Adding upstream version 110.0.1.upstream/110.0.1upstream

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

66 files changed, 14152 insertions, 0 deletions

diff --git a/dom/system/IOUtils.cpp b/dom/system/IOUtils.cpp
new file mode 100644
index 0000000000..3c12b4016f
--- /dev/null
+++ b/dom/system/IOUtils.cpp
@@ -0,0 +1,2884 @@
+/* -*- 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 "IOUtils.h"
+
+#include <cstdint>
+
+#include "ErrorList.h"
+#include "js/ArrayBuffer.h"
+#include "js/JSON.h"
+#include "js/Utility.h"
+#include "js/experimental/TypedData.h"
+#include "jsfriendapi.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/AutoRestore.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/Compression.h"
+#include "mozilla/Encoding.h"
+#include "mozilla/EndianUtils.h"
+#include "mozilla/ErrorNames.h"
+#include "mozilla/FileUtils.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Services.h"
+#include "mozilla/Span.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "mozilla/dom/IOUtilsBinding.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/WorkerCommon.h"
+#include "mozilla/dom/WorkerRef.h"
+#include "PathUtils.h"
+#include "nsCOMPtr.h"
+#include "nsError.h"
+#include "nsFileStreams.h"
+#include "nsIDirectoryEnumerator.h"
+#include "nsIFile.h"
+#include "nsIGlobalObject.h"
+#include "nsIInputStream.h"
+#include "nsISupports.h"
+#include "nsLocalFile.h"
+#include "nsNetUtil.h"
+#include "nsNSSComponent.h"
+#include "nsPrintfCString.h"
+#include "nsReadableUtils.h"
+#include "nsString.h"
+#include "nsStringFwd.h"
+#include "nsTArray.h"
+#include "nsThreadManager.h"
+#include "nsXULAppAPI.h"
+#include "prerror.h"
+#include "prio.h"
+#include "prtime.h"
+#include "prtypes.h"
+#include "ScopedNSSTypes.h"
+#include "secoidt.h"
+
+#if defined(XP_UNIX) && !defined(ANDROID)
+#  include "nsSystemInfo.h"
+#endif
+
+#if defined(XP_WIN)
+#  include "nsILocalFileWin.h"
+#elif defined(XP_MACOSX)
+#  include "nsILocalFileMac.h"
+#endif
+
+#ifdef XP_UNIX
+#  include "base/process_util.h"
+#endif
+
+#define REJECT_IF_INIT_PATH_FAILED(_file, _path, _promise)            \
+  do {                                                                \
+    if (nsresult _rv = PathUtils::InitFileWithPath((_file), (_path)); \
+        NS_FAILED(_rv)) {                                             \
+      (_promise)->MaybeRejectWithOperationError(                      \
+          FormatErrorMessage(_rv, "Could not parse path (%s)",        \
+                             NS_ConvertUTF16toUTF8(_path).get()));    \
+      return;                                                         \
+    }                                                                 \
+  } while (0)
+
+static constexpr auto SHUTDOWN_ERROR =
+    "IOUtils: Shutting down and refusing additional I/O tasks"_ns;
+
+namespace mozilla::dom {
+
+// static helper functions
+
+/**
+ * Platform-specific (e.g. Windows, Unix) implementations of XPCOM APIs may
+ * report I/O errors inconsistently. For convenience, this function will attempt
+ * to match a |nsresult| against known results which imply a file cannot be
+ * found.
+ *
+ * @see nsLocalFileWin.cpp
+ * @see nsLocalFileUnix.cpp
+ */
+static bool IsFileNotFound(nsresult aResult) {
+  return aResult == NS_ERROR_FILE_NOT_FOUND;
+}
+/**
+ * Like |IsFileNotFound|, but checks for known results that suggest a file
+ * is not a directory.
+ */
+static bool IsNotDirectory(nsresult aResult) {
+  return aResult == NS_ERROR_FILE_DESTINATION_NOT_DIR ||
+         aResult == NS_ERROR_FILE_NOT_DIRECTORY;
+}
+
+/**
+ * Formats an error message and appends the error name to the end.
+ */
+template <typename... Args>
+static nsCString FormatErrorMessage(nsresult aError, const char* const aMessage,
+                                    Args... aArgs) {
+  nsPrintfCString msg(aMessage, aArgs...);
+
+  if (const char* errName = GetStaticErrorName(aError)) {
+    msg.AppendPrintf(": %s", errName);
+  } else {
+    // In the exceptional case where there is no error name, print the literal
+    // integer value of the nsresult as an upper case hex value so it can be
+    // located easily in searchfox.
+    msg.AppendPrintf(": 0x%" PRIX32, static_cast<uint32_t>(aError));
+  }
+
+  return std::move(msg);
+}
+
+static nsCString FormatErrorMessage(nsresult aError,
+                                    const char* const aMessage) {
+  const char* errName = GetStaticErrorName(aError);
+  if (errName) {
+    return nsPrintfCString("%s: %s", aMessage, errName);
+  }
+  // In the exceptional case where there is no error name, print the literal
+  // integer value of the nsresult as an upper case hex value so it can be
+  // located easily in searchfox.
+  return nsPrintfCString("%s: 0x%" PRIX32, aMessage,
+                         static_cast<uint32_t>(aError));
+}
+
+[[nodiscard]] inline bool ToJSValue(
+    JSContext* aCx, const IOUtils::InternalFileInfo& aInternalFileInfo,
+    JS::MutableHandle<JS::Value> aValue) {
+  FileInfo info;
+  info.mPath.Construct(aInternalFileInfo.mPath);
+  info.mType.Construct(aInternalFileInfo.mType);
+  info.mSize.Construct(aInternalFileInfo.mSize);
+
+  if (aInternalFileInfo.mCreationTime.isSome()) {
+    info.mCreationTime.Construct(aInternalFileInfo.mCreationTime.ref());
+  }
+  info.mLastAccessed.Construct(aInternalFileInfo.mLastAccessed);
+  info.mLastModified.Construct(aInternalFileInfo.mLastModified);
+
+  info.mPermissions.Construct(aInternalFileInfo.mPermissions);
+
+  return ToJSValue(aCx, info, aValue);
+}
+
+template <typename T>
+static void ResolveJSPromise(Promise* aPromise, T&& aValue) {
+  if constexpr (std::is_same_v<T, Ok>) {
+    aPromise->MaybeResolveWithUndefined();
+  } else if constexpr (std::is_same_v<T, nsTArray<uint8_t>>) {
+    TypedArrayCreator<Uint8Array> array(aValue);
+    aPromise->MaybeResolve(array);
+  } else {
+    aPromise->MaybeResolve(std::forward<T>(aValue));
+  }
+}
+
+static void RejectJSPromise(Promise* aPromise, const IOUtils::IOError& aError) {
+  const auto& errMsg = aError.Message();
+
+  switch (aError.Code()) {
+    case NS_ERROR_FILE_UNRESOLVABLE_SYMLINK:
+      [[fallthrough]];  // to NS_ERROR_FILE_INVALID_PATH
+    case NS_ERROR_FILE_NOT_FOUND:
+      [[fallthrough]];  // to NS_ERROR_FILE_INVALID_PATH
+    case NS_ERROR_FILE_INVALID_PATH:
+      aPromise->MaybeRejectWithNotFoundError(errMsg.refOr("File not found"_ns));
+      break;
+    case NS_ERROR_FILE_IS_LOCKED:
+      [[fallthrough]];  // to NS_ERROR_FILE_ACCESS_DENIED
+    case NS_ERROR_FILE_ACCESS_DENIED:
+      aPromise->MaybeRejectWithNotAllowedError(
+          errMsg.refOr("Access was denied to the target file"_ns));
+      break;
+    case NS_ERROR_FILE_TOO_BIG:
+      aPromise->MaybeRejectWithNotReadableError(
+          errMsg.refOr("Target file is too big"_ns));
+      break;
+    case NS_ERROR_FILE_NO_DEVICE_SPACE:
+      aPromise->MaybeRejectWithNotReadableError(
+          errMsg.refOr("Target device is full"_ns));
+      break;
+    case NS_ERROR_FILE_ALREADY_EXISTS:
+      aPromise->MaybeRejectWithNoModificationAllowedError(
+          errMsg.refOr("Target file already exists"_ns));
+      break;
+    case NS_ERROR_FILE_COPY_OR_MOVE_FAILED:
+      aPromise->MaybeRejectWithOperationError(
+          errMsg.refOr("Failed to copy or move the target file"_ns));
+      break;
+    case NS_ERROR_FILE_READ_ONLY:
+      aPromise->MaybeRejectWithReadOnlyError(
+          errMsg.refOr("Target file is read only"_ns));
+      break;
+    case NS_ERROR_FILE_NOT_DIRECTORY:
+      [[fallthrough]];  // to NS_ERROR_FILE_DESTINATION_NOT_DIR
+    case NS_ERROR_FILE_DESTINATION_NOT_DIR:
+      aPromise->MaybeRejectWithInvalidAccessError(
+          errMsg.refOr("Target file is not a directory"_ns));
+      break;
+    case NS_ERROR_FILE_IS_DIRECTORY:
+      aPromise->MaybeRejectWithInvalidAccessError(
+          errMsg.refOr("Target file is a directory"_ns));
+      break;
+    case NS_ERROR_FILE_UNKNOWN_TYPE:
+      aPromise->MaybeRejectWithInvalidAccessError(
+          errMsg.refOr("Target file is of unknown type"_ns));
+      break;
+    case NS_ERROR_FILE_NAME_TOO_LONG:
+      aPromise->MaybeRejectWithOperationError(
+          errMsg.refOr("Target file path is too long"_ns));
+      break;
+    case NS_ERROR_FILE_UNRECOGNIZED_PATH:
+      aPromise->MaybeRejectWithOperationError(
+          errMsg.refOr("Target file path is not recognized"_ns));
+      break;
+    case NS_ERROR_FILE_DIR_NOT_EMPTY:
+      aPromise->MaybeRejectWithOperationError(
+          errMsg.refOr("Target directory is not empty"_ns));
+      break;
+    case NS_ERROR_FILE_DEVICE_FAILURE:
+      [[fallthrough]];  // to NS_ERROR_FILE_FS_CORRUPTED
+    case NS_ERROR_FILE_FS_CORRUPTED:
+      aPromise->MaybeRejectWithNotReadableError(
+          errMsg.refOr("Target file system may be corrupt or unavailable"_ns));
+      break;
+    case NS_ERROR_FILE_CORRUPTED:
+      aPromise->MaybeRejectWithNotReadableError(
+          errMsg.refOr("Target file could not be read and may be corrupt"_ns));
+      break;
+    case NS_ERROR_ILLEGAL_INPUT:
+      [[fallthrough]];  // NS_ERROR_ILLEGAL_VALUE
+    case NS_ERROR_ILLEGAL_VALUE:
+      aPromise->MaybeRejectWithDataError(
+          errMsg.refOr("Argument is not allowed"_ns));
+      break;
+    case NS_ERROR_NOT_AVAILABLE:
+      aPromise->MaybeRejectWithNotFoundError(errMsg.refOr("Unavailable"_ns));
+      break;
+    case NS_ERROR_ABORT:
+      aPromise->MaybeRejectWithAbortError(errMsg.refOr("Operation aborted"_ns));
+      break;
+    default:
+      aPromise->MaybeRejectWithUnknownError(FormatErrorMessage(
+          aError.Code(), errMsg.refOr("Unexpected error"_ns).get()));
+  }
+}
+
+static void RejectShuttingDown(Promise* aPromise) {
+  RejectJSPromise(aPromise,
+                  IOUtils::IOError(NS_ERROR_ABORT).WithMessage(SHUTDOWN_ERROR));
+}
+
+static bool AssertParentProcessWithCallerLocationImpl(GlobalObject& aGlobal,
+                                                      nsCString& reason) {
+  if (MOZ_LIKELY(XRE_IsParentProcess())) {
+    return true;
+  }
+
+  AutoJSAPI jsapi;
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  MOZ_ALWAYS_TRUE(global);
+  MOZ_ALWAYS_TRUE(jsapi.Init(global));
+
+  JSContext* cx = jsapi.cx();
+
+  JS::AutoFilename scriptFilename;
+  unsigned lineNo = 0;
+  unsigned colNo = 0;
+
+  NS_ENSURE_TRUE(
+      JS::DescribeScriptedCaller(cx, &scriptFilename, &lineNo, &colNo), false);
+
+  NS_ENSURE_TRUE(scriptFilename.get(), false);
+
+  reason.AppendPrintf(" Called from %s:%d:%d.", scriptFilename.get(), lineNo,
+                      colNo);
+  return false;
+}
+
+static void AssertParentProcessWithCallerLocation(GlobalObject& aGlobal) {
+  nsCString reason = "IOUtils can only be used in the parent process."_ns;
+  if (!AssertParentProcessWithCallerLocationImpl(aGlobal, reason)) {
+    MOZ_CRASH_UNSAFE_PRINTF("%s", reason.get());
+  }
+}
+
+// IOUtils implementation
+/* static */
+IOUtils::StateMutex IOUtils::sState{"IOUtils::sState"};
+
+/* static */
+template <typename Fn>
+already_AddRefed<Promise> IOUtils::WithPromiseAndState(GlobalObject& aGlobal,
+                                                       ErrorResult& aError,
+                                                       Fn aFn) {
+  AssertParentProcessWithCallerLocation(aGlobal);
+
+  RefPtr<Promise> promise = CreateJSPromise(aGlobal, aError);
+  if (!promise) {
+    return nullptr;
+  }
+
+  if (auto state = GetState()) {
+    aFn(promise, state.ref());
+  } else {
+    RejectShuttingDown(promise);
+  }
+  return promise.forget();
+}
+
+/* static */
+template <typename OkT, typename Fn>
+void IOUtils::DispatchAndResolve(IOUtils::EventQueue* aQueue, Promise* aPromise,
+                                 Fn aFunc) {
+  RefPtr<StrongWorkerRef> workerRef;
+  if (!NS_IsMainThread()) {
+    // We need to manually keep the worker alive until the promise returned by
+    // Dispatch() resolves or rejects.
+    workerRef = StrongWorkerRef::CreateForcibly(GetCurrentThreadWorkerPrivate(),
+                                                __func__);
+  }
+
+  if (RefPtr<IOPromise<OkT>> p = aQueue->Dispatch<OkT, Fn>(std::move(aFunc))) {
+    p->Then(
+        GetCurrentSerialEventTarget(), __func__,
+        [workerRef, promise = RefPtr(aPromise)](OkT&& ok) {
+          ResolveJSPromise(promise, std::forward<OkT>(ok));
+        },
+        [workerRef, promise = RefPtr(aPromise)](const IOError& err) {
+          RejectJSPromise(promise, err);
+        });
+  }
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Read(GlobalObject& aGlobal,
+                                        const nsAString& aPath,
+                                        const ReadOptions& aOptions,
+                                        ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        Maybe<uint32_t> toRead = Nothing();
+        if (!aOptions.mMaxBytes.IsNull()) {
+          if (aOptions.mMaxBytes.Value() == 0) {
+            // Resolve with an empty buffer.
+            nsTArray<uint8_t> arr(0);
+            promise->MaybeResolve(TypedArrayCreator<Uint8Array>(arr));
+            return;
+          }
+          toRead.emplace(aOptions.mMaxBytes.Value());
+        }
+
+        DispatchAndResolve<JsBuffer>(
+            state->mEventQueue, promise,
+            [file = std::move(file), offset = aOptions.mOffset, toRead,
+             decompress = aOptions.mDecompress]() {
+              return ReadSync(file, offset, toRead, decompress,
+                              BufferKind::Uint8Array);
+            });
+      });
+}
+
+/* static */
+RefPtr<SyncReadFile> IOUtils::OpenFileForSyncReading(GlobalObject& aGlobal,
+                                                     const nsAString& aPath,
+                                                     ErrorResult& aRv) {
+  MOZ_DIAGNOSTIC_ASSERT(XRE_IsParentProcess());
+
+  // This API is only exposed to workers, so we should not be on the main
+  // thread here.
+  MOZ_RELEASE_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsIFile> file = new nsLocalFile();
+  if (nsresult rv = PathUtils::InitFileWithPath(file, aPath); NS_FAILED(rv)) {
+    aRv.ThrowOperationError(FormatErrorMessage(
+        rv, "Could not parse path (%s)", NS_ConvertUTF16toUTF8(aPath).get()));
+    return nullptr;
+  }
+
+  RefPtr<nsFileRandomAccessStream> stream = new nsFileRandomAccessStream();
+  if (nsresult rv =
+          stream->Init(file, PR_RDONLY | nsIFile::OS_READAHEAD, 0666, 0);
+      NS_FAILED(rv)) {
+    aRv.ThrowOperationError(
+        FormatErrorMessage(rv, "Could not open the file at %s",
+                           NS_ConvertUTF16toUTF8(aPath).get()));
+    return nullptr;
+  }
+
+  int64_t size = 0;
+  if (nsresult rv = stream->GetSize(&size); NS_FAILED(rv)) {
+    aRv.ThrowOperationError(FormatErrorMessage(
+        rv, "Could not get the stream size for the file at %s",
+        NS_ConvertUTF16toUTF8(aPath).get()));
+    return nullptr;
+  }
+
+  return new SyncReadFile(aGlobal.GetAsSupports(), std::move(stream), size);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::ReadUTF8(GlobalObject& aGlobal,
+                                            const nsAString& aPath,
+                                            const ReadUTF8Options& aOptions,
+                                            ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<JsBuffer>(
+            state->mEventQueue, promise,
+            [file = std::move(file), decompress = aOptions.mDecompress]() {
+              return ReadUTF8Sync(file, decompress);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::ReadJSON(GlobalObject& aGlobal,
+                                            const nsAString& aPath,
+                                            const ReadUTF8Options& aOptions,
+                                            ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        RefPtr<StrongWorkerRef> workerRef;
+        if (!NS_IsMainThread()) {
+          // We need to manually keep the worker alive until the promise
+          // returned by Dispatch() resolves or rejects.
