mozilla/SystemPrincipal.h"
+#include "mozilla/scache/StartupCache.h"
+#include "mozilla/scache/StartupCacheUtils.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Utf8.h"  // mozilla::Utf8Unit
+#include "nsContentUtils.h"
+#include "nsString.h"
+
+using namespace mozilla::scache;
+using namespace JS;
+using namespace xpc;
+using namespace mozilla;
+using namespace mozilla::dom;
+
+class MOZ_STACK_CLASS LoadSubScriptOptions : public OptionsBase {
+ public:
+  explicit LoadSubScriptOptions(JSContext* cx = xpc_GetSafeJSContext(),
+                                JSObject* options = nullptr)
+      : OptionsBase(cx, options),
+        target(cx),
+        ignoreCache(false),
+        wantReturnValue(false) {}
+
+  virtual bool Parse() override {
+    return ParseObject("target", &target) &&
+           ParseBoolean("ignoreCache", &ignoreCache) &&
+           ParseBoolean("wantReturnValue", &wantReturnValue);
+  }
+
+  RootedObject target;
+  bool ignoreCache;
+  bool wantReturnValue;
+};
+
+/* load() error msgs, XXX localize? */
+#define LOAD_ERROR_NOSERVICE "Error creating IO Service."
+#define LOAD_ERROR_NOURI "Error creating URI (invalid URL scheme?)"
+#define LOAD_ERROR_NOSCHEME "Failed to get URI scheme.  This is bad."
+#define LOAD_ERROR_URI_NOT_LOCAL "Trying to load a non-local URI."
+#define LOAD_ERROR_NOSTREAM "Error opening input stream (invalid filename?)"
+#define LOAD_ERROR_NOCONTENT "ContentLength not available (not a local URL?)"
+#define LOAD_ERROR_BADCHARSET "Error converting to specified charset"
+#define LOAD_ERROR_NOSPEC "Failed to get URI spec.  This is bad."
+#define LOAD_ERROR_CONTENTTOOBIG "ContentLength is too large"
+
+mozJSSubScriptLoader::mozJSSubScriptLoader() = default;
+
+mozJSSubScriptLoader::~mozJSSubScriptLoader() = default;
+
+NS_IMPL_ISUPPORTS(mozJSSubScriptLoader, mozIJSSubScriptLoader)
+
+#define JSSUB_CACHE_PREFIX(aScopeType, aCompilationTarget) \
+  "jssubloader/" aScopeType "/" aCompilationTarget
+
+static void SubscriptCachePath(JSContext* cx, nsIURI* uri,
+                               JS::HandleObject targetObj,
+                               nsACString& cachePath) {
+  // StartupCache must distinguish between non-syntactic vs global when
+  // computing the cache key.
+  if (!JS_IsGlobalObject(targetObj)) {
+    PathifyURI(JSSUB_CACHE_PREFIX("non-syntactic", "script"), uri, cachePath);
+  } else {
+    PathifyURI(JSSUB_CACHE_PREFIX("global", "script"), uri, cachePath);
+  }
+}
+
+static void ReportError(JSContext* cx, const nsACString& msg) {
+  NS_ConvertUTF8toUTF16 ucMsg(msg);
+
+  RootedValue exn(cx);
+  if (xpc::NonVoidStringToJsval(cx, ucMsg, &exn)) {
+    JS_SetPendingException(cx, exn);
+  }
+}
+
+static void ReportError(JSContext* cx, const char* origMsg, nsIURI* uri) {
+  if (!uri) {
+    ReportError(cx, nsDependentCString(origMsg));
+    return;
+  }
+
+  nsAutoCString spec;
+  nsresult rv = uri->GetSpec(spec);
+  if (NS_FAILED(rv)) {
+    spec.AssignLiteral("(unknown)");
+  }
+
+  nsAutoCString msg(origMsg);
+  msg.AppendLiteral(": ");
+  msg.Append(spec);
+  ReportError(cx, msg);
+}
+
+static bool EvalStencil(JSContext* cx, HandleObject targetObj,
+                        HandleObject loadScope, MutableHandleValue retval,
+                        nsIURI* uri, bool storeIntoStartupCache,
+                        bool storeIntoPreloadCache, JS::Stencil* stencil) {
+  MOZ_ASSERT(!js::IsWrapper(targetObj));
+
+  JS::InstantiateOptions options;
+  JS::RootedScript script(cx,
+                          JS::InstantiateGlobalStencil(cx, options, stencil));
+  if (!script) {
+    return false;
+  }
+
+  if (JS_IsGlobalObject(targetObj)) {
+    if (!JS_ExecuteScript(cx, script, retval)) {
+      return false;
+    }
+  } else if (JS::IsJSMEnvironment(targetObj)) {
+    if (!JS::ExecuteInJSMEnvironment(cx, script, targetObj)) {
+      return false;
+    }
+    retval.setUndefined();
+  } else {
+    JS::RootedObjectVector envChain(cx);
+    if (!envChain.append(targetObj)) {
+      return false;
+    }
+    if (!loadScope) {
+      // A null loadScope means we are cross-realm. In this case, we should
+      // check the target isn't in the JSM loader shared-global or we will
+      // contaminate all JSMs in the realm.
+      //
+      // NOTE: If loadScope is already a shared-global JSM, we can't
+      // determine which JSM the target belongs to and have to assume it
+      // is in our JSM.
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      JSObject* targetGlobal = JS::GetNonCCWObjectGlobal(targetObj);
+      MOZ_DIAGNOSTIC_ASSERT(
+          !mozJSModuleLoader::Get()->IsLoaderGlobal(targetGlobal),
+          "Don't load subscript into target in a shared-global JSM");
+#endif
+      if (!JS_ExecuteScript(cx, envChain, script, retval)) {
+        return false;
+      }
+    } else if (JS_IsGlobalObject(loadScope)) {
+      if (!JS_ExecuteScript(cx, envChain, script, retval)) {
+        return false;
+      }
+    } else {
+      MOZ_ASSERT(JS::IsJSMEnvironment(loadScope));
+      if (!JS::ExecuteInJSMEnvironment(cx, script, loadScope, envChain)) {
+        return false;
+      }
+      retval.setUndefined();
+    }
+  }
+
+  JSAutoRealm rar(cx, targetObj);
+  if (!JS_WrapValue(cx, retval)) {
+    return false;
+  }
+
+  if (script && (storeIntoStartupCache || storeIntoPreloadCache)) {
+    nsAutoCString cachePath;
+    SubscriptCachePath(cx, uri, targetObj, cachePath);
+
+    nsCString uriStr;
+    if (storeIntoPreloadCache && NS_SUCCEEDED(uri->GetSpec(uriStr))) {
+      ScriptPreloader::GetSingleton().NoteStencil(uriStr, cachePath, stencil);
+    }
+
+    if (storeIntoStartupCache) {
+      JSAutoRealm ar(cx, script);
+      WriteCachedStencil(StartupCache::GetSingleton(), cachePath, cx, stencil);
+    }
+  }
+
+  return true;
+}
+
+bool mozJSSubScriptLoader::ReadStencil(
+    JS::Stencil** stencilOut, nsIURI* uri, JSContext* cx,
+    const JS::ReadOnlyCompileOptions& options, nsIIOService* serv,
+    bool useCompilationScope) {
+  // We create a channel and call SetContentType, to avoid expensive MIME type
+  // lookups (bug 632490).
+  nsCOMPtr<nsIChannel> chan;
+  nsCOMPtr<nsIInputStream> instream;
+  nsresult rv;
+  rv = NS_NewChannel(getter_AddRefs(chan), uri,
+                     nsContentUtils::GetSystemPrincipal(),
+                     nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+                     nsIContentPolicy::TYPE_OTHER,
+                     nullptr,  // nsICookieJarSettings
+                     nullptr,  // PerformanceStorage
+                     nullptr,  // aLoadGroup
+                     nullptr,  // aCallbacks
+                     nsIRequest::LOAD_NORMAL, serv);
+
+  if (NS_SUCCEEDED(rv)) {
+    chan->SetContentType("application/javascript"_ns);
+    rv = chan->Open(getter_AddRefs(instream));
+  }
+
+  if (NS_FAILED(rv)) {
+    ReportError(cx, LOAD_ERROR_NOSTREAM, uri);
+    return false;
+  }
+
+  int64_t len = -1;
+
+  rv = chan->GetContentLength(&len);
+  if (NS_FAILED(rv)) {
+    ReportError(cx, LOAD_ERROR_NOCONTENT, uri);
+    return false;
+  }
+
+  if (len > INT32_MAX) {
+    ReportError(cx, LOAD_ERROR_CONTENTTOOBIG, uri);
+    return false;
+  }
+
+  nsCString buf;
+  rv = NS_ReadInputStreamToString(instream, buf, len);
+  NS_ENSURE_SUCCESS(rv, false);
+
+  if (len < 0) {
+    len = buf.Length();
+  }
+
+  Maybe<JSAutoRealm> ar;
+
+  // Note that when using the ScriptPreloader cache with loadSubScript, there
+  // will be a side-effect of keeping the global that the script was compiled
+  // for alive. See note above in EvalScript().