+          workerRef = StrongWorkerRef::CreateForcibly(
+              GetCurrentThreadWorkerPrivate(), __func__);
+        }
+
+        state->mEventQueue
+            ->template Dispatch<JsBuffer>(
+                [file, decompress = aOptions.mDecompress]() {
+                  return ReadUTF8Sync(file, decompress);
+                })
+            ->Then(
+                GetCurrentSerialEventTarget(), __func__,
+                [workerRef, promise = RefPtr{promise},
+                 file](JsBuffer&& aBuffer) {
+                  AutoJSAPI jsapi;
+                  if (NS_WARN_IF(!jsapi.Init(promise->GetGlobalObject()))) {
+                    promise->MaybeRejectWithUnknownError(
+                        "Could not initialize JS API");
+                    return;
+                  }
+                  JSContext* cx = jsapi.cx();
+
+                  JS::Rooted<JSString*> jsonStr(
+                      cx,
+                      IOUtils::JsBuffer::IntoString(cx, std::move(aBuffer)));
+                  if (!jsonStr) {
+                    RejectJSPromise(promise, IOError(NS_ERROR_OUT_OF_MEMORY));
+                    return;
+                  }
+
+                  JS::Rooted<JS::Value> val(cx);
+                  if (!JS_ParseJSON(cx, jsonStr, &val)) {
+                    JS::Rooted<JS::Value> exn(cx);
+                    if (JS_GetPendingException(cx, &exn)) {
+                      JS_ClearPendingException(cx);
+                      promise->MaybeReject(exn);
+                    } else {
+                      RejectJSPromise(
+                          promise,
+                          IOError(NS_ERROR_DOM_UNKNOWN_ERR)
+                              .WithMessage(
+                                  "ParseJSON threw an uncatchable exception "
+                                  "while parsing file(%s)",
+                                  file->HumanReadablePath().get()));
+                    }
+
+                    return;
+                  }
+
+                  promise->MaybeResolve(val);
+                },
+                [workerRef, promise = RefPtr{promise}](const IOError& aErr) {
+                  RejectJSPromise(promise, aErr);
+                });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Write(GlobalObject& aGlobal,
+                                         const nsAString& aPath,
+                                         const Uint8Array& aData,
+                                         const WriteOptions& aOptions,
+                                         ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        aData.ComputeState();
+        auto buf =
+            Buffer<uint8_t>::CopyFrom(Span(aData.Data(), aData.Length()));
+        if (buf.isNothing()) {
+          promise->MaybeRejectWithOperationError(
+              "Out of memory: Could not allocate buffer while writing to file");
+          return;
+        }
+
+        auto opts = InternalWriteOpts::FromBinding(aOptions);
+        if (opts.isErr()) {
+          RejectJSPromise(promise, opts.unwrapErr());
+          return;
+        }
+
+        DispatchAndResolve<uint32_t>(
+            state->mEventQueue, promise,
+            [file = std::move(file), buf = std::move(*buf),
+             opts = opts.unwrap()]() { return WriteSync(file, buf, opts); });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::WriteUTF8(GlobalObject& aGlobal,
+                                             const nsAString& aPath,
+                                             const nsACString& aString,
+                                             const WriteOptions& aOptions,
+                                             ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        auto opts = InternalWriteOpts::FromBinding(aOptions);
+        if (opts.isErr()) {
+          RejectJSPromise(promise, opts.unwrapErr());
+          return;
+        }
+
+        DispatchAndResolve<uint32_t>(
+            state->mEventQueue, promise,
+            [file = std::move(file), str = nsCString(aString),
+             opts = opts.unwrap()]() {
+              return WriteSync(file, AsBytes(Span(str)), opts);
+            });
+      });
+}
+
+static bool AppendJsonAsUtf8(const char16_t* aData, uint32_t aLen, void* aStr) {
+  nsCString* str = static_cast<nsCString*>(aStr);
+  return AppendUTF16toUTF8(Span<const char16_t>(aData, aLen), *str, fallible);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::WriteJSON(GlobalObject& aGlobal,
+                                             const nsAString& aPath,
+                                             JS::Handle<JS::Value> aValue,
+                                             const WriteOptions& aOptions,
+                                             ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        auto opts = InternalWriteOpts::FromBinding(aOptions);
+        if (opts.isErr()) {
+          RejectJSPromise(promise, opts.unwrapErr());
+          return;
+        }
+
+        if (opts.inspect().mMode == WriteMode::Append ||
+            opts.inspect().mMode == WriteMode::AppendOrCreate) {
+          promise->MaybeRejectWithNotSupportedError(
+              "IOUtils.writeJSON does not support appending to files."_ns);
+          return;
+        }
+
+        JSContext* cx = aGlobal.Context();
+        JS::Rooted<JS::Value> rootedValue(cx, aValue);
+        nsCString utf8Str;
+
+        if (!JS_Stringify(cx, &rootedValue, nullptr, JS::NullHandleValue,
+                          AppendJsonAsUtf8, &utf8Str)) {
+          JS::Rooted<JS::Value> exn(cx, JS::UndefinedValue());
+          if (JS_GetPendingException(cx, &exn)) {
+            JS_ClearPendingException(cx);
+            promise->MaybeReject(exn);
+          } else {
+            RejectJSPromise(
+                promise,
+                IOError(NS_ERROR_DOM_UNKNOWN_ERR)
+                    .WithMessage("Could not serialize object to JSON"));
+          }
+          return;
+        }
+
+        DispatchAndResolve<uint32_t>(
+            state->mEventQueue, promise,
+            [file = std::move(file), utf8Str = std::move(utf8Str),
+             opts = opts.unwrap()]() {
+              return WriteSync(file, AsBytes(Span(utf8Str)), opts);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Move(GlobalObject& aGlobal,
+                                        const nsAString& aSourcePath,
+                                        const nsAString& aDestPath,
+                                        const MoveOptions& aOptions,
+                                        ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> sourceFile = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(sourceFile, aSourcePath, promise);
+
+        nsCOMPtr<nsIFile> destFile = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(destFile, aDestPath, promise);
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [sourceFile = std::move(sourceFile), destFile = std::move(destFile),
+             noOverwrite = aOptions.mNoOverwrite]() {
+              return MoveSync(sourceFile, destFile, noOverwrite);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Remove(GlobalObject& aGlobal,
+                                          const nsAString& aPath,
+                                          const RemoveOptions& aOptions,
+                                          ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [file = std::move(file), ignoreAbsent = aOptions.mIgnoreAbsent,
+             recursive = aOptions.mRecursive]() {
+              return RemoveSync(file, ignoreAbsent, recursive);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::MakeDirectory(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const MakeDirectoryOptions& aOptions, ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<Ok>(state->mEventQueue, promise,
+                               [file = std::move(file),
+                                createAncestors = aOptions.mCreateAncestors,
+                                ignoreExisting = aOptions.mIgnoreExisting,
+                                permissions = aOptions.mPermissions]() {
+                                 return MakeDirectorySync(file, createAncestors,
+                                                          ignoreExisting,
+                                                          permissions);
+                               });
+      });
+}
+
+already_AddRefed<Promise> IOUtils::Stat(GlobalObject& aGlobal,
+                                        const nsAString& aPath,
+                                        ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<InternalFileInfo>(
+            state->mEventQueue, promise,
+            [file = std::move(file)]() { return StatSync(file); });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Copy(GlobalObject& aGlobal,
+                                        const nsAString& aSourcePath,
+                                        const nsAString& aDestPath,
+                                        const CopyOptions& aOptions,
+                                        ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> sourceFile = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(sourceFile, aSourcePath, promise);
+
+        nsCOMPtr<nsIFile> destFile = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(destFile, aDestPath, promise);
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [sourceFile = std::move(sourceFile), destFile = std::move(destFile),
+             noOverwrite = aOptions.mNoOverwrite,
+             recursive = aOptions.mRecursive]() {
+              return CopySync(sourceFile, destFile, noOverwrite, recursive);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetAccessTime(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const Optional<int64_t>& aAccess, ErrorResult& aError) {
+  return SetTime(aGlobal, aPath, aAccess, &nsIFile::SetLastAccessedTime,
+                 aError);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetModificationTime(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const Optional<int64_t>& aModification, ErrorResult& aError) {
+  return SetTime(aGlobal, aPath, aModification, &nsIFile::SetLastModifiedTime,
+                 aError);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetTime(GlobalObject& aGlobal,
+                                           const nsAString& aPath,
+                                           const Optional<int64_t>& aNewTime,
+                                           IOUtils::SetTimeFn aSetTimeFn,
+                                           ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        int64_t newTime = aNewTime.WasPassed() ? aNewTime.Value()
+                                               : PR_Now() / PR_USEC_PER_MSEC;
+        DispatchAndResolve<int64_t>(
+            state->mEventQueue, promise,
+            [file = std::move(file), aSetTimeFn, newTime]() {
+              return SetTimeSync(file, aSetTimeFn, newTime);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::GetChildren(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const GetChildrenOptions& aOptions, ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<nsTArray<nsString>>(
+            state->mEventQueue, promise,
+            [file = std::move(file), ignoreAbsent = aOptions.mIgnoreAbsent]() {
+              return GetChildrenSync(file, ignoreAbsent);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetPermissions(GlobalObject& aGlobal,
+                                                  const nsAString& aPath,
+                                                  uint32_t aPermissions,
+                                                  const bool aHonorUmask,
+                                                  ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+#if defined(XP_UNIX) && !defined(ANDROID)
+        if (aHonorUmask) {
+          aPermissions &= ~nsSystemInfo::gUserUmask;
+        }
+#endif
+
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [file = std::move(file), permissions = aPermissions]() {
+              return SetPermissionsSync(file, permissions);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::Exists(GlobalObject& aGlobal,
+                                          const nsAString& aPath,
+                                          ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<bool>(
+            state->mEventQueue, promise,
+            [file = std::move(file)]() { return ExistsSync(file); });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::CreateUniqueFile(GlobalObject& aGlobal,
+                                                    const nsAString& aParent,
+                                                    const nsAString& aPrefix,
+                                                    const uint32_t aPermissions,
+                                                    ErrorResult& aError) {
+  return CreateUnique(aGlobal, aParent, aPrefix, nsIFile::NORMAL_FILE_TYPE,
+                      aPermissions, aError);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::CreateUniqueDirectory(
+    GlobalObject& aGlobal, const nsAString& aParent, const nsAString& aPrefix,
+    const uint32_t aPermissions, ErrorResult& aError) {
+  return CreateUnique(aGlobal, aParent, aPrefix, nsIFile::DIRECTORY_TYPE,
+                      aPermissions, aError);
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::CreateUnique(GlobalObject& aGlobal,
+                                                const nsAString& aParent,
+                                                const nsAString& aPrefix,
+                                                const uint32_t aFileType,
+                                                const uint32_t aPermissions,
+                                                ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aParent, promise);
+
+        if (nsresult rv = file->Append(aPrefix); NS_FAILED(rv)) {
+          RejectJSPromise(promise,
+                          IOError(rv).WithMessage(
+                              "Could not append prefix `%s' to parent `%s'",
+                              NS_ConvertUTF16toUTF8(aPrefix).get(),
+                              file->HumanReadablePath().get()));
+          return;
+        }
+
+        DispatchAndResolve<nsString>(
+            state->mEventQueue, promise,
+            [file = std::move(file), aPermissions, aFileType]() {
+              return CreateUniqueSync(file, aFileType, aPermissions);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::ComputeHexDigest(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const HashAlgorithm aAlgorithm, ErrorResult& aError) {
+  const bool nssInitialized = EnsureNSSInitializedChromeOrContent();
+
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        if (!nssInitialized) {
+          RejectJSPromise(promise,
+                          IOError(NS_ERROR_UNEXPECTED)
+                              .WithMessage("Could not initialize NSS"));
+          return;
+        }
+
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<nsCString>(state->mEventQueue, promise,
+                                      [file = std::move(file), aAlgorithm]() {
+                                        return ComputeHexDigestSync(file,
+                                                                    aAlgorithm);
+                                      });
+      });
+}
+
+#if defined(XP_WIN)
+
+/* static */
+already_AddRefed<Promise> IOUtils::GetWindowsAttributes(GlobalObject& aGlobal,
+                                                        const nsAString& aPath,
+                                                        ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        RefPtr<StrongWorkerRef> workerRef;
+        if (!NS_IsMainThread()) {
+          // We need to manually keep the worker alive until the promise
+          // returned by Dispatch() resolves or rejects.
+          workerRef = StrongWorkerRef::CreateForcibly(
+              GetCurrentThreadWorkerPrivate(), __func__);
+        }
+
+        state->mEventQueue
+            ->template Dispatch<uint32_t>([file = std::move(file)]() {
+              return GetWindowsAttributesSync(file);
+            })
+            ->Then(
+                GetCurrentSerialEventTarget(), __func__,
+                [workerRef, promise = RefPtr{promise}](const uint32_t aAttrs) {
+                  WindowsFileAttributes attrs;
+
+                  attrs.mReadOnly.Construct(aAttrs & FILE_ATTRIBUTE_READONLY);
+                  attrs.mHidden.Construct(aAttrs & FILE_ATTRIBUTE_HIDDEN);
+                  attrs.mSystem.Construct(aAttrs & FILE_ATTRIBUTE_SYSTEM);
+
+                  promise->MaybeResolve(attrs);
+                },
+                [workerRef, promise = RefPtr{promise}](const IOError& aErr) {
+                  RejectJSPromise(promise, aErr);
+                });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetWindowsAttributes(
+    GlobalObject& aGlobal, const nsAString& aPath,
+    const WindowsFileAttributes& aAttrs, ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        uint32_t setAttrs = 0;
+        uint32_t clearAttrs = 0;
+
+        if (aAttrs.mReadOnly.WasPassed()) {
+          if (aAttrs.mReadOnly.Value()) {
+            setAttrs |= FILE_ATTRIBUTE_READONLY;
+          } else {
+            clearAttrs |= FILE_ATTRIBUTE_READONLY;
+          }
+        }
+
+        if (aAttrs.mHidden.WasPassed()) {
+          if (aAttrs.mHidden.Value()) {
+            setAttrs |= FILE_ATTRIBUTE_HIDDEN;
+          } else {
+            clearAttrs |= FILE_ATTRIBUTE_HIDDEN;
+          }
+        }
+
+        if (aAttrs.mSystem.WasPassed()) {
+          if (aAttrs.mSystem.Value()) {
+            setAttrs |= FILE_ATTRIBUTE_SYSTEM;
+          } else {
+            clearAttrs |= FILE_ATTRIBUTE_SYSTEM;
+          }
+        }
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [file = std::move(file), setAttrs, clearAttrs]() {
+              return SetWindowsAttributesSync(file, setAttrs, clearAttrs);
+            });
+      });
+}
+
+#elif defined(XP_MACOSX)
+
+/* static */
+already_AddRefed<Promise> IOUtils::HasMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<bool>(
+            state->mEventQueue, promise,
+            [file = std::move(file), attr = nsCString(aAttr)]() {
+              return HasMacXAttrSync(file, attr);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::GetMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<nsTArray<uint8_t>>(
+            state->mEventQueue, promise,
+            [file = std::move(file), attr = nsCString(aAttr)]() {
+              return GetMacXAttrSync(file, attr);
+            });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::SetMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               const Uint8Array& aValue,
+                                               ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        aValue.ComputeState();
+        nsTArray<uint8_t> value;
+
+        if (!value.AppendElements(aValue.Data(), aValue.Length(), fallible)) {
+          RejectJSPromise(
+              promise,
+              IOError(NS_ERROR_OUT_OF_MEMORY)
+                  .WithMessage(
+                      "Could not allocate buffer to set extended attribute"));
+          return;
+        }
+
+        DispatchAndResolve<Ok>(state->mEventQueue, promise,
+                               [file = std::move(file), attr = nsCString(aAttr),
+                                value = std::move(value)] {
+                                 return SetMacXAttrSync(file, attr, value);
+                               });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::DelMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        nsCOMPtr<nsIFile> file = new nsLocalFile();
+        REJECT_IF_INIT_PATH_FAILED(file, aPath, promise);
+
+        DispatchAndResolve<Ok>(
+            state->mEventQueue, promise,
+            [file = std::move(file), attr = nsCString(aAttr)] {
+              return DelMacXAttrSync(file, attr);
+            });
+      });
+}
+
+#endif
+
+/* static */
+already_AddRefed<Promise> IOUtils::GetFile(
+    GlobalObject& aGlobal, const Sequence<nsString>& aComponents,
+    ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        ErrorResult joinErr;
+        nsCOMPtr<nsIFile> file = PathUtils::Join(aComponents, joinErr);
+        if (joinErr.Failed()) {
+          promise->MaybeReject(std::move(joinErr));
+          return;
+        }
+
+        nsCOMPtr<nsIFile> parent;
+        if (nsresult rv = file->GetParent(getter_AddRefs(parent));
+            NS_FAILED(rv)) {
+          RejectJSPromise(promise, IOError(rv).WithMessage(
+                                       "Could not get parent directory"));
+          return;
+        }
+
+        state->mEventQueue
+            ->template Dispatch<Ok>([parent = std::move(parent)]() {
+              return MakeDirectorySync(parent, /* aCreateAncestors = */ true,
+                                       /* aIgnoreExisting = */ true, 0755);
+            })
+            ->Then(
+                GetCurrentSerialEventTarget(), __func__,
+                [file = std::move(file), promise = RefPtr(promise)](const Ok&) {
+                  promise->MaybeResolve(file);
+                },
+                [promise = RefPtr(promise)](const IOError& err) {
+                  RejectJSPromise(promise, err);
+                });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::GetDirectory(
+    GlobalObject& aGlobal, const Sequence<nsString>& aComponents,
+    ErrorResult& aError) {
+  return WithPromiseAndState(
+      aGlobal, aError, [&](Promise* promise, auto& state) {
+        ErrorResult joinErr;
+        nsCOMPtr<nsIFile> dir = PathUtils::Join(aComponents, joinErr);
+        if (joinErr.Failed()) {
+          promise->MaybeReject(std::move(joinErr));
+          return;
+        }
+
+        state->mEventQueue
+            ->template Dispatch<Ok>([dir]() {
+              return MakeDirectorySync(dir, /* aCreateAncestors = */ true,
+                                       /* aIgnoreExisting = */ true, 0755);
+            })
+            ->Then(
+                GetCurrentSerialEventTarget(), __func__,
+                [dir, promise = RefPtr(promise)](const Ok&) {
+                  promise->MaybeResolve(dir);
+                },
+                [promise = RefPtr(promise)](const IOError& err) {
+                  RejectJSPromise(promise, err);
+                });
+      });
+}
+
+/* static */
+already_AddRefed<Promise> IOUtils::CreateJSPromise(GlobalObject& aGlobal,
+                                                   ErrorResult& aError) {
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<Promise> promise = Promise::Create(global, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+  MOZ_ASSERT(promise);
+  return do_AddRef(promise);
+}
+
+/* static */
+Result<IOUtils::JsBuffer, IOUtils::IOError> IOUtils::ReadSync(
+    nsIFile* aFile, const uint64_t aOffset, const Maybe<uint32_t> aMaxBytes,
+    const bool aDecompress, IOUtils::BufferKind aBufferKind) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  if (aMaxBytes.isSome() && aDecompress) {
+    return Err(
+        IOError(NS_ERROR_ILLEGAL_INPUT)
+            .WithMessage(
+                "The `maxBytes` and `decompress` options are not compatible"));
+  }
+
+  if (aOffset > static_cast<uint64_t>(INT64_MAX)) {
+    return Err(IOError(NS_ERROR_ILLEGAL_INPUT)
+                   .WithMessage("Requested offset is too large (%" PRIu64
+                                " > %" PRId64 ")",
+                                aOffset, INT64_MAX));
+  }
+
+  const int64_t offset = static_cast<int64_t>(aOffset);
+
+  RefPtr<nsFileRandomAccessStream> stream = new nsFileRandomAccessStream();
+  if (nsresult rv =
+          stream->Init(aFile, PR_RDONLY | nsIFile::OS_READAHEAD, 0666, 0);
+      NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage("Could not open the file at %s",
+                                       aFile->HumanReadablePath().get()));
+  }
+
+  uint32_t bufSize = 0;
+
+  if (aMaxBytes.isNothing()) {
+    // Limitation: We cannot read more than the maximum size of a TypedArray
+    //            (UINT32_MAX bytes). Reject if we have been requested to
+    //            perform too large of a read.