+  //
+  // This will compile the script in XPConnect compilation scope. When the
+  // script is evaluated, it will be cloned into the target scope to be
+  // executed, avoiding leaks on the first session when we don't have a
+  // startup cache.
+  if (useCompilationScope) {
+    ar.emplace(cx, xpc::CompilationScope());
+  }
+
+  JS::SourceText<Utf8Unit> srcBuf;
+  if (!srcBuf.init(cx, buf.get(), len, JS::SourceOwnership::Borrowed)) {
+    return false;
+  }
+
+  RefPtr<JS::Stencil> stencil =
+      JS::CompileGlobalScriptToStencil(cx, options, srcBuf);
+  stencil.forget(stencilOut);
+  return *stencilOut;
+}
+
+NS_IMETHODIMP
+mozJSSubScriptLoader::LoadSubScript(const nsAString& url, HandleValue target,
+                                    JSContext* cx, MutableHandleValue retval) {
+  /*
+   * Loads a local url, referring to UTF-8-encoded data, and evals it into the
+   * current cx.  Synchronous. ChromeUtils.compileScript() should be used for
+   * async loads.
+   *   url: The url to load.  Must be local so that it can be loaded
+   *        synchronously.
+   *   targetObj: Optional object to eval the script onto (defaults to context
+   *              global)
+   *   returns: Whatever jsval the script pointed to by the url returns.
+   * Should ONLY (O N L Y !) be called from JavaScript code.
+   */
+  LoadSubScriptOptions options(cx);
+  options.target = target.isObject() ? &target.toObject() : nullptr;
+  return DoLoadSubScriptWithOptions(url, options, cx, retval);
+}
+
+NS_IMETHODIMP
+mozJSSubScriptLoader::LoadSubScriptWithOptions(const nsAString& url,
+                                               HandleValue optionsVal,
+                                               JSContext* cx,
+                                               MutableHandleValue retval) {
+  if (!optionsVal.isObject()) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  LoadSubScriptOptions options(cx, &optionsVal.toObject());
+  if (!options.Parse()) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  return DoLoadSubScriptWithOptions(url, options, cx, retval);
+}
+
+nsresult mozJSSubScriptLoader::DoLoadSubScriptWithOptions(
+    const nsAString& url, LoadSubScriptOptions& options, JSContext* cx,
+    MutableHandleValue retval) {
+  nsresult rv = NS_OK;
+  RootedObject targetObj(cx);
+  RootedObject loadScope(cx);
+  mozJSModuleLoader* loader = mozJSModuleLoader::Get();
+  loader->FindTargetObject(cx, &loadScope);
+
+  if (options.target) {
+    targetObj = options.target;
+  } else {
+    targetObj = loadScope;
+  }
+
+  targetObj = JS_FindCompilationScope(cx, targetObj);
+  if (!targetObj || !loadScope) {
+    return NS_ERROR_FAILURE;
+  }
+
+  MOZ_ASSERT(!js::IsWrapper(targetObj), "JS_FindCompilationScope must unwrap");
+
+  if (js::GetNonCCWObjectRealm(loadScope) !=
+      js::GetNonCCWObjectRealm(targetObj)) {
+    loadScope = nullptr;
+  }
+
+  /* load up the url.  From here on, failures are reflected as ``custom''
+   * js exceptions */
+  nsCOMPtr<nsIURI> uri;
+  nsAutoCString uriStr;
+  nsAutoCString scheme;
+
+  // Figure out who's calling us
+  JS::AutoFilename filename;
+  if (!JS::DescribeScriptedCaller(cx, &filename)) {
+    // No scripted frame means we don't know who's calling, bail.