+
+    int64_t rawStreamSize = -1;
+    if (nsresult rv = stream->GetSize(&rawStreamSize); NS_FAILED(rv)) {
+      return Err(IOError(NS_ERROR_FILE_ACCESS_DENIED)
+                     .WithMessage("Could not get info for the file at %s",
+                                  aFile->HumanReadablePath().get()));
+    }
+    MOZ_RELEASE_ASSERT(rawStreamSize >= 0);
+
+    uint64_t streamSize = static_cast<uint64_t>(rawStreamSize);
+    if (aOffset >= streamSize) {
+      bufSize = 0;
+    } else {
+      if (streamSize - offset > static_cast<int64_t>(UINT32_MAX)) {
+        return Err(IOError(NS_ERROR_FILE_TOO_BIG)
+                       .WithMessage(
+                           "Could not read the file at %s with offset %" PRIu32
+                           " because it is too large(size=%" PRIu64 " bytes)",
+                           aFile->HumanReadablePath().get(), offset,
+                           streamSize));
+      }
+
+      bufSize = static_cast<uint32_t>(streamSize - offset);
+    }
+  } else {
+    bufSize = aMaxBytes.value();
+  }
+
+  if (offset > 0) {
+    if (nsresult rv = stream->Seek(PR_SEEK_SET, offset); NS_FAILED(rv)) {
+      return Err(IOError(rv).WithMessage(
+          "Could not seek to position %" PRId64 " in file %s", offset,
+          aFile->HumanReadablePath().get()));
+    }
+  }
+
+  JsBuffer buffer = JsBuffer::CreateEmpty(aBufferKind);
+
+  if (bufSize > 0) {
+    auto result = JsBuffer::Create(aBufferKind, bufSize);
+    if (result.isErr()) {
+      return result.propagateErr();
+    }
+    buffer = result.unwrap();
+    Span<char> toRead = buffer.BeginWriting();
+
+    // Read the file from disk.
+    uint32_t totalRead = 0;
+    while (totalRead != bufSize) {
+      // Read no more than INT32_MAX on each call to stream->Read, otherwise it
+      // returns an error.
+      uint32_t bytesToReadThisChunk =
+          std::min<uint32_t>(bufSize - totalRead, INT32_MAX);
+      uint32_t bytesRead = 0;
+      if (nsresult rv =
+              stream->Read(toRead.Elements(), bytesToReadThisChunk, &bytesRead);
+          NS_FAILED(rv)) {
+        return Err(IOError(rv).WithMessage(
+            "Encountered an unexpected error while reading file(%s)",
+            aFile->HumanReadablePath().get()));
+      }
+      if (bytesRead == 0) {
+        break;
+      }
+      totalRead += bytesRead;
+      toRead = toRead.From(bytesRead);
+    }
+
+    buffer.SetLength(totalRead);
+  }
+
+  // Decompress the file contents, if required.
+  if (aDecompress) {
+    return MozLZ4::Decompress(AsBytes(buffer.BeginReading()), aBufferKind);
+  }
+
+  return std::move(buffer);
+}
+
+/* static */
+Result<IOUtils::JsBuffer, IOUtils::IOError> IOUtils::ReadUTF8Sync(
+    nsIFile* aFile, bool aDecompress) {
+  auto result = ReadSync(aFile, 0, Nothing{}, aDecompress, BufferKind::String);
+  if (result.isErr()) {
+    return result.propagateErr();
+  }
+
+  JsBuffer buffer = result.unwrap();
+  if (!IsUtf8(buffer.BeginReading())) {
+    return Err(
+        IOError(NS_ERROR_FILE_CORRUPTED)
+            .WithMessage(
+                "Could not read file(%s) because it is not UTF-8 encoded",
+                aFile->HumanReadablePath().get()));
+  }
+
+  return buffer;
+}
+
+/* static */
+Result<uint32_t, IOUtils::IOError> IOUtils::WriteSync(
+    nsIFile* aFile, const Span<const uint8_t>& aByteArray,
+    const IOUtils::InternalWriteOpts& aOptions) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsIFile* backupFile = aOptions.mBackupFile;
+  nsIFile* tempFile = aOptions.mTmpFile;
+
+  bool exists = false;
+  MOZ_TRY(aFile->Exists(&exists));
+
+  if (exists && aOptions.mMode == WriteMode::Create) {
+    return Err(IOError(NS_ERROR_FILE_ALREADY_EXISTS)
+                   .WithMessage("Refusing to overwrite the file at %s\n"
+                                "Specify `mode: \"overwrite\"` to allow "
+                                "overwriting the destination",
+                                aFile->HumanReadablePath().get()));
+  }
+
+  // If backupFile was specified, perform the backup as a move.
+  if (exists && backupFile) {
+    // We copy `destFile` here to a new `nsIFile` because
+    // `nsIFile::MoveToFollowingLinks` will update the path of the file. If we
+    // did not do this, we would end up having `destFile` point to the same
+    // location as `backupFile`. Then, when we went to write to `destFile`, we
+    // would end up overwriting `backupFile` and never actually write to the
+    // file we were supposed to.
+    nsCOMPtr<nsIFile> toMove;
+    MOZ_ALWAYS_SUCCEEDS(aFile->Clone(getter_AddRefs(toMove)));
+
+    bool noOverwrite = aOptions.mMode == WriteMode::Create;
+
+    if (MoveSync(toMove, backupFile, noOverwrite).isErr()) {
+      return Err(IOError(NS_ERROR_FILE_COPY_OR_MOVE_FAILED)
+                     .WithMessage("Failed to backup the source file(%s) to %s",
+                                  aFile->HumanReadablePath().get(),
+                                  backupFile->HumanReadablePath().get()));
+    }
+  }
+
+  // If tempFile was specified, we will write to there first, then perform a
+  // move to ensure the file ends up at the final requested destination.
+  nsIFile* writeFile;
+
+  if (tempFile) {
+    writeFile = tempFile;
+  } else {
+    writeFile = aFile;
+  }
+
+  int32_t flags = PR_WRONLY;
+
+  switch (aOptions.mMode) {
+    case WriteMode::Overwrite:
+      flags |= PR_TRUNCATE | PR_CREATE_FILE;
+      break;
+
+    case WriteMode::Append:
+      flags |= PR_APPEND;
+      break;
+
+    case WriteMode::AppendOrCreate:
+      flags |= PR_APPEND | PR_CREATE_FILE;
+      break;
+
+    case WriteMode::Create:
+      flags |= PR_CREATE_FILE | PR_EXCL;
+      break;
+
+    default:
+      MOZ_CRASH("IOUtils: unknown write mode");
+  }
+
+  if (aOptions.mFlush) {
+    flags |= PR_SYNC;
+  }
+
+  // Try to perform the write and ensure that the file is closed before
+  // continuing.
+  uint32_t totalWritten = 0;
+  {
+    // Compress the byte array if required.
+    nsTArray<uint8_t> compressed;
+    Span<const char> bytes;
+    if (aOptions.mCompress) {
+      auto rv = MozLZ4::Compress(aByteArray);
+      if (rv.isErr()) {
+        return rv.propagateErr();
+      }
+      compressed = rv.unwrap();
+      bytes = Span(reinterpret_cast<const char*>(compressed.Elements()),
+                   compressed.Length());
+    } else {
+      bytes = Span(reinterpret_cast<const char*>(aByteArray.Elements()),
+                   aByteArray.Length());
+    }
+
+    RefPtr<nsFileOutputStream> stream = new nsFileOutputStream();
+    if (nsresult rv = stream->Init(writeFile, flags, 0666, 0); NS_FAILED(rv)) {
+      // Normalize platform-specific errors for opening a directory to an access
+      // denied error.
+      if (rv == nsresult::NS_ERROR_FILE_IS_DIRECTORY) {
+        rv = NS_ERROR_FILE_ACCESS_DENIED;
+      }
+      return Err(
+          IOError(rv).WithMessage("Could not open the file at %s for writing",
+                                  writeFile->HumanReadablePath().get()));
+    }
+
+    // nsFileRandomAccessStream::Write uses PR_Write under the hood, which
+    // accepts a *int32_t* for the chunk size.
+    uint32_t chunkSize = INT32_MAX;
+    Span<const char> pendingBytes = bytes;
+
+    while (pendingBytes.Length() > 0) {
+      if (pendingBytes.Length() < chunkSize) {
+        chunkSize = pendingBytes.Length();
+      }
+
+      uint32_t bytesWritten = 0;
+      if (nsresult rv =
+              stream->Write(pendingBytes.Elements(), chunkSize, &bytesWritten);
+          NS_FAILED(rv)) {
+        return Err(IOError(rv).WithMessage(
+            "Could not write chunk (size = %" PRIu32
+            ") to file %s. The file may be corrupt.",
+            chunkSize, writeFile->HumanReadablePath().get()));
+      }
+      pendingBytes = pendingBytes.From(bytesWritten);
+      totalWritten += bytesWritten;
+    }
+  }
+
+  // If tempFile was passed, check destFile against writeFile and, if they
+  // differ, the operation is finished by performing a move.
+  if (tempFile) {
+    nsAutoStringN<256> destPath;
+    nsAutoStringN<256> writePath;
+
+    MOZ_ALWAYS_SUCCEEDS(aFile->GetPath(destPath));
+    MOZ_ALWAYS_SUCCEEDS(writeFile->GetPath(writePath));
+
+    // nsIFile::MoveToFollowingLinks will only update the path of the file if
+    // the move succeeds.
+    if (destPath != writePath) {
+      if (aOptions.mTmpFile) {
+        bool isDir = false;
+        if (nsresult rv = aFile->IsDirectory(&isDir);
+            NS_FAILED(rv) && !IsFileNotFound(rv)) {
+          return Err(IOError(rv).WithMessage("Could not stat the file at %s",
+                                             aFile->HumanReadablePath().get()));
+        }
+
+        // If we attempt to write to a directory *without* a temp file, we get a
+        // permission error.
+        //
+        // However, if we are writing to a temp file first, when we copy the
+        // temp file over the destination file, we actually end up copying it
+        // inside the directory, which is not what we want. In this case, we are
+        // just going to bail out early.
+        if (isDir) {
+          return Err(
+              IOError(NS_ERROR_FILE_ACCESS_DENIED)
+                  .WithMessage("Could not open the file at %s for writing",
+                               aFile->HumanReadablePath().get()));
+        }
+      }
+
+      if (MoveSync(writeFile, aFile, /* aNoOverwrite = */ false).isErr()) {
+        return Err(
+            IOError(NS_ERROR_FILE_COPY_OR_MOVE_FAILED)
+                .WithMessage(
+                    "Could not move temporary file(%s) to destination(%s)",
+                    writeFile->HumanReadablePath().get(),
+                    aFile->HumanReadablePath().get()));
+      }
+    }
+  }
+  return totalWritten;
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::MoveSync(nsIFile* aSourceFile,
+                                               nsIFile* aDestFile,
+                                               bool aNoOverwrite) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  // Ensure the source file exists before continuing. If it doesn't exist,
+  // subsequent operations can fail in different ways on different platforms.
+  bool srcExists = false;
+  MOZ_TRY(aSourceFile->Exists(&srcExists));
+  if (!srcExists) {
+    return Err(
+        IOError(NS_ERROR_FILE_NOT_FOUND)
+            .WithMessage(
+                "Could not move source file(%s) because it does not exist",
+                aSourceFile->HumanReadablePath().get()));
+  }
+
+  return CopyOrMoveSync(&nsIFile::MoveToFollowingLinks, "move", aSourceFile,
+                        aDestFile, aNoOverwrite);
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::CopySync(nsIFile* aSourceFile,
+                                               nsIFile* aDestFile,
+                                               bool aNoOverwrite,
+                                               bool aRecursive) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  // Ensure the source file exists before continuing. If it doesn't exist,
+  // subsequent operations can fail in different ways on different platforms.
+  bool srcExists;
+  MOZ_TRY(aSourceFile->Exists(&srcExists));
+  if (!srcExists) {
+    return Err(
+        IOError(NS_ERROR_FILE_NOT_FOUND)
+            .WithMessage(
+                "Could not copy source file(%s) because it does not exist",
+                aSourceFile->HumanReadablePath().get()));
+  }
+
+  // If source is a directory, fail immediately unless the recursive option is
+  // true.
+  bool srcIsDir = false;
+  MOZ_TRY(aSourceFile->IsDirectory(&srcIsDir));
+  if (srcIsDir && !aRecursive) {
+    return Err(
+        IOError(NS_ERROR_FILE_COPY_OR_MOVE_FAILED)
+            .WithMessage(
+                "Refused to copy source directory(%s) to the destination(%s)\n"
+                "Specify the `recursive: true` option to allow copying "
+                "directories",
+                aSourceFile->HumanReadablePath().get(),
+                aDestFile->HumanReadablePath().get()));
+  }
+
+  return CopyOrMoveSync(&nsIFile::CopyToFollowingLinks, "copy", aSourceFile,
+                        aDestFile, aNoOverwrite);
+}
+
+/* static */
+template <typename CopyOrMoveFn>
+Result<Ok, IOUtils::IOError> IOUtils::CopyOrMoveSync(CopyOrMoveFn aMethod,
+                                                     const char* aMethodName,
+                                                     nsIFile* aSource,
+                                                     nsIFile* aDest,
+                                                     bool aNoOverwrite) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  // Case 1: Destination is an existing directory. Copy/move source into dest.
+  bool destIsDir = false;
+  bool destExists = true;
+
+  nsresult rv = aDest->IsDirectory(&destIsDir);
+  if (NS_SUCCEEDED(rv) && destIsDir) {
+    rv = (aSource->*aMethod)(aDest, u""_ns);
+    if (NS_FAILED(rv)) {
+      return Err(IOError(rv).WithMessage(
+          "Could not %s source file(%s) to destination directory(%s)",
+          aMethodName, aSource->HumanReadablePath().get(),
+          aDest->HumanReadablePath().get()));
+    }
+    return Ok();
+  }
+
+  if (NS_FAILED(rv)) {
+    if (!IsFileNotFound(rv)) {
+      // It's ok if the dest file doesn't exist. Case 2 handles this below.
+      // Bail out early for any other kind of error though.
+      return Err(IOError(rv));
+    }
+    destExists = false;
+  }
+
+  // Case 2: Destination is a file which may or may not exist.
+  //         Try to copy or rename the source to the destination.
+  //         If the destination exists and the source is not a regular file,
+  //         then this may fail.
+  if (aNoOverwrite && destExists) {
+    return Err(
+        IOError(NS_ERROR_FILE_ALREADY_EXISTS)
+            .WithMessage(
+                "Could not %s source file(%s) to destination(%s) because the "
+                "destination already exists and overwrites are not allowed\n"
+                "Specify the `noOverwrite: false` option to mitigate this "
+                "error",
+                aMethodName, aSource->HumanReadablePath().get(),
+                aDest->HumanReadablePath().get()));
+  }
+  if (destExists && !destIsDir) {
+    // If the source file is a directory, but the target is a file, abort early.
+    // Different implementations of |CopyTo| and |MoveTo| seem to handle this
+    // error case differently (or not at all), so we explicitly handle it here.
+    bool srcIsDir = false;
+    MOZ_TRY(aSource->IsDirectory(&srcIsDir));
+    if (srcIsDir) {
+      return Err(IOError(NS_ERROR_FILE_DESTINATION_NOT_DIR)
+                     .WithMessage("Could not %s the source directory(%s) to "
+                                  "the destination(%s) because the destination "
+                                  "is not a directory",
+                                  aMethodName,
+                                  aSource->HumanReadablePath().get(),
+                                  aDest->HumanReadablePath().get()));
+    }
+  }
+
+  nsCOMPtr<nsIFile> destDir;
+  nsAutoString destName;
+  MOZ_TRY(aDest->GetLeafName(destName));
+  MOZ_TRY(aDest->GetParent(getter_AddRefs(destDir)));
+
+  // We know `destName` is a file and therefore must have a parent directory.
+  MOZ_RELEASE_ASSERT(destDir);
+
+  // NB: if destDir doesn't exist, then |CopyToFollowingLinks| or
+  // |MoveToFollowingLinks| will create it.
+  rv = (aSource->*aMethod)(destDir, destName);
+  if (NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage(
+        "Could not %s the source file(%s) to the destination(%s)", aMethodName,
+        aSource->HumanReadablePath().get(), aDest->HumanReadablePath().get()));
+  }
+  return Ok();
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::RemoveSync(nsIFile* aFile,
+                                                 bool aIgnoreAbsent,
+                                                 bool aRecursive) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsresult rv = aFile->Remove(aRecursive);
+  if (aIgnoreAbsent && IsFileNotFound(rv)) {
+    return Ok();
+  }
+  if (NS_FAILED(rv)) {
+    IOError err(rv);
+    if (IsFileNotFound(rv)) {
+      return Err(err.WithMessage(
+          "Could not remove the file at %s because it does not exist.\n"
+          "Specify the `ignoreAbsent: true` option to mitigate this error",
+          aFile->HumanReadablePath().get()));
+    }
+    if (rv == NS_ERROR_FILE_DIR_NOT_EMPTY) {
+      return Err(err.WithMessage(
+          "Could not remove the non-empty directory at %s.\n"
+          "Specify the `recursive: true` option to mitigate this error",
+          aFile->HumanReadablePath().get()));
+    }
+    return Err(err.WithMessage("Could not remove the file at %s",
+                               aFile->HumanReadablePath().get()));
+  }
+  return Ok();
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::MakeDirectorySync(nsIFile* aFile,
+                                                        bool aCreateAncestors,
+                                                        bool aIgnoreExisting,
+                                                        int32_t aMode) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsIFile> parent;
+  MOZ_TRY(aFile->GetParent(getter_AddRefs(parent)));
+  if (!parent) {
+    // If we don't have a parent directory, we were called with a
+    // root directory. If the directory doesn't already exist (e.g., asking
+    // for a drive on Windows that does not exist), we will not be able to
+    // create it.
+    //
+    // Calling `nsLocalFile::Create()` on Windows can fail with
+    // `NS_ERROR_ACCESS_DENIED` trying to create a root directory, but we
+    // would rather the call succeed, so return early if the directory exists.
+    //
+    // Otherwise, we fall through to `nsiFile::Create()` and let it fail there
+    // instead.
+    bool exists = false;
+    MOZ_TRY(aFile->Exists(&exists));
+    if (exists) {
+      return Ok();
+    }
+  }
+
+  nsresult rv =
+      aFile->Create(nsIFile::DIRECTORY_TYPE, aMode, !aCreateAncestors);
+  if (NS_FAILED(rv)) {
+    if (rv == NS_ERROR_FILE_ALREADY_EXISTS) {
+      // NB: We may report a success only if the target is an existing
+      // directory. We don't want to silence errors that occur if the target is
+      // an existing file, since trying to create a directory where a regular
+      // file exists may be indicative of a logic error.
+      bool isDirectory;
+      MOZ_TRY(aFile->IsDirectory(&isDirectory));
+      if (!isDirectory) {
+        return Err(IOError(NS_ERROR_FILE_NOT_DIRECTORY)
+                       .WithMessage("Could not create directory because the "
+                                    "target file(%s) exists "
+                                    "and is not a directory",
+                                    aFile->HumanReadablePath().get()));
+      }
+      // The directory exists.
+      // The caller may suppress this error.
+      if (aIgnoreExisting) {
+        return Ok();
+      }
+      // Otherwise, forward it.
+      return Err(IOError(rv).WithMessage(
+          "Could not create directory because it already exists at %s\n"
+          "Specify the `ignoreExisting: true` option to mitigate this "
+          "error",
+          aFile->HumanReadablePath().get()));
+    }
+    return Err(IOError(rv).WithMessage("Could not create directory at %s",
+                                       aFile->HumanReadablePath().get()));
+  }
+  return Ok();
+}
+
+Result<IOUtils::InternalFileInfo, IOUtils::IOError> IOUtils::StatSync(
+    nsIFile* aFile) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  InternalFileInfo info;
+  MOZ_ALWAYS_SUCCEEDS(aFile->GetPath(info.mPath));
+
+  bool isRegular = false;
+  // IsFile will stat and cache info in the file object. If the file doesn't
+  // exist, or there is an access error, we'll discover it here.
+  // Any subsequent errors are unexpected and will just be forwarded.
+  nsresult rv = aFile->IsFile(&isRegular);
+  if (NS_FAILED(rv)) {
+    IOError err(rv);
+    if (IsFileNotFound(rv)) {
+      return Err(
+          err.WithMessage("Could not stat file(%s) because it does not exist",
+                          aFile->HumanReadablePath().get()));
+    }
+    return Err(err);
+  }
+
+  // Now we can populate the info object by querying the file.
+  info.mType = FileType::Regular;
+  if (!isRegular) {
+    bool isDir = false;
+    MOZ_TRY(aFile->IsDirectory(&isDir));
+    info.mType = isDir ? FileType::Directory : FileType::Other;
+  }
+
+  int64_t size = -1;
+  if (info.mType == FileType::Regular) {
+    MOZ_TRY(aFile->GetFileSize(&size));
+  }
+  info.mSize = size;
+
+  PRTime creationTime = 0;
+  if (nsresult rv = aFile->GetCreationTime(&creationTime); NS_SUCCEEDED(rv)) {
+    info.mCreationTime.emplace(static_cast<int64_t>(creationTime));
+  } else if (NS_FAILED(rv) && rv != NS_ERROR_NOT_IMPLEMENTED) {
+    // This field is only supported on some platforms.
+    return Err(IOError(rv));
+  }
+
+  PRTime lastAccessed = 0;
+  MOZ_TRY(aFile->GetLastAccessedTime(&lastAccessed));
+  info.mLastAccessed = static_cast<int64_t>(lastAccessed);
+
+  PRTime lastModified = 0;
+  MOZ_TRY(aFile->GetLastModifiedTime(&lastModified));
+  info.mLastModified = static_cast<int64_t>(lastModified);
+
+  MOZ_TRY(aFile->GetPermissions(&info.mPermissions));
+
+  return info;
+}
+
+/* static */
+Result<int64_t, IOUtils::IOError> IOUtils::SetTimeSync(
+    nsIFile* aFile, IOUtils::SetTimeFn aSetTimeFn, int64_t aNewTime) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  // nsIFile::SetLastModifiedTime will *not* do what is expected when passed 0
+  // as an argument. Rather than setting the time to 0, it will recalculate the
+  // system time and set it to that value instead. We explicit forbid this,
+  // because this side effect is surprising.
+  //
+  // If it ever becomes possible to set a file time to 0, this check should be
+  // removed, though this use case seems rare.
+  if (aNewTime == 0) {
+    return Err(
+        IOError(NS_ERROR_ILLEGAL_VALUE)
+            .WithMessage(
+                "Refusing to set the modification time of file(%s) to 0.\n"
+                "To use the current system time, call `setModificationTime` "
+                "with no arguments",
+                aFile->HumanReadablePath().get()));
+  }
+
+  nsresult rv = (aFile->*aSetTimeFn)(aNewTime);
+
+  if (NS_FAILED(rv)) {
+    IOError err(rv);
+    if (IsFileNotFound(rv)) {
+      return Err(
+          err.WithMessage("Could not set modification time of file(%s) "
+                          "because it does not exist",
+                          aFile->HumanReadablePath().get()));
+    }
+    return Err(err);
+  }
+  return aNewTime;
+}
+
+/* static */
+Result<nsTArray<nsString>, IOUtils::IOError> IOUtils::GetChildrenSync(
+    nsIFile* aFile, bool aIgnoreAbsent) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsTArray<nsString> children;
+  nsCOMPtr<nsIDirectoryEnumerator> iter;
+  nsresult rv = aFile->GetDirectoryEntries(getter_AddRefs(iter));
+  if (aIgnoreAbsent && IsFileNotFound(rv)) {
+    return children;
+  }
+  if (NS_FAILED(rv)) {
+    IOError err(rv);
+    if (IsFileNotFound(rv)) {
+      return Err(err.WithMessage(
+          "Could not get children of file(%s) because it does not exist",
+          aFile->HumanReadablePath().get()));
+    }
+    if (IsNotDirectory(rv)) {
+      return Err(err.WithMessage(
+          "Could not get children of file(%s) because it is not a directory",
+          aFile->HumanReadablePath().get()));
+    }
+    return Err(err);
+  }
+
+  bool hasMoreElements = false;
+  MOZ_TRY(iter->HasMoreElements(&hasMoreElements));
+  while (hasMoreElements) {
+    nsCOMPtr<nsIFile> child;
+    MOZ_TRY(iter->GetNextFile(getter_AddRefs(child)));
+    if (child) {
+      nsString path;
+      MOZ_TRY(child->GetPath(path));
+      children.AppendElement(path);
+    }
+    MOZ_TRY(iter->HasMoreElements(&hasMoreElements));
+  }
+
+  return children;
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::SetPermissionsSync(
+    nsIFile* aFile, const uint32_t aPermissions) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  MOZ_TRY(aFile->SetPermissions(aPermissions));
+  return Ok{};
+}
+
+/* static */
+Result<bool, IOUtils::IOError> IOUtils::ExistsSync(nsIFile* aFile) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  bool exists = false;
+  MOZ_TRY(aFile->Exists(&exists));
+
+  return exists;
+}
+
+/* static */
+Result<nsString, IOUtils::IOError> IOUtils::CreateUniqueSync(
+    nsIFile* aFile, const uint32_t aFileType, const uint32_t aPermissions) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  if (nsresult rv = aFile->CreateUnique(aFileType, aPermissions);
+      NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage("Could not create unique path"));
+  }
+
+  nsString path;
+  MOZ_ALWAYS_SUCCEEDS(aFile->GetPath(path));
+
+  return path;
+}
+
+/* static */
+Result<nsCString, IOUtils::IOError> IOUtils::ComputeHexDigestSync(
+    nsIFile* aFile, const HashAlgorithm aAlgorithm) {
+  static constexpr size_t BUFFER_SIZE = 8192;
+
+  SECOidTag alg;
+  switch (aAlgorithm) {
+    case HashAlgorithm::Sha1:
+      alg = SEC_OID_SHA1;
+      break;
+
+    case HashAlgorithm::Sha256:
+      alg = SEC_OID_SHA256;
+      break;
+
+    case HashAlgorithm::Sha384:
+      alg = SEC_OID_SHA384;
+      break;
+
+    case HashAlgorithm::Sha512:
+      alg = SEC_OID_SHA512;
+      break;
+
+    default:
+      MOZ_RELEASE_ASSERT(false, "Unexpected HashAlgorithm");
+  }
+
+  Digest digest;
+  if (nsresult rv = digest.Begin(alg); NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage("Could not hash file at %s",
+                                       aFile->HumanReadablePath().get()));
+  }
+
+  RefPtr<nsIInputStream> stream;
+  if (nsresult rv = NS_NewLocalFileInputStream(getter_AddRefs(stream), aFile);
+      NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage("Could not open the file at %s",
+                                       aFile->HumanReadablePath().get()));
+  }
+
+  char buffer[BUFFER_SIZE];
+  uint32_t read = 0;
+  for (;;) {
+    if (nsresult rv = stream->Read(buffer, BUFFER_SIZE, &read); NS_FAILED(rv)) {
+      return Err(IOError(rv).WithMessage(
+          "Encountered an unexpected error while reading file(%s)",
+          aFile->HumanReadablePath().get()));
+    }
+    if (read == 0) {
+      break;
+    }
+
+    if (nsresult rv =
+            digest.Update(reinterpret_cast<unsigned char*>(buffer), read);
+        NS_FAILED(rv)) {
+      return Err(IOError(rv).WithMessage("Could not hash file at %s",
+                                         aFile->HumanReadablePath().get()));
+    }
+  }
+
+  AutoTArray<uint8_t, SHA512_LENGTH> rawDigest;
+  if (nsresult rv = digest.End(rawDigest); NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage("Could not hash file at %s",
+                                       aFile->HumanReadablePath().get()));
+  }
+
+  nsCString hexDigest;
+  if (!hexDigest.SetCapacity(2 * rawDigest.Length(), fallible)) {
+    return Err(IOError(NS_ERROR_OUT_OF_MEMORY));
+  }
+
+  const char HEX[] = "0123456789abcdef";
+  for (uint8_t b : rawDigest) {
+    hexDigest.Append(HEX[(b >> 4) & 0xF]);
+    hexDigest.Append(HEX[b & 0xF]);
+  }
+
+  return hexDigest;
+}
+
+#if defined(XP_WIN)
+
+Result<uint32_t, IOUtils::IOError> IOUtils::GetWindowsAttributesSync(
+    nsIFile* aFile) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  uint32_t attrs = 0;
+
+  nsCOMPtr<nsILocalFileWin> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  if (nsresult rv = file->GetWindowsFileAttributes(&attrs); NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage(
+        "Could not get Windows file attributes for the file at `%s'",
+        aFile->HumanReadablePath().get()));
+  }
+  return attrs;
+}
+
+Result<Ok, IOUtils::IOError> IOUtils::SetWindowsAttributesSync(
+    nsIFile* aFile, const uint32_t aSetAttrs, const uint32_t aClearAttrs) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsILocalFileWin> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  if (nsresult rv = file->SetWindowsFileAttributes(aSetAttrs, aClearAttrs);
+      NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage(
+        "Could not set Windows file attributes for the file at `%s'",
+        aFile->HumanReadablePath().get()));
+  }
+
+  return Ok{};
+}
+
+#elif defined(XP_MACOSX)
+
+/* static */
+Result<bool, IOUtils::IOError> IOUtils::HasMacXAttrSync(
+    nsIFile* aFile, const nsCString& aAttr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsILocalFileMac> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  bool hasAttr = false;
+  if (nsresult rv = file->HasXAttr(aAttr, &hasAttr); NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage(
+        "Could not read the extended attribute `%s' from the file `%s'",
+        aAttr.get(), aFile->HumanReadablePath().get()));
+  }
+
+  return hasAttr;
+}
+
+/* static */
+Result<nsTArray<uint8_t>, IOUtils::IOError> IOUtils::GetMacXAttrSync(
+    nsIFile* aFile, const nsCString& aAttr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsILocalFileMac> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  nsTArray<uint8_t> value;
+  if (nsresult rv = file->GetXAttr(aAttr, value); NS_FAILED(rv)) {
+    auto err = IOError(rv);
+
+    if (rv == NS_ERROR_NOT_AVAILABLE) {
+      return Err(err.WithMessage(
+          "The file `%s' does not have an extended attribute `%s'",
+          aFile->HumanReadablePath().get(), aAttr.get()));
+    }
+
+    return Err(err.WithMessage(
+        "Could not read the extended attribute `%s' from the file `%s'",
+        aAttr.get(), aFile->HumanReadablePath().get()));
+  }
+
+  return value;
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::SetMacXAttrSync(
+    nsIFile* aFile, const nsCString& aAttr, const nsTArray<uint8_t>& aValue) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsILocalFileMac> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  if (nsresult rv = file->SetXAttr(aAttr, aValue); NS_FAILED(rv)) {
+    return Err(IOError(rv).WithMessage(
+        "Could not set extended attribute `%s' on file `%s'", aAttr.get(),
+        aFile->HumanReadablePath().get()));
+  }
+
+  return Ok{};
+}
+
+/* static */
+Result<Ok, IOUtils::IOError> IOUtils::DelMacXAttrSync(nsIFile* aFile,
+                                                      const nsCString& aAttr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  nsCOMPtr<nsILocalFileMac> file = do_QueryInterface(aFile);
+  MOZ_ASSERT(file);
+
+  if (nsresult rv = file->DelXAttr(aAttr); NS_FAILED(rv)) {
+    auto err = IOError(rv);
+
+    if (rv == NS_ERROR_NOT_AVAILABLE) {
+      return Err(err.WithMessage(
+          "The file `%s' does not have an extended attribute `%s'",
+          aFile->HumanReadablePath().get(), aAttr.get()));
+    }
+
+    return Err(IOError(rv).WithMessage(
+        "Could not delete extended attribute `%s' on file `%s'", aAttr.get(),
+        aFile->HumanReadablePath().get()));
+  }
+
+  return Ok{};
+}
+
+#endif
+
+/* static */
+void IOUtils::GetProfileBeforeChange(GlobalObject& aGlobal,
+                                     JS::MutableHandle<JS::Value> aClient,
+                                     ErrorResult& aRv) {
+  return GetShutdownClient(aGlobal, aClient, aRv,
+                           ShutdownPhase::ProfileBeforeChange);
+}
+
+/* static */
+void IOUtils::GetSendTelemetry(GlobalObject& aGlobal,
+                               JS::MutableHandle<JS::Value> aClient,
+                               ErrorResult& aRv) {
+  return GetShutdownClient(aGlobal, aClient, aRv, ShutdownPhase::SendTelemetry);
+}
+
+/**
+ * Assert that the given phase has a shutdown client exposed by IOUtils
+ *
+ * There is no shutdown client exposed for XpcomWillShutdown.