+    return NS_ERROR_FAILURE;
+  }
+
+  JSAutoRealm ar(cx, targetObj);
+
+  nsCOMPtr<nsIIOService> serv = do_GetService(NS_IOSERVICE_CONTRACTID);
+  if (!serv) {
+    ReportError(cx, nsLiteralCString(LOAD_ERROR_NOSERVICE));
+    return NS_OK;
+  }
+
+  NS_LossyConvertUTF16toASCII asciiUrl(url);
+  const nsDependentCSubstring profilerUrl =
+      Substring(asciiUrl, 0, std::min(size_t(128), asciiUrl.Length()));
+  AUTO_PROFILER_LABEL_DYNAMIC_NSCSTRING_NONSENSITIVE(
+      "mozJSSubScriptLoader::DoLoadSubScriptWithOptions", OTHER, profilerUrl);
+  AUTO_PROFILER_MARKER_TEXT("SubScript", JS,
+                            MarkerOptions(MarkerStack::Capture(),
+                                          MarkerInnerWindowIdFromJSContext(cx)),
+                            profilerUrl);
+
+  // Make sure to explicitly create the URI, since we'll need the
+  // canonicalized spec.
+  rv = NS_NewURI(getter_AddRefs(uri), asciiUrl);
+  if (NS_FAILED(rv)) {
+    ReportError(cx, nsLiteralCString(LOAD_ERROR_NOURI));
+    return NS_OK;
+  }
+
+  rv = uri->GetSpec(uriStr);
+  if (NS_FAILED(rv)) {
+    ReportError(cx, nsLiteralCString(LOAD_ERROR_NOSPEC));
+    return NS_OK;
+  }
+
+  rv = uri->GetScheme(scheme);
+  if (NS_FAILED(rv)) {
+    ReportError(cx, LOAD_ERROR_NOSCHEME, uri);
+    return NS_OK;
+  }
+
+  // Suppress caching if we're compiling as content or if we're loading a
+  // blob: URI.
+  bool useCompilationScope = false;
+  auto* principal = BasePrincipal::Cast(GetObjectPrincipal(targetObj));
+  bool isSystem = principal->Is<SystemPrincipal>();
+  if (!isSystem && principal->Is<ContentPrincipal>()) {
+    nsAutoCString scheme;
+    principal->GetScheme(scheme);
+
+    // We want to enable caching for scripts with Activity Stream's
+    // codebase URLs.
+    if (scheme.EqualsLiteral("about")) {
+      nsAutoCString filePath;
+      principal->GetFilePath(filePath);
+
+      useCompilationScope = filePath.EqualsLiteral("home") ||
+                            filePath.EqualsLiteral("newtab") ||
+                            filePath.EqualsLiteral("welcome");
+      isSystem = true;
+    }
+  }
+  bool ignoreCache =
+      options.ignoreCache || !isSystem || scheme.EqualsLiteral("blob");
+
+  StartupCache* cache = ignoreCache ? nullptr : StartupCache::GetSingleton();
+
+  nsAutoCString cachePath;
+  SubscriptCachePath(cx, uri, targetObj, cachePath);
+
+  JS::DecodeOptions decodeOptions;
+  ScriptPreloader::FillDecodeOptionsForCachedStencil(decodeOptions);
+
+  RefPtr<JS::Stencil> stencil;
+  if (!options.ignoreCache) {
+    if (!options.wantReturnValue) {
+      // NOTE: If we need the return value, we cannot use ScriptPreloader.
+      stencil = ScriptPreloader::GetSingleton().GetCachedStencil(
+          cx, decodeOptions, cachePath);
+    }
+    if (!stencil && cache) {
+      rv = ReadCachedStencil(cache, cachePath, cx, decodeOptions,
+                             getter_AddRefs(stencil));
+      if (NS_FAILED(rv) || !stencil) {
+        JS_ClearPendingException(cx);
+      }
+    }
+  }
+
+  bool storeIntoStartupCache = false;
+  if (!stencil) {
+    // Store into startup cache only when the script isn't come from any cache.
+    storeIntoStartupCache = cache;
+
+    JS::CompileOptions compileOptions(cx);
+    ScriptPreloader::FillCompileOptionsForCachedStencil(compileOptions);
+    compileOptions.setFileAndLine(uriStr.get(), 1);
+    compileOptions.setNonSyntacticScope(!JS_IsGlobalObject(targetObj));
+
+    if (options.wantReturnValue) {
+      compileOptions.setNoScriptRval(false);
+    }
+
+    if (!ReadStencil(getter_AddRefs(stencil), uri, cx, compileOptions, serv,
+                     useCompilationScope)) {
+      return NS_OK;
+    }
+
+#ifdef DEBUG
+    // The above shouldn't touch any options for instantiation.