+ */
+static void AssertHasShutdownClient(const IOUtils::ShutdownPhase aPhase) {
+  MOZ_RELEASE_ASSERT(aPhase >= IOUtils::ShutdownPhase::ProfileBeforeChange &&
+                     aPhase < IOUtils::ShutdownPhase::XpcomWillShutdown);
+}
+
+/* static */
+void IOUtils::GetShutdownClient(GlobalObject& aGlobal,
+                                JS::MutableHandle<JS::Value> aClient,
+                                ErrorResult& aRv,
+                                const IOUtils::ShutdownPhase aPhase) {
+  MOZ_RELEASE_ASSERT(XRE_IsParentProcess());
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+  AssertHasShutdownClient(aPhase);
+
+  if (auto state = GetState()) {
+    MOZ_RELEASE_ASSERT(state.ref()->mBlockerStatus !=
+                       ShutdownBlockerStatus::Uninitialized);
+
+    if (state.ref()->mBlockerStatus == ShutdownBlockerStatus::Failed) {
+      aRv.ThrowAbortError("IOUtils: could not register shutdown blockers");
+      return;
+    }
+
+    MOZ_RELEASE_ASSERT(state.ref()->mBlockerStatus ==
+                       ShutdownBlockerStatus::Initialized);
+    auto result = state.ref()->mEventQueue->GetShutdownClient(aPhase);
+    if (result.isErr()) {
+      aRv.ThrowAbortError("IOUtils: could not get shutdown client");
+      return;
+    }
+
+    RefPtr<nsIAsyncShutdownClient> client = result.unwrap();
+    MOZ_RELEASE_ASSERT(client);
+    if (nsresult rv = client->GetJsclient(aClient); NS_FAILED(rv)) {
+      aRv.ThrowAbortError("IOUtils: Could not get shutdown jsclient");
+    }
+    return;
+  }
+
+  aRv.ThrowAbortError(
+      "IOUtils: profileBeforeChange phase has already finished");
+}
+
+/* sstatic */
+Maybe<IOUtils::StateMutex::AutoLock> IOUtils::GetState() {
+  auto state = sState.Lock();
+  if (state->mQueueStatus == EventQueueStatus::Shutdown) {
+    return Nothing{};
+  }
+
+  if (state->mQueueStatus == EventQueueStatus::Uninitialized) {
+    MOZ_RELEASE_ASSERT(!state->mEventQueue);
+    state->mEventQueue = new EventQueue();
+    state->mQueueStatus = EventQueueStatus::Initialized;
+
+    MOZ_RELEASE_ASSERT(state->mBlockerStatus ==
+                       ShutdownBlockerStatus::Uninitialized);
+  }
+
+  if (NS_IsMainThread() &&
+      state->mBlockerStatus == ShutdownBlockerStatus::Uninitialized) {
+    state->SetShutdownHooks();
+  }
+
+  return Some(std::move(state));
+}
+
+IOUtils::EventQueue::EventQueue() {
+  MOZ_ALWAYS_SUCCEEDS(NS_CreateBackgroundTaskQueue(
+      "IOUtils::EventQueue", getter_AddRefs(mBackgroundEventTarget)));
+
+  MOZ_RELEASE_ASSERT(mBackgroundEventTarget);
+}
+
+void IOUtils::State::SetShutdownHooks() {
+  if (mBlockerStatus != ShutdownBlockerStatus::Uninitialized) {
+    return;
+  }
+
+  if (NS_WARN_IF(NS_FAILED(mEventQueue->SetShutdownHooks()))) {
+    mBlockerStatus = ShutdownBlockerStatus::Failed;
+  } else {
+    mBlockerStatus = ShutdownBlockerStatus::Initialized;
+  }
+
+  if (mBlockerStatus != ShutdownBlockerStatus::Initialized) {
+    NS_WARNING("IOUtils: could not register shutdown blockers.");
+  }
+}
+
+nsresult IOUtils::EventQueue::SetShutdownHooks() {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+
+  constexpr static auto STACK = u"IOUtils::EventQueue::SetShutdownHooks"_ns;
+  constexpr static auto FILE = NS_LITERAL_STRING_FROM_CSTRING(__FILE__);
+
+  nsCOMPtr<nsIAsyncShutdownService> svc = services::GetAsyncShutdownService();
+  if (!svc) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  nsCOMPtr<nsIAsyncShutdownBlocker> profileBeforeChangeBlocker;
+
+  // Create a shutdown blocker for the profile-before-change phase.
+  {
+    profileBeforeChangeBlocker =
+        new IOUtilsShutdownBlocker(ShutdownPhase::ProfileBeforeChange);
+
+    nsCOMPtr<nsIAsyncShutdownClient> globalClient;
+    MOZ_TRY(svc->GetProfileBeforeChange(getter_AddRefs(globalClient)));
+    MOZ_RELEASE_ASSERT(globalClient);
+
+    MOZ_TRY(globalClient->AddBlocker(profileBeforeChangeBlocker, FILE, __LINE__,
+                                     STACK));
+  }
+
+  // Create the shutdown barrier for profile-before-change so that consumers can
+  // register shutdown blockers.
+  //
+  // The blocker we just created will wait for all clients registered on this
+  // barrier to finish.
+  {
+    nsCOMPtr<nsIAsyncShutdownBarrier> barrier;
+
+    // It is okay for this to fail. The created shutdown blocker won't await
+    // anything and shutdown will proceed.
+    MOZ_TRY(svc->MakeBarrier(
+        u"IOUtils: waiting for profileBeforeChange IO to complete"_ns,
+        getter_AddRefs(barrier)));
+    MOZ_RELEASE_ASSERT(barrier);
+
+    mBarriers[ShutdownPhase::ProfileBeforeChange] = std::move(barrier);
+  }
+
+  // Create a shutdown blocker for the profile-before-change-telemetry phase.
+  nsCOMPtr<nsIAsyncShutdownBlocker> sendTelemetryBlocker;
+  {
+    sendTelemetryBlocker =
+        new IOUtilsShutdownBlocker(ShutdownPhase::SendTelemetry);
+
+    nsCOMPtr<nsIAsyncShutdownClient> globalClient;
+    MOZ_TRY(svc->GetSendTelemetry(getter_AddRefs(globalClient)));
+    MOZ_RELEASE_ASSERT(globalClient);
+
+    MOZ_TRY(
+        globalClient->AddBlocker(sendTelemetryBlocker, FILE, __LINE__, STACK));
+  }
+
+  // Create the shutdown barrier for profile-before-change-telemetry so that
+  // consumers can register shutdown blockers.
+  //
+  // The blocker we just created will wait for all clients registered on this
+  // barrier to finish.
+  {
+    nsCOMPtr<nsIAsyncShutdownBarrier> barrier;
+
+    MOZ_TRY(svc->MakeBarrier(
+        u"IOUtils: waiting for sendTelemetry IO to complete"_ns,
+        getter_AddRefs(barrier)));
+    MOZ_RELEASE_ASSERT(barrier);
+
+    // Add a blocker on the previous shutdown phase.
+    nsCOMPtr<nsIAsyncShutdownClient> client;
+    MOZ_TRY(barrier->GetClient(getter_AddRefs(client)));
+
+    MOZ_TRY(
+        client->AddBlocker(profileBeforeChangeBlocker, FILE, __LINE__, STACK));
+
+    mBarriers[ShutdownPhase::SendTelemetry] = std::move(barrier);
+  }
+
+  // Create a shutdown blocker for the xpcom-will-shutdown phase.
+  {
+    nsCOMPtr<nsIAsyncShutdownClient> globalClient;
+    MOZ_TRY(svc->GetXpcomWillShutdown(getter_AddRefs(globalClient)));
+    MOZ_RELEASE_ASSERT(globalClient);
+
+    nsCOMPtr<nsIAsyncShutdownBlocker> blocker =
+        new IOUtilsShutdownBlocker(ShutdownPhase::XpcomWillShutdown);
+    MOZ_TRY(globalClient->AddBlocker(
+        blocker, FILE, __LINE__, u"IOUtils::EventQueue::SetShutdownHooks"_ns));
+  }
+
+  // Create a shutdown barrier for the xpcom-will-shutdown phase.
+  //
+  // The blocker we just created will wait for all clients registered on this
+  // barrier to finish.
+  //
+  // The only client registered on this barrier should be a blocker for the
+  // previous phase. This is to ensure that all shutdown IO happens when
+  // shutdown phases do not happen (e.g., in xpcshell tests where
+  // profile-before-change does not occur).
+  {
+    nsCOMPtr<nsIAsyncShutdownBarrier> barrier;
+
+    MOZ_TRY(svc->MakeBarrier(
+        u"IOUtils: waiting for xpcomWillShutdown IO to complete"_ns,
+        getter_AddRefs(barrier)));
+    MOZ_RELEASE_ASSERT(barrier);
+
+    // Add a blocker on the previous shutdown phase.
+    nsCOMPtr<nsIAsyncShutdownClient> client;
+    MOZ_TRY(barrier->GetClient(getter_AddRefs(client)));
+
+    client->AddBlocker(sendTelemetryBlocker, FILE, __LINE__,
+                       u"IOUtils::EventQueue::SetShutdownHooks"_ns);
+
+    mBarriers[ShutdownPhase::XpcomWillShutdown] = std::move(barrier);
+  }
+
+  return NS_OK;
+}
+
+template <typename OkT, typename Fn>
+RefPtr<IOUtils::IOPromise<OkT>> IOUtils::EventQueue::Dispatch(Fn aFunc) {
+  MOZ_RELEASE_ASSERT(mBackgroundEventTarget);
+
+  auto promise =
+      MakeRefPtr<typename IOUtils::IOPromise<OkT>::Private>(__func__);
+  mBackgroundEventTarget->Dispatch(
+      NS_NewRunnableFunction("IOUtils::EventQueue::Dispatch",
+                             [promise, func = std::move(aFunc)] {
+                               Result<OkT, IOError> result = func();
+                               if (result.isErr()) {
+                                 promise->Reject(result.unwrapErr(), __func__);
+                               } else {
+                                 promise->Resolve(result.unwrap(), __func__);
+                               }
+                             }),
+      NS_DISPATCH_EVENT_MAY_BLOCK);
+  return promise;
+};
+
+Result<already_AddRefed<nsIAsyncShutdownBarrier>, nsresult>
+IOUtils::EventQueue::GetShutdownBarrier(const IOUtils::ShutdownPhase aPhase) {
+  if (!mBarriers[aPhase]) {
+    return Err(NS_ERROR_NOT_AVAILABLE);
+  }
+
+  return do_AddRef(mBarriers[aPhase]);
+}
+
+Result<already_AddRefed<nsIAsyncShutdownClient>, nsresult>
+IOUtils::EventQueue::GetShutdownClient(const IOUtils::ShutdownPhase aPhase) {
+  AssertHasShutdownClient(aPhase);
+
+  if (!mBarriers[aPhase]) {
+    return Err(NS_ERROR_NOT_AVAILABLE);
+  }
+
+  nsCOMPtr<nsIAsyncShutdownClient> client;
+  MOZ_TRY(mBarriers[aPhase]->GetClient(getter_AddRefs(client)));
+
+  return do_AddRef(client);
+}
+
+/* static */
+Result<nsTArray<uint8_t>, IOUtils::IOError> IOUtils::MozLZ4::Compress(
+    Span<const uint8_t> aUncompressed) {
+  nsTArray<uint8_t> result;
+  size_t worstCaseSize =
+      Compression::LZ4::maxCompressedSize(aUncompressed.Length()) + HEADER_SIZE;
+  if (!result.SetCapacity(worstCaseSize, fallible)) {
+    return Err(IOError(NS_ERROR_OUT_OF_MEMORY)
+                   .WithMessage("Could not allocate buffer to compress data"));
+  }
+  result.AppendElements(Span(MAGIC_NUMBER.data(), MAGIC_NUMBER.size()));
+  std::array<uint8_t, sizeof(uint32_t)> contentSizeBytes{};
+  LittleEndian::writeUint32(contentSizeBytes.data(), aUncompressed.Length());
+  result.AppendElements(Span(contentSizeBytes.data(), contentSizeBytes.size()));
+
+  if (aUncompressed.Length() == 0) {
+    // Don't try to compress an empty buffer.
+    // Just return the correctly formed header.
+    result.SetLength(HEADER_SIZE);
+    return result;
+  }
+
+  size_t compressed = Compression::LZ4::compress(
+      reinterpret_cast<const char*>(aUncompressed.Elements()),
+      aUncompressed.Length(),
+      reinterpret_cast<char*>(result.Elements()) + HEADER_SIZE);
+  if (!compressed) {
+    return Err(
+        IOError(NS_ERROR_UNEXPECTED).WithMessage("Could not compress data"));
+  }
+  result.SetLength(HEADER_SIZE + compressed);
+  return result;
+}
+
+/* static */
+Result<IOUtils::JsBuffer, IOUtils::IOError> IOUtils::MozLZ4::Decompress(
+    Span<const uint8_t> aFileContents, IOUtils::BufferKind aBufferKind) {
+  if (aFileContents.LengthBytes() < HEADER_SIZE) {
+    return Err(
+        IOError(NS_ERROR_FILE_CORRUPTED)
+            .WithMessage(
+                "Could not decompress file because the buffer is too short"));
+  }
+  auto header = aFileContents.To(HEADER_SIZE);
+  if (!std::equal(std::begin(MAGIC_NUMBER), std::end(MAGIC_NUMBER),
+                  std::begin(header))) {
+    nsCString magicStr;
+    uint32_t i = 0;
+    for (; i < header.Length() - 1; ++i) {
+      magicStr.AppendPrintf("%02X ", header.at(i));
+    }
+    magicStr.AppendPrintf("%02X", header.at(i));
+
+    return Err(IOError(NS_ERROR_FILE_CORRUPTED)
+                   .WithMessage("Could not decompress file because it has an "
+                                "invalid LZ4 header (wrong magic number: '%s')",
+                                magicStr.get()));
+  }
+  size_t numBytes = sizeof(uint32_t);
+  Span<const uint8_t> sizeBytes = header.Last(numBytes);
+  uint32_t expectedDecompressedSize =
+      LittleEndian::readUint32(sizeBytes.data());
+  if (expectedDecompressedSize == 0) {
+    return JsBuffer::CreateEmpty(aBufferKind);
+  }
+  auto contents = aFileContents.From(HEADER_SIZE);
+  auto result = JsBuffer::Create(aBufferKind, expectedDecompressedSize);
+  if (result.isErr()) {
+    return result.propagateErr();
+  }
+
+  JsBuffer decompressed = result.unwrap();
+  size_t actualSize = 0;
+  if (!Compression::LZ4::decompress(
+          reinterpret_cast<const char*>(contents.Elements()), contents.Length(),
+          reinterpret_cast<char*>(decompressed.Elements()),
+          expectedDecompressedSize, &actualSize)) {
+    return Err(
+        IOError(NS_ERROR_FILE_CORRUPTED)
+            .WithMessage(
+                "Could not decompress file contents, the file may be corrupt"));
+  }
+  decompressed.SetLength(actualSize);
+  return decompressed;
+}
+
+NS_IMPL_ISUPPORTS(IOUtilsShutdownBlocker, nsIAsyncShutdownBlocker,
+                  nsIAsyncShutdownCompletionCallback);
+
+NS_IMETHODIMP IOUtilsShutdownBlocker::GetName(nsAString& aName) {
+  aName = u"IOUtils Blocker ("_ns;
+  aName.Append(PHASE_NAMES[mPhase]);
+  aName.Append(')');
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP IOUtilsShutdownBlocker::BlockShutdown(
+    nsIAsyncShutdownClient* aBarrierClient) {
+  using EventQueueStatus = IOUtils::EventQueueStatus;
+  using ShutdownPhase = IOUtils::ShutdownPhase;
+
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+
+  nsCOMPtr<nsIAsyncShutdownBarrier> barrier;
+
+  {
+    auto state = IOUtils::sState.Lock();
+    if (state->mQueueStatus == EventQueueStatus::Shutdown) {
+      // If the previous blockers have already run, then the event queue is
+      // already torn down and we have nothing to do.
+
+      MOZ_RELEASE_ASSERT(mPhase == ShutdownPhase::XpcomWillShutdown);
+      MOZ_RELEASE_ASSERT(!state->mEventQueue);
+
+      Unused << NS_WARN_IF(NS_FAILED(aBarrierClient->RemoveBlocker(this)));
+      mParentClient = nullptr;
+
+      return NS_OK;
+    }
+
+    MOZ_RELEASE_ASSERT(state->mEventQueue);
+
+    mParentClient = aBarrierClient;
+
+    barrier = state->mEventQueue->GetShutdownBarrier(mPhase).unwrapOr(nullptr);
+  }
+
+  // We cannot barrier->Wait() while holding the mutex because it will lead to
+  // deadlock.
+  if (!barrier || NS_WARN_IF(NS_FAILED(barrier->Wait(this)))) {
+    // If we don't have a barrier, we still need to flush the IOUtils event
+    // queue and disable task submission.
+    //
+    // Likewise, if waiting on the barrier failed, we are going to make our best
+    // attempt to clean up.
+    Unused << Done();
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP IOUtilsShutdownBlocker::Done() {
+  using EventQueueStatus = IOUtils::EventQueueStatus;
+  using ShutdownPhase = IOUtils::ShutdownPhase;
+
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+
+  bool didFlush = false;
+
+  {
+    auto state = IOUtils::sState.Lock();
+
+    if (state->mEventQueue) {
+      MOZ_RELEASE_ASSERT(state->mQueueStatus == EventQueueStatus::Initialized);
+
+      // This method is called once we have served all shutdown clients. Now we
+      // flush the remaining IO queue. This ensures any straggling IO that was
+      // not part of the shutdown blocker finishes before we move to the next
+      // phase.
+      state->mEventQueue->Dispatch<Ok>([]() { return Ok{}; })
+          ->Then(GetMainThreadSerialEventTarget(), __func__,
+                 [self = RefPtr(this)]() { self->OnFlush(); });
+
+      // And if we're the last shutdown phase to allow IO, disable the event
+      // queue to disallow further IO requests.
+      if (mPhase >= LAST_IO_PHASE) {
+        state->mQueueStatus = EventQueueStatus::Shutdown;
+      }
+
+      didFlush = true;
+    }
+  }
+
+  // If we have already shut down the event loop, then call OnFlush to stop
+  // blocking our parent shutdown client.
+  if (!didFlush) {
+    MOZ_RELEASE_ASSERT(mPhase == ShutdownPhase::XpcomWillShutdown);
+    OnFlush();
+  }
+
+  return NS_OK;
+}
+
+void IOUtilsShutdownBlocker::OnFlush() {
+  if (mParentClient) {
+    (void)NS_WARN_IF(NS_FAILED(mParentClient->RemoveBlocker(this)));
+    mParentClient = nullptr;
+
+    // If we are past the last shutdown phase that allows IO,
+    // we can shutdown the event queue here because no additional IO requests
+    // will be allowed (see |Done()|).
+    if (mPhase >= LAST_IO_PHASE) {
+      auto state = IOUtils::sState.Lock();
+      if (state->mEventQueue) {
+        state->mEventQueue = nullptr;
+      }
+    }
+  }
+}
+
+NS_IMETHODIMP IOUtilsShutdownBlocker::GetState(nsIPropertyBag** aState) {
+  return NS_OK;
+}
+
+Result<IOUtils::InternalWriteOpts, IOUtils::IOError>
+IOUtils::InternalWriteOpts::FromBinding(const WriteOptions& aOptions) {
+  InternalWriteOpts opts;
+  opts.mFlush = aOptions.mFlush;
+  opts.mMode = aOptions.mMode;
+
+  if (aOptions.mBackupFile.WasPassed()) {
+    opts.mBackupFile = new nsLocalFile();
+    if (nsresult rv = PathUtils::InitFileWithPath(opts.mBackupFile,
+                                                  aOptions.mBackupFile.Value());
+        NS_FAILED(rv)) {
+      return Err(IOUtils::IOError(rv).WithMessage(
+          "Could not parse path of backupFile (%s)",
+          NS_ConvertUTF16toUTF8(aOptions.mBackupFile.Value()).get()));
+    }
+  }
+
+  if (aOptions.mTmpPath.WasPassed()) {
+    opts.mTmpFile = new nsLocalFile();
+    if (nsresult rv = PathUtils::InitFileWithPath(opts.mTmpFile,
+                                                  aOptions.mTmpPath.Value());
+        NS_FAILED(rv)) {
+      return Err(IOUtils::IOError(rv).WithMessage(
+          "Could not parse path of temp file (%s)",
+          NS_ConvertUTF16toUTF8(aOptions.mTmpPath.Value()).get()));
+    }
+  }
+
+  opts.mCompress = aOptions.mCompress;
+  return opts;
+}
+
+/* static */
+Result<IOUtils::JsBuffer, IOUtils::IOError> IOUtils::JsBuffer::Create(
+    IOUtils::BufferKind aBufferKind, size_t aCapacity) {
+  JsBuffer buffer(aBufferKind, aCapacity);
+  if (aCapacity != 0 && !buffer.mBuffer) {
+    return Err(IOError(NS_ERROR_OUT_OF_MEMORY)
+                   .WithMessage("Could not allocate buffer"));
+  }
+  return buffer;
+}
+
+/* static */
+IOUtils::JsBuffer IOUtils::JsBuffer::CreateEmpty(
+    IOUtils::BufferKind aBufferKind) {
+  JsBuffer buffer(aBufferKind, 0);
+  MOZ_RELEASE_ASSERT(buffer.mBuffer == nullptr);
+  return buffer;
+}
+
+IOUtils::JsBuffer::JsBuffer(IOUtils::BufferKind aBufferKind, size_t aCapacity)
+    : mBufferKind(aBufferKind), mCapacity(aCapacity), mLength(0) {
+  if (mCapacity) {
+    if (aBufferKind == BufferKind::String) {
+      mBuffer = JS::UniqueChars(
+          js_pod_arena_malloc<char>(js::StringBufferArena, mCapacity));
+    } else {
+      MOZ_RELEASE_ASSERT(aBufferKind == BufferKind::Uint8Array);
+      mBuffer = JS::UniqueChars(
+          js_pod_arena_malloc<char>(js::ArrayBufferContentsArena, mCapacity));
+    }
+  }
+}
+
+IOUtils::JsBuffer::JsBuffer(IOUtils::JsBuffer&& aOther) noexcept
+    : mBufferKind(aOther.mBufferKind),
+      mCapacity(aOther.mCapacity),
+      mLength(aOther.mLength),
+      mBuffer(std::move(aOther.mBuffer)) {
+  aOther.mCapacity = 0;
+  aOther.mLength = 0;
+}
+
+IOUtils::JsBuffer& IOUtils::JsBuffer::operator=(
+    IOUtils::JsBuffer&& aOther) noexcept {
+  mBufferKind = aOther.mBufferKind;
+  mCapacity = aOther.mCapacity;
+  mLength = aOther.mLength;
+  mBuffer = std::move(aOther.mBuffer);
+
+  // Invalidate aOther.
+  aOther.mCapacity = 0;
+  aOther.mLength = 0;
+
+  return *this;
+}
+
+/* static */
+JSString* IOUtils::JsBuffer::IntoString(JSContext* aCx, JsBuffer aBuffer) {
+  MOZ_RELEASE_ASSERT(aBuffer.mBufferKind == IOUtils::BufferKind::String);
+
+  if (!aBuffer.mCapacity) {
+    return JS_GetEmptyString(aCx);
+  }
+
+  if (IsAscii(aBuffer.BeginReading())) {
+    // If the string is just plain ASCII, then we can hand the buffer off to
+    // JavaScript as a Latin1 string (since ASCII is a subset of Latin1).
+    JS::UniqueLatin1Chars asLatin1(
+        reinterpret_cast<JS::Latin1Char*>(aBuffer.mBuffer.release()));
+    return JS_NewLatin1String(aCx, std::move(asLatin1), aBuffer.mLength);
+  }
+
+  // If the string is encodable as Latin1, we need to deflate the string to a
+  // Latin1 string to accoutn for UTF-8 characters that are encoded as more than
+  // a single byte.
+  //
+  // Otherwise, the string contains characters outside Latin1 so we have to
+  // inflate to UTF-16.
+  return JS_NewStringCopyUTF8N(
+      aCx, JS::UTF8Chars(aBuffer.mBuffer.get(), aBuffer.mLength));
+}
+
+/* static */
+JSObject* IOUtils::JsBuffer::IntoUint8Array(JSContext* aCx, JsBuffer aBuffer) {
+  MOZ_RELEASE_ASSERT(aBuffer.mBufferKind == IOUtils::BufferKind::Uint8Array);
+
+  if (!aBuffer.mCapacity) {
+    return JS_NewUint8Array(aCx, 0);
+  }
+
+  char* rawBuffer = aBuffer.mBuffer.release();
+  MOZ_RELEASE_ASSERT(rawBuffer);
+  JS::Rooted<JSObject*> arrayBuffer(
+      aCx, JS::NewArrayBufferWithContents(aCx, aBuffer.mLength,
+                                          reinterpret_cast<void*>(rawBuffer)));
+
+  if (!arrayBuffer) {
+    // The array buffer does not take ownership of the data pointer unless
+    // creation succeeds. We are still on the hook to free it.
+    //
+    // aBuffer will be destructed at end of scope, but its destructor does not
+    // take into account |mCapacity| or |mLength|, so it is OK for them to be
+    // non-zero here with a null |mBuffer|.
+    js_free(rawBuffer);
+    return nullptr;
+  }
+
+  return JS_NewUint8ArrayWithBuffer(aCx, arrayBuffer, 0, aBuffer.mLength);
+}
+
+[[nodiscard]] bool ToJSValue(JSContext* aCx, IOUtils::JsBuffer&& aBuffer,
+                             JS::MutableHandle<JS::Value> aValue) {
+  if (aBuffer.mBufferKind == IOUtils::BufferKind::String) {
+    JSString* str = IOUtils::JsBuffer::IntoString(aCx, std::move(aBuffer));
+    if (!str) {
+      return false;
+    }
+
+    aValue.setString(str);
+    return true;
+  }
+
+  JSObject* array = IOUtils::JsBuffer::IntoUint8Array(aCx, std::move(aBuffer));
+  if (!array) {
+    return false;
+  }
+
+  aValue.setObject(*array);
+  return true;
+}
+
+// SyncReadFile
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(SyncReadFile)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(SyncReadFile)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(SyncReadFile)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(SyncReadFile, mParent)
+
+SyncReadFile::SyncReadFile(nsISupports* aParent,
+                           RefPtr<nsFileRandomAccessStream>&& aStream,
+                           int64_t aSize)
+    : mParent(aParent), mStream(std::move(aStream)), mSize(aSize) {
+  MOZ_RELEASE_ASSERT(mSize >= 0);
+}
+
+SyncReadFile::~SyncReadFile() = default;
+
+JSObject* SyncReadFile::WrapObject(JSContext* aCx,
+                                   JS::Handle<JSObject*> aGivenProto) {
+  return SyncReadFile_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+void SyncReadFile::ReadBytesInto(const Uint8Array& aDestArray,
+                                 const int64_t aOffset, ErrorResult& aRv) {
+  if (!mStream) {
+    return aRv.ThrowOperationError("SyncReadFile is closed");
+  }
+
+  aDestArray.ComputeState();
+
+  auto rangeEnd = CheckedInt64(aOffset) + aDestArray.Length();
+  if (!rangeEnd.isValid()) {
+    return aRv.ThrowOperationError("Requested range overflows i64");
+  }
+
+  if (rangeEnd.value() > mSize) {
+    return aRv.ThrowOperationError(
+        "Requested range overflows SyncReadFile size");
+  }
+
+  uint32_t readLen{aDestArray.Length()};
+  if (readLen == 0) {
+    return;
+  }
+
+  if (nsresult rv = mStream->Seek(PR_SEEK_SET, aOffset); NS_FAILED(rv)) {
+    return aRv.ThrowOperationError(
+        FormatErrorMessage(rv, "Could not seek to position %lld", aOffset));
+  }
+
+  Span<char> toRead(reinterpret_cast<char*>(aDestArray.Data()), readLen);
+
+  uint32_t totalRead = 0;
+  while (totalRead != readLen) {
+    // Read no more than INT32_MAX on each call to mStream->Read, otherwise it
+    // returns an error.
+    uint32_t bytesToReadThisChunk =
+        std::min<uint32_t>(readLen - totalRead, INT32_MAX);
+
+    uint32_t bytesRead = 0;
+    if (nsresult rv =
+            mStream->Read(toRead.Elements(), bytesToReadThisChunk, &bytesRead);
+        NS_FAILED(rv)) {
+      return aRv.ThrowOperationError(FormatErrorMessage(
+          rv, "Encountered an unexpected error while reading file stream"));
+    }
+    if (bytesRead == 0) {
+      return aRv.ThrowOperationError(
+          "Reading stopped before the entire array was filled");
+    }
+    totalRead += bytesRead;
+    toRead = toRead.From(bytesRead);
+  }
+}
+
+void SyncReadFile::Close() { mStream = nullptr; }
+
+#ifdef XP_UNIX
+namespace {
+
+static nsCString FromUnixString(const IOUtils::UnixString& aString) {
+  if (aString.IsUTF8String()) {
+    return aString.GetAsUTF8String();
+  }
+  if (aString.IsUint8Array()) {
+    const auto& u8a = aString.GetAsUint8Array();
+    u8a.ComputeState();
+    // Cast to deal with char signedness
+    return nsCString(reinterpret_cast<const char*>(u8a.Data()), u8a.Length());
+  }
+  MOZ_CRASH("unreachable");
+}
+
+}  // namespace
+
+// static
+uint32_t IOUtils::LaunchProcess(GlobalObject& aGlobal,
+                                const Sequence<UnixString>& aArgv,
+                                const LaunchOptions& aOptions,
+                                ErrorResult& aRv) {
+  // The binding is worker-only, so should always be off-main-thread.
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  // This generally won't work in child processes due to sandboxing.
+  AssertParentProcessWithCallerLocation(aGlobal);
+
+  std::vector<std::string> argv;
+  base::LaunchOptions options;
+
+  for (const auto& arg : aArgv) {
+    argv.push_back(FromUnixString(arg).get());
+  }
+
+  size_t envLen = aOptions.mEnvironment.Length();
+  base::EnvironmentArray envp(new char*[envLen + 1]);
+  for (size_t i = 0; i < envLen; ++i) {
+    // EnvironmentArray is a UniquePtr instance which will `free`
+    // these strings.
+    envp[i] = strdup(FromUnixString(aOptions.mEnvironment[i]).get());
+  }
+  envp[envLen] = nullptr;
+  options.full_env = std::move(envp);
+
+  if (aOptions.mWorkdir.WasPassed()) {
+    options.workdir = FromUnixString(aOptions.mWorkdir.Value()).get();
+  }
+
+  if (aOptions.mFdMap.WasPassed()) {
+    for (const auto& fdItem : aOptions.mFdMap.Value()) {
+      options.fds_to_remap.push_back({fdItem.mSrc, fdItem.mDst});
+    }
+  }
+
+#  ifdef XP_MACOSX
+  options.disclaim = aOptions.mDisclaim;
+#  endif
+
+  base::ProcessHandle pid;
+  static_assert(sizeof(pid) <= sizeof(uint32_t),
+                "WebIDL long should be large enough for a pid");
+  bool ok = base::LaunchApp(argv, options, &pid);
+  if (!ok) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return 0;
+  }
+
+  MOZ_ASSERT(pid >= 0);
+  return static_cast<uint32_t>(pid);
+}
+#endif  // XP_UNIX
+
+}  // namespace mozilla::dom
+
+#undef REJECT_IF_INIT_PATH_FAILED
diff --git a/dom/system/IOUtils.h b/dom/system/IOUtils.h
new file mode 100644
index 0000000000..a5fea23c60
--- /dev/null
+++ b/dom/system/IOUtils.h
@@ -0,0 +1,921 @@
+/* -*- 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_dom_IOUtils__
+#define mozilla_dom_IOUtils__
+
+#include "js/Utility.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/DataMutex.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/Result.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/IOUtilsBinding.h"
+#include "mozilla/dom/TypedArray.h"
+#include "nsIAsyncShutdown.h"
+#include "nsIFile.h"
+#include "nsISerialEventTarget.h"
+#include "nsPrintfCString.h"
+#include "nsProxyRelease.h"
+#include "nsString.h"
+#include "nsStringFwd.h"
+#include "nsTArray.h"
+#include "prio.h"
+
+class nsFileRandomAccessStream;
+
+namespace mozilla {
+
+/**
+ * Utility class to be used with |UniquePtr| to automatically close NSPR file
+ * descriptors when they go out of scope.
+ *
+ * Example:
+ *
+ *   UniquePtr<PRFileDesc, PR_CloseDelete> fd = PR_Open(path, flags, mode);
+ */
+class PR_CloseDelete {
+ public:
+  constexpr PR_CloseDelete() = default;
+  PR_CloseDelete(const PR_CloseDelete& aOther) = default;
+  PR_CloseDelete(PR_CloseDelete&& aOther) = default;
+  PR_CloseDelete& operator=(const PR_CloseDelete& aOther) = default;
+  PR_CloseDelete& operator=(PR_CloseDelete&& aOther) = default;
+
+  void operator()(PRFileDesc* aPtr) const { PR_Close(aPtr); }
+};
+
+namespace dom {
+
+/**
+ * Implementation for the Web IDL interface at dom/chrome-webidl/IOUtils.webidl.
+ * Methods of this class must only be called from the parent process.