+    JS::InstantiateOptions instantiateOptions(compileOptions);
+    instantiateOptions.assertDefault();
+#endif
+  }
+
+  // As a policy choice, we don't store scripts that want return values
+  // into the preload cache.
+  bool storeIntoPreloadCache = !ignoreCache && !options.wantReturnValue;
+
+  Unused << EvalStencil(cx, targetObj, loadScope, retval, uri,
+                        storeIntoStartupCache, storeIntoPreloadCache, stencil);
+  return NS_OK;
+}
diff --git a/js/xpconnect/loader/mozJSSubScriptLoader.h b/js/xpconnect/loader/mozJSSubScriptLoader.h
new file mode 100644
index 0000000000..01909da10c
--- /dev/null
+++ b/js/xpconnect/loader/mozJSSubScriptLoader.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsCOMPtr.h"
+#include "mozIJSSubScriptLoader.h"
+
+#include "js/experimental/JSStencil.h"
+#include "js/CompileOptions.h"  // JS::ReadOnlyCompileOptions
+
+class nsIPrincipal;
+class nsIURI;
+class LoadSubScriptOptions;
+
+#define MOZ_JSSUBSCRIPTLOADER_CID                    \
+  { /* 829814d6-1dd2-11b2-8e08-82fa0a339b00 */       \
+    0x929814d6, 0x1dd2, 0x11b2, {                    \
+      0x8e, 0x08, 0x82, 0xfa, 0x0a, 0x33, 0x9b, 0x00 \
+    }                                                \
+  }
+
+class nsIIOService;
+
+class mozJSSubScriptLoader : public mozIJSSubScriptLoader {
+ public:
+  mozJSSubScriptLoader();
+
+  // all the interface method declarations...
+  NS_DECL_ISUPPORTS
+  NS_DECL_MOZIJSSUBSCRIPTLOADER
+
+ private:
+  virtual ~mozJSSubScriptLoader();
+
+  bool ReadStencil(JS::Stencil** stencilOut, nsIURI* uri, JSContext* cx,
+                   const JS::ReadOnlyCompileOptions& options,
+                   nsIIOService* serv, bool useCompilationScope);
+
+  nsresult ReadScriptAsync(nsIURI* uri, JS::HandleObject targetObj,
+                           JS::HandleObject loadScope, nsIIOService* serv,
+                           bool wantReturnValue, bool cache,
+                           JS::MutableHandleValue retval);
+
+  nsresult DoLoadSubScriptWithOptions(const nsAString& url,
+                                      LoadSubScriptOptions& options,
+                                      JSContext* cx,
+                                      JS::MutableHandleValue retval);
+};
diff --git a/js/xpconnect/loader/nsImportModule.cpp b/js/xpconnect/loader/nsImportModule.cpp
new file mode 100644
index 0000000000..a313c44388
--- /dev/null
+++ b/js/xpconnect/loader/nsImportModule.cpp
@@ -0,0 +1,113 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsImportModule.h"
+
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozJSModuleLoader.h"
+#include "nsContentUtils.h"
+#include "nsExceptionHandler.h"
+#include "nsPrintfCString.h"
+#include "xpcpublic.h"
+#include "xpcprivate.h"
+#include "js/PropertyAndElement.h"  // JS_GetProperty
+
+using mozilla::dom::AutoJSAPI;
+
+namespace mozilla {
+namespace loader {
+
+static void AnnotateCrashReportWithJSException(JSContext* aCx,
+                                               const char* aURI) {
+  JS::RootedValue exn(aCx);
+  if (JS_GetPendingException(aCx, &exn)) {
+    JS_ClearPendingException(aCx);
+
+    JSAutoRealm ar(aCx, xpc::PrivilegedJunkScope());
+    JS_WrapValue(aCx, &exn);
+
+    nsAutoCString file;
+    uint32_t line;
+    uint32_t column;
+    nsAutoString msg;
+    nsContentUtils::ExtractErrorValues(aCx, exn, file, &line, &column, msg);
+
+    nsPrintfCString errorString("Failed to load module \"%s\": %s:%u:%u: %s",
+                                aURI, file.get(), line, column,
+                                NS_ConvertUTF16toUTF8(msg).get());
+
+    CrashReporter::AnnotateCrashReport(
+        CrashReporter::Annotation::JSModuleLoadError, errorString);
+  }
+}
+
+nsresult ImportModule(const char* aURI, const char* aExportName,
+                      const nsIID& aIID, void** aResult, bool aInfallible) {
+  AutoJSAPI jsapi;
+  MOZ_ALWAYS_TRUE(jsapi.Init(xpc::PrivilegedJunkScope()));
+  JSContext* cx = jsapi.cx();
+
+  JS::RootedObject global(cx);
+  JS::RootedObject exports(cx);
+  nsresult rv = mozJSModuleLoader::Get()->Import(cx, nsDependentCString(aURI),
+                                                 &global, &exports);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    if (aInfallible) {
+      AnnotateCrashReportWithJSException(cx, aURI);
+
+      MOZ_CRASH_UNSAFE_PRINTF("Failed to load critical module \"%s\"", aURI);
+    }
+    return rv;
+  }
+
+  if (aExportName) {
+    JS::RootedValue namedExport(cx);
+    if (!JS_GetProperty(cx, exports, aExportName, &namedExport)) {
+      return NS_ERROR_FAILURE;
+    }
+    if (!namedExport.isObject()) {
+      return NS_ERROR_XPC_BAD_CONVERT_JS;
+    }
+    exports.set(&namedExport.toObject());
+  }
+
+  return nsXPConnect::XPConnect()->WrapJS(cx, exports, aIID, aResult);
+}
+
+nsresult ImportESModule(const char* aURI, const char* aExportName,
+                        const nsIID& aIID, void** aResult, bool aInfallible) {
+  AutoJSAPI jsapi;
+  MOZ_ALWAYS_TRUE(jsapi.Init(xpc::PrivilegedJunkScope()));
+  JSContext* cx = jsapi.cx();
+
+  JS::RootedObject moduleNamespace(cx);
+  nsresult rv = mozJSModuleLoader::Get()->ImportESModule(
+      cx, nsDependentCString(aURI), &moduleNamespace);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    if (aInfallible) {
+      AnnotateCrashReportWithJSException(cx, aURI);
+
+      MOZ_CRASH_UNSAFE_PRINTF("Failed to load critical module \"%s\"", aURI);
+    }
+    return rv;
+  }
+
+  if (aExportName) {
+    JS::RootedValue namedExport(cx);
+    if (!JS_GetProperty(cx, moduleNamespace, aExportName, &namedExport)) {
+      return NS_ERROR_FAILURE;
+    }
+    if (!namedExport.isObject()) {
+      return NS_ERROR_XPC_BAD_CONVERT_JS;
+    }
+    moduleNamespace.set(&namedExport.toObject());
+  }
+
+  return nsXPConnect::XPConnect()->WrapJS(cx, moduleNamespace, aIID, aResult);
+}
+
+}  // namespace loader
+}  // namespace mozilla
diff --git a/js/xpconnect/loader/nsImportModule.h b/js/xpconnect/loader/nsImportModule.h
new file mode 100644
index 0000000000..31f6f8c7c1
--- /dev/null
+++ b/js/xpconnect/loader/nsImportModule.h
@@ -0,0 +1,240 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsImportModule_h
+#define nsImportModule_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+#include "nsCOMPtr.h"
+#include "mozilla/RefPtr.h"
+
+namespace mozilla {
+namespace loader {
+
+nsresult ImportModule(const char* aURI, const char* aExportName,
+                      const nsIID& aIID, void** aResult, bool aInfallible);
+
+nsresult ImportESModule(const char* aURI, const char* aExportName,
+                        const nsIID& aIID, void** aResult, bool aInfallible);
+
+}  // namespace loader
+}  // namespace mozilla
+
+class MOZ_STACK_CLASS nsImportModule final : public nsCOMPtr_helper {
+ public:
+  nsImportModule(const char* aURI, const char* aExportName, nsresult* aErrorPtr,
+                 bool aInfallible)
+      : mURI(aURI),
+        mExportName(aExportName),
+        mErrorPtr(aErrorPtr),
+        mInfallible(aInfallible) {
+    MOZ_ASSERT_IF(mErrorPtr, !mInfallible);
+  }
+
+  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID,
+                                          void** aResult) const override {
+    nsresult rv = ::mozilla::loader::ImportModule(mURI, mExportName, aIID,
+                                                  aResult, mInfallible);
+    if (mErrorPtr) {
+      *mErrorPtr = rv;
+    }
+    return rv;
+  }
+
+ private:
+  const char* mURI;
+  const char* mExportName;
+  nsresult* mErrorPtr;
+  bool mInfallible;
+};
+
+/**
+ * These helpers make it considerably easier for C++ code to import a JS module
+ * and wrap it in an appropriately-defined XPIDL interface for its exports.