+ */
+class IOUtils final {
+ public:
+  class IOError;
+
+  enum class ShutdownPhase : uint8_t {
+    ProfileBeforeChange,
+    SendTelemetry,
+    XpcomWillShutdown,
+    Count,
+  };
+
+  template <typename T>
+  using PhaseArray =
+      EnumeratedArray<IOUtils::ShutdownPhase, IOUtils::ShutdownPhase::Count, T>;
+
+  static already_AddRefed<Promise> Read(GlobalObject& aGlobal,
+                                        const nsAString& aPath,
+                                        const ReadOptions& aOptions,
+                                        ErrorResult& aError);
+
+  static already_AddRefed<Promise> ReadUTF8(GlobalObject& aGlobal,
+                                            const nsAString& aPath,
+                                            const ReadUTF8Options& aOptions,
+                                            ErrorResult& aError);
+
+  static already_AddRefed<Promise> ReadJSON(GlobalObject& aGlobal,
+                                            const nsAString& aPath,
+                                            const ReadUTF8Options& aOptions,
+                                            ErrorResult& aError);
+
+  static already_AddRefed<Promise> Write(GlobalObject& aGlobal,
+                                         const nsAString& aPath,
+                                         const Uint8Array& aData,
+                                         const WriteOptions& aOptions,
+                                         ErrorResult& aError);
+
+  static already_AddRefed<Promise> WriteUTF8(GlobalObject& aGlobal,
+                                             const nsAString& aPath,
+                                             const nsACString& aString,
+                                             const WriteOptions& aOptions,
+                                             ErrorResult& aError);
+
+  static already_AddRefed<Promise> WriteJSON(GlobalObject& aGlobal,
+                                             const nsAString& aPath,
+                                             JS::Handle<JS::Value> aValue,
+                                             const WriteOptions& aOptions,
+                                             ErrorResult& aError);
+
+  static already_AddRefed<Promise> Move(GlobalObject& aGlobal,
+                                        const nsAString& aSourcePath,
+                                        const nsAString& aDestPath,
+                                        const MoveOptions& aOptions,
+                                        ErrorResult& aError);
+
+  static already_AddRefed<Promise> Remove(GlobalObject& aGlobal,
+                                          const nsAString& aPath,
+                                          const RemoveOptions& aOptions,
+                                          ErrorResult& aError);
+
+  static already_AddRefed<Promise> MakeDirectory(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const MakeDirectoryOptions& aOptions, ErrorResult& aError);
+
+  static already_AddRefed<Promise> Stat(GlobalObject& aGlobal,
+                                        const nsAString& aPath,
+                                        ErrorResult& aError);
+
+  static already_AddRefed<Promise> Copy(GlobalObject& aGlobal,
+                                        const nsAString& aSourcePath,
+                                        const nsAString& aDestPath,
+                                        const CopyOptions& aOptions,
+                                        ErrorResult& aError);
+
+  static already_AddRefed<Promise> SetAccessTime(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const Optional<int64_t>& aAccess, ErrorResult& aError);
+
+  static already_AddRefed<Promise> SetModificationTime(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const Optional<int64_t>& aModification, ErrorResult& aError);
+
+ private:
+  using SetTimeFn = decltype(&nsIFile::SetLastAccessedTime);
+
+  static_assert(
+      std::is_same_v<SetTimeFn, decltype(&nsIFile::SetLastModifiedTime)>);
+
+  static already_AddRefed<Promise> SetTime(GlobalObject& aGlobal,
+                                           const nsAString& aPath,
+                                           const Optional<int64_t>& aNewTime,
+                                           SetTimeFn aSetTimeFn,
+                                           ErrorResult& aError);
+
+ public:
+  static already_AddRefed<Promise> GetChildren(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const GetChildrenOptions& aOptions, ErrorResult& aError);
+
+  static already_AddRefed<Promise> SetPermissions(GlobalObject& aGlobal,
+                                                  const nsAString& aPath,
+                                                  uint32_t aPermissions,
+                                                  const bool aHonorUmask,
+                                                  ErrorResult& aError);
+
+  static already_AddRefed<Promise> Exists(GlobalObject& aGlobal,
+                                          const nsAString& aPath,
+                                          ErrorResult& aError);
+
+  static already_AddRefed<Promise> CreateUniqueFile(GlobalObject& aGlobal,
+                                                    const nsAString& aParent,
+                                                    const nsAString& aPrefix,
+                                                    const uint32_t aPermissions,
+                                                    ErrorResult& aError);
+  static already_AddRefed<Promise> CreateUniqueDirectory(
+      GlobalObject& aGlobal, const nsAString& aParent, const nsAString& aPrefix,
+      const uint32_t aPermissions, ErrorResult& aError);
+
+ private:
+  /**
+   * A helper method for CreateUniqueFile and CreateUniqueDirectory.
+   */
+  static already_AddRefed<Promise> CreateUnique(GlobalObject& aGlobal,
+                                                const nsAString& aParent,
+                                                const nsAString& aPrefix,
+                                                const uint32_t aFileType,
+                                                const uint32_t aPermissions,
+                                                ErrorResult& aError);
+
+ public:
+  static already_AddRefed<Promise> ComputeHexDigest(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const HashAlgorithm aAlgorithm, ErrorResult& aError);
+
+#if defined(XP_WIN)
+  static already_AddRefed<Promise> GetWindowsAttributes(GlobalObject& aGlobal,
+                                                        const nsAString& aPath,
+                                                        ErrorResult& aError);
+
+  static already_AddRefed<Promise> SetWindowsAttributes(
+      GlobalObject& aGlobal, const nsAString& aPath,
+      const mozilla::dom::WindowsFileAttributes& aAttrs, ErrorResult& aError);
+#elif defined(XP_MACOSX)
+  static already_AddRefed<Promise> HasMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError);
+  static already_AddRefed<Promise> GetMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError);
+  static already_AddRefed<Promise> SetMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               const Uint8Array& aValue,
+                                               ErrorResult& aError);
+  static already_AddRefed<Promise> DelMacXAttr(GlobalObject& aGlobal,
+                                               const nsAString& aPath,
+                                               const nsACString& aAttr,
+                                               ErrorResult& aError);
+#endif
+
+#ifdef XP_UNIX
+  using UnixString = OwningUTF8StringOrUint8Array;
+  static uint32_t LaunchProcess(GlobalObject& aGlobal,
+                                const Sequence<UnixString>& aArgv,
+                                const LaunchOptions& aOptions,
+                                ErrorResult& aRv);
+#endif
+
+  static already_AddRefed<Promise> GetFile(
+      GlobalObject& aGlobal, const Sequence<nsString>& aComponents,
+      ErrorResult& aError);
+
+  static already_AddRefed<Promise> GetDirectory(
+      GlobalObject& aGlobal, const Sequence<nsString>& aComponents,
+      ErrorResult& aError);
+
+  static void GetProfileBeforeChange(GlobalObject& aGlobal,
+                                     JS::MutableHandle<JS::Value>,
+                                     ErrorResult& aRv);
+
+  static void GetSendTelemetry(GlobalObject& aGlobal,
+                               JS::MutableHandle<JS::Value>, ErrorResult& aRv);
+
+  static RefPtr<SyncReadFile> OpenFileForSyncReading(GlobalObject& aGlobal,
+                                                     const nsAString& aPath,
+                                                     ErrorResult& aRv);
+
+  class JsBuffer;
+
+  /**
+   * The kind of buffer to allocate.
+   *
+   * This controls what kind of JS object (a JSString or a Uint8Array) is
+   * returned by |ToJSValue()|.
+   */
+  enum class BufferKind {
+    String,
+    Uint8Array,
+  };
+
+ private:
+  ~IOUtils() = default;
+
+  template <typename T>
+  using IOPromise = MozPromise<T, IOError, true>;
+
+  friend class IOUtilsShutdownBlocker;
+  struct InternalFileInfo;
+  struct InternalWriteOpts;
+  class MozLZ4;
+  class EventQueue;
+  class State;
+
+  template <typename Fn>
+  static already_AddRefed<Promise> WithPromiseAndState(GlobalObject& aGlobal,
+                                                       ErrorResult& aError,
+                                                       Fn aFn);
+
+  /**
+   * Dispatch a task on the event queue and resolve or reject the associated
+   * promise based on the result.
+   *
+   * NB: If the calling thread is a worker, this function takes care of keepting
+   *     it alive until the |IOPromise| can complete.
+   *
+   * @param aPromise The promise corresponding to the task running on the event
+   * queue.
+   * @param aFunc The task to run.
+   */
+  template <typename OkT, typename Fn>
+  static void DispatchAndResolve(EventQueue* aQueue, Promise* aPromise,
+                                 Fn aFunc);
+
+  /**
+   * Creates a new JS Promise.
+   *
+   * @return The new promise, or |nullptr| on failure.
+   */
+  static already_AddRefed<Promise> CreateJSPromise(GlobalObject& aGlobal,
+                                                   ErrorResult& aError);
+
+  // Allow conversion of |InternalFileInfo| with |ToJSValue|.
+  friend bool ToJSValue(JSContext* aCx,
+                        const InternalFileInfo& aInternalFileInfo,
+                        JS::MutableHandle<JS::Value> aValue);
+
+  /**
+   * Attempts to read the entire file at |aPath| into a buffer.
+   *
+   * @param aFile       The location of the file.
+   * @param aOffset     The offset to start reading from.
+   * @param aMaxBytes   If |Some|, then only read up this this number of bytes,
+   *                    otherwise attempt to read the whole file.
+   * @param aDecompress If true, decompress the bytes read from disk before
+   *                    returning the result to the caller.
+   * @param aBufferKind The kind of buffer to allocate.
+   *
+   * @return A buffer containing the entire (decompressed) file contents, or an
+   *         error.
+   */
+  static Result<JsBuffer, IOError> ReadSync(nsIFile* aFile,
+                                            const uint64_t aOffset,
+                                            const Maybe<uint32_t> aMaxBytes,
+                                            const bool aDecompress,
+                                            BufferKind aBufferKind);
+
+  /*
+   * Attempts to read the entire file at |aPath| as a UTF-8 string.
+   *
+   * @param aFile       The location of the file.
+   * @param aDecompress If true, decompress the bytes read from disk before
+   *                    returning the result to the caller.
+   *
+   * @return The (decompressed) contents of the file re-encoded as a UTF-16
+   *         string.
+   */
+  static Result<JsBuffer, IOError> ReadUTF8Sync(nsIFile* aFile,
+                                                const bool aDecompress);
+
+  /**
+   * Attempt to write the entirety of |aByteArray| to the file at |aPath|.
+   * This may occur by writing to an intermediate destination and performing a
+   * move, depending on |aOptions|.
+   *
+   * @param aFile  The location of the file.
+   * @param aByteArray The data to write to the file.
+   * @param aOptions   Options to modify the way the write is completed.
+   *
+   * @return The number of bytes written to the file, or an error if the write
+   *         failed or was incomplete.
+   */
+  static Result<uint32_t, IOError> WriteSync(
+      nsIFile* aFile, const Span<const uint8_t>& aByteArray,
+      const InternalWriteOpts& aOptions);
+
+  /**
+   * Attempts to move the file located at |aSourceFile| to |aDestFile|.
+   *
+   * @param aSourceFile  The location of the file to move.
+   * @param aDestFile    The destination for the file.
+   * @param aNoOverWrite If true, abort with an error if a file already exists
+   * at |aDestFile|. Otherwise, the file will be overwritten by the move.
+   *
+   * @return Ok if the file was moved successfully, or an error.
+   */
+  static Result<Ok, IOError> MoveSync(nsIFile* aSourceFile, nsIFile* aDestFile,
+                                      bool aNoOverwrite);
+
+  /**
+   * Attempts to copy the file at |aSourceFile| to |aDestFile|.
+   *
+   * @param aSourceFile The location of the file to copy.
+   * @param aDestFile   The destination that the file will be copied to.
+   *
+   * @return Ok if the operation was successful, or an error.
+   */
+  static Result<Ok, IOError> CopySync(nsIFile* aSourceFile, nsIFile* aDestFile,
+                                      bool aNoOverWrite, bool aRecursive);
+
+  /**
+   * Provides the implementation for |CopySync| and |MoveSync|.
+   *
+   * @param aMethod      A pointer to one of |nsIFile::MoveTo| or |CopyTo|
+   *                     instance methods.
+   * @param aMethodName  The name of the method to the performed. Either "move"
+   *                     or "copy".
+   * @param aSource      The source file to be copied or moved.
+   * @param aDest        The destination file.
+   * @param aNoOverwrite If true, allow overwriting |aDest| during the copy or
+   *                     move. Otherwise, abort with an error if the file would
+   *                     be overwritten.
+   *
+   * @return Ok if the operation was successful, or an error.
+   */
+  template <typename CopyOrMoveFn>
+  static Result<Ok, IOError> CopyOrMoveSync(CopyOrMoveFn aMethod,
+                                            const char* aMethodName,
+                                            nsIFile* aSource, nsIFile* aDest,
+                                            bool aNoOverwrite);
+
+  /**
+   * Attempts to remove the file located at |aFile|.
+   *
+   * @param aFile         The location of the file.
+   * @param aIgnoreAbsent If true, suppress errors due to an absent target file.
+   * @param aRecursive    If true, attempt to recursively remove descendant
+   *                      files. This option is safe to use even if the target
+   *                      is not a directory.
+   *
+   * @return Ok if the file was removed successfully, or an error.
+   */
+  static Result<Ok, IOError> RemoveSync(nsIFile* aFile, bool aIgnoreAbsent,
+                                        bool aRecursive);
+
+  /**
+   * Attempts to create a new directory at |aFile|.
+   *
+   * @param aFile             The location of the directory to create.
+   * @param aCreateAncestors  If true, create missing ancestor directories as
+   *                          needed. Otherwise, report an error if the target
+   *                          has non-existing ancestor directories.
+   * @param aIgnoreExisting   If true, suppress errors that occur if the target
+   *                          directory already exists. Otherwise, propagate the
+   *                          error if it occurs.
+   * @param aMode             Optional file mode. Defaults to 0777 to allow the
+   *                          system umask to compute the best mode for the new
+   *                          directory.
+   *
+   * @return Ok if the directory was created successfully, or an error.
+   */
+  static Result<Ok, IOError> MakeDirectorySync(nsIFile* aFile,
+                                               bool aCreateAncestors,
+                                               bool aIgnoreExisting,
+                                               int32_t aMode = 0777);
+
+  /**
+   * Attempts to stat a file at |aFile|.
+   *
+   * @param aFile The location of the file.
+   *
+   * @return An |InternalFileInfo| struct if successful, or an error.
+   */
+  static Result<IOUtils::InternalFileInfo, IOError> StatSync(nsIFile* aFile);
+
+  /**
+   * Attempts to update the last access or modification time of the file at
+   * |aFile|.
+   *
+   * @param aFile     The location of the file.
+   * @param SetTimeFn A member function pointer to either
+   *                  nsIFile::SetLastAccessedTime or
+   *                  nsIFile::SetLastModifiedTime.
+   * @param aNewTime  Some value in milliseconds since Epoch.
+   *
+   * @return Timestamp of the file if the operation was successful, or an error.
+   */
+  static Result<int64_t, IOError> SetTimeSync(nsIFile* aFile,
+                                              SetTimeFn aSetTimeFn,
+                                              int64_t aNewTime);
+
+  /**
+   * Returns the immediate children of the directory at |aFile|, if any.
+   *
+   * @param aFile The location of the directory.
+   *
+   * @return An array of absolute paths identifying the children of |aFile|.
+   *         If there are no children, an empty array. Otherwise, an error.
+   */
+  static Result<nsTArray<nsString>, IOError> GetChildrenSync(
+      nsIFile* aFile, bool aIgnoreAbsent);
+
+  /**
+   * Set the permissions of the given file.
+   *
+   * Windows does not make a distinction between user, group, and other
+   * permissions like UNICES do. If a permission flag is set for any of user,
+   * group, or other has a permission, then all users will have that
+   * permission.
+   *
+   * @param aFile        The location of the file.
+   * @param aPermissions The permissions to set, as a UNIX file mode.
+   *
+   * @return |Ok| if the permissions were successfully set, or an error.
+   */
+  static Result<Ok, IOError> SetPermissionsSync(nsIFile* aFile,
+                                                const uint32_t aPermissions);
+
+  /**
+   * Return whether or not the file exists.
+   *
+   * @param aFile The location of the file.
+   *
+   * @return Whether or not the file exists.
+   */
+  static Result<bool, IOError> ExistsSync(nsIFile* aFile);
+
+  /**
+   * Create a file or directory with a unique path.
+   *
+   * @param aFile     The location of the file or directory (including prefix)
+   * @param aFileType One of |nsIFile::NORMAL_FILE_TYPE| or
+   *                  |nsIFile::DIRECTORY_TYPE|.
+   * @param aperms    The permissions to create the file or directory with.
+   *
+   * @return A unique path.
+   */
+  static Result<nsString, IOError> CreateUniqueSync(
+      nsIFile* aFile, const uint32_t aFileType, const uint32_t aPermissions);
+
+  /**
+   * Compute the hash of a file.
+   *
+   * @param aFile      The file to hash.
+   * @param aAlgorithm The hashing algorithm to use.
+   *
+   * @return The hash of the file, as a hex digest.
+   */
+  static Result<nsCString, IOError> ComputeHexDigestSync(
+      nsIFile* aFile, const HashAlgorithm aAlgorithm);
+
+#if defined(XP_WIN)
+  /**
+   * Return the Windows-specific attributes of the file.
+   *
+   * @param aFile The location of the file.
+   *
+   * @return The Windows-specific attributes of the file.
+   */
+  static Result<uint32_t, IOError> GetWindowsAttributesSync(nsIFile* aFile);
+
+  /**
+   * Set the Windows-specific attributes of the file.
+   *
+   * @param aFile  The location of the file.
+   * @param aAttrs The attributes to set on the file.
+   *
+   * @return |Ok| if the attributes were successfully set, or an error.
+   */
+  static Result<Ok, IOError> SetWindowsAttributesSync(
+      nsIFile* aFile, const uint32_t aSetAttrs, const uint32_t aClearAttrs);
+#elif defined(XP_MACOSX)
+  static Result<bool, IOError> HasMacXAttrSync(nsIFile* aFile,
+                                               const nsCString& aAttr);
+  static Result<nsTArray<uint8_t>, IOError> GetMacXAttrSync(
+      nsIFile* aFile, const nsCString& aAttr);
+  static Result<Ok, IOError> SetMacXAttrSync(nsIFile* aFile,
+                                             const nsCString& aAttr,
+                                             const nsTArray<uint8_t>& aValue);
+  static Result<Ok, IOError> DelMacXAttrSync(nsIFile* aFile,
+                                             const nsCString& aAttr);
+#endif
+
+  static void GetShutdownClient(GlobalObject& aGlobal,
+                                JS::MutableHandle<JS::Value> aClient,
+                                ErrorResult& aRv, const ShutdownPhase aPhase);
+
+  enum class EventQueueStatus {
+    Uninitialized,
+    Initialized,
+    Shutdown,
+  };
+
+  enum class ShutdownBlockerStatus {
+    Uninitialized,
+    Initialized,
+    Failed,
+  };
+
+  /**
+   * Internal IOUtils state.
+   */
+  class State {
+   public:
+    StaticAutoPtr<EventQueue> mEventQueue;
+    EventQueueStatus mQueueStatus = EventQueueStatus::Uninitialized;
+    ShutdownBlockerStatus mBlockerStatus = ShutdownBlockerStatus::Uninitialized;
+
+    /**
+     * Set up shutdown hooks to free our internals at shutdown.
+     *
+     * NB: Must be called on main thread.
+     */
+    void SetShutdownHooks();
+  };
+
+  using StateMutex = StaticDataMutex<State>;
+
+  /**
+   * Lock the state mutex and return a handle. If shutdown has not yet
+   * finished, the internals will be constructed if necessary.
+   *
+   * @returns A handle to the internal state, which can be used to retrieve the
+   *          event queue.