+ * Typical usage is something like:
+ *
+ * Foo.jsm:
+ *
+ *   var EXPORTED_SYMBOLS = ["foo"];
+ *
+ *   function foo(bar) {
+ *     return bar.toString();
+ *   }
+ *
+ * mozIFoo.idl:
+ *
+ *   interface mozIFoo : nsISupports {
+ *     AString foo(double meh);
+ *   }
+ *
+ * Thing.cpp:
+ *
+ *   nsCOMPtr<mozIFoo> foo = do_ImportModule(
+ *     "resource://meh/Foo.jsm");
+ *
+ *   MOZ_TRY(foo->Foo(42));
+ *
+ * For JS modules which export all fields within a single named object, a second
+ * argument can be passed naming that object.
+ *
+ * Foo.jsm:
+ *
+ *   var EXPORTED_SYMBOLS = ["Foo"];
+ *
+ *   var Foo = {
+ *     function foo(bar) {
+ *       return bar.toString();
+ *     }
+ *   };
+ *
+ * Thing.cpp:
+ *
+ *   nsCOMPtr<mozIFoo> foo = do_ImportModule(
+ *       "resource:://meh/Foo.jsm", "Foo");
+ */
+
+template <size_t N>
+inline nsImportModule do_ImportModule(const char (&aURI)[N]) {
+  return {aURI, nullptr, nullptr, /* infallible */ true};
+}
+
+template <size_t N>
+inline nsImportModule do_ImportModule(const char (&aURI)[N],
+                                      const mozilla::fallible_t&) {
+  return {aURI, nullptr, nullptr, /* infallible */ false};
+}
+
+template <size_t N>
+inline nsImportModule do_ImportModule(const char (&aURI)[N], nsresult* aRv) {
+  return {aURI, nullptr, aRv, /* infallible */ false};
+}
+
+template <size_t N, size_t N2>
+inline nsImportModule do_ImportModule(const char (&aURI)[N],
+                                      const char (&aExportName)[N2]) {
+  return {aURI, aExportName, nullptr, /* infallible */ true};
+}
+
+template <size_t N, size_t N2>
+inline nsImportModule do_ImportModule(const char (&aURI)[N],
+                                      const char (&aExportName)[N2],
+                                      const mozilla::fallible_t&) {
+  return {aURI, aExportName, nullptr, /* infallible */ false};
+}
+
+template <size_t N, size_t N2>
+inline nsImportModule do_ImportModule(const char (&aURI)[N],
+                                      const char (&aExportName)[N2],
+                                      nsresult* aRv) {
+  return {aURI, aExportName, aRv, /* infallible */ false};
+}
+
+class MOZ_STACK_CLASS nsImportESModule final : public nsCOMPtr_helper {
+ public:
+  nsImportESModule(const char* aURI, const char* aExportName,
+                   nsresult* aErrorPtr, bool aInfallible)
+      : mURI(aURI),
+        mExportName(aExportName),
+        mErrorPtr(aErrorPtr),
+        mInfallible(aInfallible) {
+    MOZ_ASSERT_IF(mErrorPtr, !mInfallible);
+  }
+
+  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID,
+                                          void** aResult) const override {
+    nsresult rv = ::mozilla::loader::ImportESModule(mURI, mExportName, aIID,
+                                                    aResult, mInfallible);
+    if (mErrorPtr) {
+      *mErrorPtr = rv;
+    }
+    return rv;
+  }
+
+ private:
+  const char* mURI;
+  const char* mExportName;
+  nsresult* mErrorPtr;
+  bool mInfallible;
+};
+
+/**
+ * Usage with exported name:
+ *
+ * Foo.sys.mjs:
+ *
+ *   export function foo(bar) {
+ *     return bar.toString();
+ *   }
+ *
+ * mozIFoo.idl:
+ *
+ *   interface mozIFoo : nsISupports {
+ *     AString foo(double meh);
+ *   }
+ *
+ * Thing.cpp:
+ *
+ *   nsCOMPtr<mozIFoo> foo = do_ImportESModule(
+ *     "resource://meh/Foo.sys.mjs");
+ *
+ *   MOZ_TRY(foo->Foo(42));
+ *
+ * Usage with a single named object:
+ *
+ * Foo.sys.mjs:
+ *
+ *   export var Foo = {
+ *     function foo(bar) {
+ *       return bar.toString();
+ *     }
+ *   };
+ *
+ * Thing.cpp:
+ *
+ *   nsCOMPtr<mozIFoo> foo = do_ImportESModule(
+ *       "resource:://meh/Foo.sys.mjs", "Foo");
+ */
+
+template <size_t N>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N]) {
+  return {aURI, nullptr, nullptr, /* infallible */ true};
+}
+
+template <size_t N>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N],
+                                          const mozilla::fallible_t&) {
+  return {aURI, nullptr, nullptr, /* infallible */ false};
+}
+
+template <size_t N>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N],
+                                          nsresult* aRv) {
+  return {aURI, nullptr, aRv, /* infallible */ false};
+}
+
+template <size_t N, size_t N2>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N],
+                                          const char (&aExportName)[N2]) {
+  return {aURI, aExportName, nullptr, /* infallible */ true};
+}
+
+template <size_t N, size_t N2>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N],
+                                          const char (&aExportName)[N2],
+                                          const mozilla::fallible_t&) {
+  return {aURI, aExportName, nullptr, /* infallible */ false};
+}
+
+template <size_t N, size_t N2>
+inline nsImportESModule do_ImportESModule(const char (&aURI)[N],
+                                          const char (&aExportName)[N2],
+                                          nsresult* aRv) {
+  return {aURI, aExportName, aRv, /* infallible */ false};
+}
+
+#endif  // defined nsImportModule_h
diff --git a/js/xpconnect/loader/script_cache.py b/js/xpconnect/loader/script_cache.py
new file mode 100755
index 0000000000..bd3a746fcf
--- /dev/null
+++ b/js/xpconnect/loader/script_cache.py
@@ -0,0 +1,92 @@
+#!/usr/bin/env python
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import io
+import os
+import struct
+import sys
+
+MAGIC = b"mozXDRcachev002\0"
+
+
+def usage():
+    print(
+        """Usage: script_cache.py <file.bin> ...
+
+        Decodes and prints out the contents of a startup script cache file
+        (e.g., startupCache/scriptCache.bin) in human-readable form."""
+    )
+
+    sys.exit(1)
+
+
+class ProcessTypes:
+    Uninitialized = 0
+    Parent = 1
+    Web = 2
+    Extension = 3
+    Privileged = 4
+
+    def __init__(self, val):
+        self.val = val
+
+    def __str__(self):
+        res = []
+        if self.val & (1 << self.Uninitialized):
+            raise Exception("Uninitialized process type")
+        if self.val & (1 << self.Parent):
+            res.append("Parent")
+        if self.val & (1 << self.Web):
+            res.append("Web")
+        if self.val & (1 << self.Extension):
+            res.append("Extension")
+        if self.val & (1 << self.Privileged):
+            res.append("Privileged")
+        return "|".join(res)
+
+
+class InputBuffer(object):
+    def __init__(self, data):
+        self.data = data
+        self.offset = 0
+
+    @property
+    def remaining(self):
+        return len(self.data) - self.offset
+
+    def unpack(self, fmt):
+        res = struct.unpack_from(fmt, self.data, self.offset)
+        self.offset += struct.calcsize(fmt)
+        return res
+
+    def unpack_str(self):
+        (size,) = self.unpack("<H")
+        res = self.data[self.offset : self.offset + size].decode("utf-8")
+        self.offset += size
+        return res
+
+
+if len(sys.argv) < 2 or not os.path.exists(sys.argv[1]):
+    usage()
+
+for filename in sys.argv[1:]:
+    with io.open(filename, "rb") as f:
+        magic = f.read(len(MAGIC))
+        if magic != MAGIC:
+            raise Exception("Bad magic number")
+
+        (hdrSize,) = struct.unpack("<I", f.read(4))
+
+        hdr = InputBuffer(f.read(hdrSize))
+
+        i = 0
+        while hdr.remaining:
+            i += 1
+            print("{}: {}".format(i, hdr.unpack_str()))
+            print("  Key:       {}".format(hdr.unpack_str()))
+            print("  Offset:    {:>9,}".format(*hdr.unpack("<I")))
+            print("  Size:      {:>9,}".format(*hdr.unpack("<I")))
+            print("  Processes: {}".format(ProcessTypes(*hdr.unpack("B"))))
+            print("")