+   *          If |Some| is returned, |mEventQueue| is guaranteed to be
+   * initialized. If shutdown has finished, |Nothing| is returned.
+   */
+  static Maybe<StateMutex::AutoLock> GetState();
+
+  static StateMutex sState;
+};
+
+/**
+ * The IOUtils event queue.
+ */
+class IOUtils::EventQueue final {
+  friend void IOUtils::State::SetShutdownHooks();
+
+ public:
+  EventQueue();
+
+  EventQueue(const EventQueue&) = delete;
+  EventQueue(EventQueue&&) = delete;
+  EventQueue& operator=(const EventQueue&) = delete;
+  EventQueue& operator=(EventQueue&&) = delete;
+
+  /**
+   * Dispatch a task on the event queue.
+   *
+   * NB: If using this directly from |IOUtils| instead of
+   *     |IOUtils::DispatchAndResolve| *and* the calling thread is a worker, you
+   *     *must* take care to keep the worker thread alive until the |IOPromise|
+   *     resolves or rejects. See the implementation of
+   *     |IOUtils::DispatchAndResolve| or |IOUtils::GetWindowsAttributes| for an
+   *     example.
+   *
+   * @param aFunc The task to dispatch on the event queue.
+   *
+   * @return A promise that resolves to the task's return value or rejects with
+   *         an error.
+   */
+  template <typename OkT, typename Fn>
+  RefPtr<IOPromise<OkT>> Dispatch(Fn aFunc);
+
+  Result<already_AddRefed<nsIAsyncShutdownBarrier>, nsresult>
+  GetShutdownBarrier(const ShutdownPhase aPhase);
+  Result<already_AddRefed<nsIAsyncShutdownClient>, nsresult> GetShutdownClient(
+      const ShutdownPhase aPhase);
+
+ private:
+  nsresult SetShutdownHooks();
+
+  nsCOMPtr<nsISerialEventTarget> mBackgroundEventTarget;
+  IOUtils::PhaseArray<nsCOMPtr<nsIAsyncShutdownBarrier>> mBarriers;
+};
+
+/**
+ * An error class used with the |Result| type returned by most private |IOUtils|
+ * methods.
+ */
+class IOUtils::IOError {
+ public:
+  MOZ_IMPLICIT IOError(nsresult aCode) : mCode(aCode), mMessage(Nothing()) {}
+
+  /**
+   * Replaces the message associated with this error.
+   */
+  template <typename... Args>
+  IOError WithMessage(const char* const aMessage, Args... aArgs) {
+    mMessage.emplace(nsPrintfCString(aMessage, aArgs...));
+    return *this;
+  }
+  IOError WithMessage(const char* const aMessage) {
+    mMessage.emplace(nsCString(aMessage));
+    return *this;
+  }
+  IOError WithMessage(const nsCString& aMessage) {
+    mMessage.emplace(aMessage);
+    return *this;
+  }
+
+  /**
+   * Returns the |nsresult| associated with this error.
+   */
+  nsresult Code() const { return mCode; }
+
+  /**
+   * Maybe returns a message associated with this error.
+   */
+  const Maybe<nsCString>& Message() const { return mMessage; }
+
+ private:
+  nsresult mCode;
+  Maybe<nsCString> mMessage;
+};
+
+/**
+ * This is an easier to work with representation of a |mozilla::dom::FileInfo|
+ * for private use in the IOUtils implementation.
+ *
+ * Because web IDL dictionaries are not easily copy/moveable, this class is
+ * used instead, until converted to the proper |mozilla::dom::FileInfo| before
+ * returning any results to JavaScript.
+ */
+struct IOUtils::InternalFileInfo {
+  nsString mPath;
+  FileType mType = FileType::Other;
+  uint64_t mSize = 0;
+  Maybe<PRTime> mCreationTime;  // In ms since epoch.
+  PRTime mLastAccessed = 0;     // In ms since epoch.
+  PRTime mLastModified = 0;     // In ms since epoch.
+  uint32_t mPermissions = 0;
+};
+
+/**
+ * This is an easier to work with representation of a
+ * |mozilla::dom::WriteOptions| for private use in the |IOUtils|
+ * implementation.
+ *
+ * Because web IDL dictionaries are not easily copy/moveable, this class is
+ * used instead.
+ */
+struct IOUtils::InternalWriteOpts {
+  RefPtr<nsIFile> mBackupFile;
+  RefPtr<nsIFile> mTmpFile;
+  WriteMode mMode;
+  bool mFlush = false;
+  bool mCompress = false;
+
+  static Result<InternalWriteOpts, IOUtils::IOError> FromBinding(
+      const WriteOptions& aOptions);
+};
+
+/**
+ * Re-implements the file compression and decompression utilities found
+ * in toolkit/components/lz4/lz4.js
+ *
+ * This implementation uses the non-standard data layout:
+ *
+ *  - MAGIC_NUMBER (8 bytes)
+ *  - content size (uint32_t, little endian)
+ *  - content, as obtained from mozilla::Compression::LZ4::compress
+ *
+ * See bug 1209390 for more info.
+ */
+class IOUtils::MozLZ4 {
+ public:
+  static constexpr std::array<uint8_t, 8> MAGIC_NUMBER{
+      {'m', 'o', 'z', 'L', 'z', '4', '0', '\0'}};
+
+  static const uint32_t HEADER_SIZE = 8 + sizeof(uint32_t);
+
+  /**
+   * Compresses |aUncompressed| byte array, and returns a byte array with the
+   * correct format whose contents may be written to disk.
+   */
+  static Result<nsTArray<uint8_t>, IOError> Compress(
+      Span<const uint8_t> aUncompressed);
+
+  /**
+   * Checks |aFileContents| for the correct file header, and returns the
+   * decompressed content.
+   */
+  static Result<IOUtils::JsBuffer, IOError> Decompress(
+      Span<const uint8_t> aFileContents, IOUtils::BufferKind);
+};
+
+class IOUtilsShutdownBlocker : public nsIAsyncShutdownBlocker,
+                               public nsIAsyncShutdownCompletionCallback {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIASYNCSHUTDOWNBLOCKER
+  NS_DECL_NSIASYNCSHUTDOWNCOMPLETIONCALLBACK
+
+  explicit IOUtilsShutdownBlocker(const IOUtils::ShutdownPhase aPhase)
+      : mPhase(aPhase) {}
+
+ private:
+  virtual ~IOUtilsShutdownBlocker() = default;
+
+  /**
+   * Called on the main thread after the event queue has been flushed.
+   */
+  void OnFlush();
+
+  static constexpr IOUtils::PhaseArray<const char16_t*> PHASE_NAMES{
+      u"profile-before-change",
+      u"profile-before-change-telemetry",
+      u"xpcom-will-shutdown",
+  };
+
+  // The last shutdown phase before we should shut down the event loop.
+  static constexpr auto LAST_IO_PHASE = IOUtils::ShutdownPhase::SendTelemetry;
+
+  IOUtils::ShutdownPhase mPhase;
+  nsCOMPtr<nsIAsyncShutdownClient> mParentClient;
+};
+
+/**
+ * A buffer that is allocated inside one of JS heaps so that it can be converted
+ * to a JSString or Uint8Array object with at most one copy in the worst case.
+ */
+class IOUtils::JsBuffer final {
+ public:
+  /**
+   * Create a new buffer of the given kind with the requested capacity.
+   *
+   * @param aBufferKind The kind of buffer to create (either a string or an
+   *                    array).
+   * @param aCapacity The capacity of the buffer.
+   *
+   * @return Either a successfully created buffer or an error if it could not be
+   * allocated.
+   */
+  static Result<JsBuffer, IOUtils::IOError> Create(
+      IOUtils::BufferKind aBufferKind, size_t aCapacity);
+
+  /**
+   * Create a new, empty buffer.
+   *
+   * This operation cannot fail.
+   *
+   * @param aBufferKind The kind of buffer to create (either a string or an
+   *                    array).
+   *
+   * @return An empty JsBuffer.
+   */
+  static JsBuffer CreateEmpty(IOUtils::BufferKind aBufferKind);
+
+  JsBuffer(const JsBuffer&) = delete;
+  JsBuffer(JsBuffer&& aOther) noexcept;
+  JsBuffer& operator=(const JsBuffer&) = delete;
+  JsBuffer& operator=(JsBuffer&& aOther) noexcept;
+
+  size_t Length() { return mLength; }
+  char* Elements() { return mBuffer.get(); }
+  void SetLength(size_t aNewLength) {
+    MOZ_RELEASE_ASSERT(aNewLength <= mCapacity);
+    mLength = aNewLength;
+  }
+
+  /**
+   * Return a span for writing to the buffer.
+   *
+   * |SetLength| should be called after the buffer has been written to.
+   *
+   * @returns A span for writing to. The size of the span is the entire
+   *          allocated capacity.
+   */
+  Span<char> BeginWriting() {
+    MOZ_RELEASE_ASSERT(mBuffer.get());
+    return Span(mBuffer.get(), mCapacity);
+  }
+
+  /**
+   * Return a span for reading from.
+   *
+   * @returns A span for reading form. The size of the span is the set length
+   *          of the buffer.
+   */
+  Span<const char> BeginReading() const {
+    MOZ_RELEASE_ASSERT(mBuffer.get() || mLength == 0);
+    return Span(mBuffer.get(), mLength);
+  }
+
+  /**
+   * Consume the JsBuffer and convert it into a JSString.
+   *
+   * NOTE: This method asserts the buffer was allocated as a string buffer.
+   *
+   * @param aBuffer The buffer to convert to a string. After this call, the
+   *                buffer will be invaldated and |IntoString| cannot be called
+   *                again.
+   *
+   * @returns A JSString with the contents of |aBuffer|.
+   */
+  static JSString* IntoString(JSContext* aCx, JsBuffer aBuffer);
+
+  /**
+   * Consume the JsBuffer and convert it into a Uint8Array.
+   *
+   * NOTE: This method asserts the buffer was allocated as an array buffer.
+   *
+   * @param aBuffer The buffer to convert to an array. After this call, the
+   *                buffer will be invalidated and |IntoUint8Array| cannot be
+   *                called again.
+   *
+   * @returns A JSBuffer
+   */
+  static JSObject* IntoUint8Array(JSContext* aCx, JsBuffer aBuffer);
+
+  friend bool ToJSValue(JSContext* aCx, JsBuffer&& aBuffer,
+                        JS::MutableHandle<JS::Value> aValue);
+
+ private:
+  IOUtils::BufferKind mBufferKind;
+  size_t mCapacity;
+  size_t mLength;
+  JS::UniqueChars mBuffer;
+
+  JsBuffer(BufferKind aBufferKind, size_t aCapacity);
+};
+
+class SyncReadFile : public nsISupports, public nsWrapperCache {
+ public:
+  SyncReadFile(nsISupports* aParent, RefPtr<nsFileRandomAccessStream>&& aStream,
+               int64_t aSize);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(SyncReadFile)
+
+  nsISupports* GetParentObject() const { return mParent; }
+
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override;
+
+  int64_t Size() const { return mSize; }
+  void ReadBytesInto(const Uint8Array&, const int64_t, ErrorResult& aRv);
+  void Close();
+
+ private:
+  virtual ~SyncReadFile();
+
+  nsCOMPtr<nsISupports> mParent;
+  RefPtr<nsFileRandomAccessStream> mStream;
+  int64_t mSize = 0;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif
diff --git a/dom/system/NetworkGeolocationProvider.jsm b/dom/system/NetworkGeolocationProvider.jsm
new file mode 100644
index 0000000000..b20765227a
--- /dev/null
+++ b/dom/system/NetworkGeolocationProvider.jsm
@@ -0,0 +1,511 @@
+/* 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/. */
+
+"use strict";
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  clearTimeout: "resource://gre/modules/Timer.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  LocationHelper: "resource://gre/modules/LocationHelper.jsm",
+});
+
+// GeolocationPositionError has no interface object, so we can't use that here.
+const POSITION_UNAVAILABLE = 2;
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "gLoggingEnabled",
+  "geo.provider.network.logging.enabled",
+  false
+);
+
+function LOG(aMsg) {
+  if (lazy.gLoggingEnabled) {
+    dump("*** WIFI GEO: " + aMsg + "\n");
+  }
+}
+
+function CachedRequest(loc, cellInfo, wifiList) {
+  this.location = loc;
+
+  let wifis = new Set();
+  if (wifiList) {
+    for (let i = 0; i < wifiList.length; i++) {
+      wifis.add(wifiList[i].macAddress);
+    }
+  }
+
+  // Use only these values for equality
+  // (the JSON will contain additional values in future)
+  function makeCellKey(cell) {
+    return (
+      "" +
+      cell.radio +
+      ":" +
+      cell.mobileCountryCode +
+      ":" +
+      cell.mobileNetworkCode +
+      ":" +
+      cell.locationAreaCode +
+      ":" +
+      cell.cellId
+    );
+  }
+
+  let cells = new Set();
+  if (cellInfo) {
+    for (let i = 0; i < cellInfo.length; i++) {
+      cells.add(makeCellKey(cellInfo[i]));
+    }
+  }
+
+  this.hasCells = () => cells.size > 0;
+
+  this.hasWifis = () => wifis.size > 0;
+
+  // if fields match
+  this.isCellEqual = function(cellInfo) {
+    if (!this.hasCells()) {
+      return false;
+    }
+
+    let len1 = cells.size;
+    let len2 = cellInfo.length;
+
+    if (len1 != len2) {
+      LOG("cells not equal len");
+      return false;
+    }
+
+    for (let i = 0; i < len2; i++) {
+      if (!cells.has(makeCellKey(cellInfo[i]))) {
+        return false;
+      }
+    }
+    return true;
+  };
+
+  // if 50% of the SSIDS match
+  this.isWifiApproxEqual = function(wifiList) {
+    if (!this.hasWifis()) {
+      return false;
+    }
+
+    // if either list is a 50% subset of the other, they are equal
+    let common = 0;
+    for (let i = 0; i < wifiList.length; i++) {
+      if (wifis.has(wifiList[i].macAddress)) {
+        common++;
+      }
+    }
+    let kPercentMatch = 0.5;
+    return common >= Math.max(wifis.size, wifiList.length) * kPercentMatch;
+  };
+
+  this.isGeoip = function() {
+    return !this.hasCells() && !this.hasWifis();
+  };
+
+  this.isCellAndWifi = function() {
+    return this.hasCells() && this.hasWifis();
+  };
+
+  this.isCellOnly = function() {
+    return this.hasCells() && !this.hasWifis();
+  };
+
+  this.isWifiOnly = function() {
+    return this.hasWifis() && !this.hasCells();
+  };
+}
+
+var gCachedRequest = null;
+var gDebugCacheReasoning = ""; // for logging the caching logic
+
+// This function serves two purposes:
+// 1) do we have a cached request
+// 2) is the cached request better than what newCell and newWifiList will obtain
+// If the cached request exists, and we know it to have greater accuracy
+// by the nature of its origin (wifi/cell/geoip), use its cached location.
+//
+// If there is more source info than the cached request had, return false
+// In other cases, MLS is known to produce better/worse accuracy based on the
+// inputs, so base the decision on that.
+function isCachedRequestMoreAccurateThanServerRequest(newCell, newWifiList) {
+  gDebugCacheReasoning = "";
+  let isNetworkRequestCacheEnabled = true;
+  try {
+    // Mochitest needs this pref to simulate request failure
+    isNetworkRequestCacheEnabled = Services.prefs.getBoolPref(
+      "geo.provider.network.debug.requestCache.enabled"
+    );
+    if (!isNetworkRequestCacheEnabled) {
+      gCachedRequest = null;
+    }
+  } catch (e) {}
+
+  if (!gCachedRequest || !isNetworkRequestCacheEnabled) {
+    gDebugCacheReasoning = "No cached data";
+    return false;
+  }
+
+  if (!newCell && !newWifiList) {
+    gDebugCacheReasoning = "New req. is GeoIP.";
+    return true;
+  }
+
+  if (
+    newCell &&
+    newWifiList &&
+    (gCachedRequest.isCellOnly() || gCachedRequest.isWifiOnly())
+  ) {
+    gDebugCacheReasoning = "New req. is cell+wifi, cache only cell or wifi.";
+    return false;
+  }
+
+  if (newCell && gCachedRequest.isWifiOnly()) {
+    // In order to know if a cell-only request should trump a wifi-only request
+    // need to know if wifi is low accuracy. >5km would be VERY low accuracy,
+    // it is worth trying the cell
+    var isHighAccuracyWifi = gCachedRequest.location.coords.accuracy < 5000;
+    gDebugCacheReasoning =
+      "Req. is cell, cache is wifi, isHigh:" + isHighAccuracyWifi;
+    return isHighAccuracyWifi;
+  }
+
+  let hasEqualCells = false;
+  if (newCell) {
+    hasEqualCells = gCachedRequest.isCellEqual(newCell);
+  }
+
+  let hasEqualWifis = false;
+  if (newWifiList) {
+    hasEqualWifis = gCachedRequest.isWifiApproxEqual(newWifiList);
+  }
+
+  gDebugCacheReasoning =
+    "EqualCells:" + hasEqualCells + " EqualWifis:" + hasEqualWifis;
+
+  if (gCachedRequest.isCellOnly()) {
+    gDebugCacheReasoning += ", Cell only.";
+    if (hasEqualCells) {
+      return true;
+    }
+  } else if (gCachedRequest.isWifiOnly() && hasEqualWifis) {
+    gDebugCacheReasoning += ", Wifi only.";
+    return true;
+  } else if (gCachedRequest.isCellAndWifi()) {
+    gDebugCacheReasoning += ", Cache has Cell+Wifi.";
+    if (
+      (hasEqualCells && hasEqualWifis) ||
+      (!newWifiList && hasEqualCells) ||
+      (!newCell && hasEqualWifis)
+    ) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+function NetworkGeoCoordsObject(lat, lon, acc) {
+  this.latitude = lat;
+  this.longitude = lon;
+  this.accuracy = acc;
+
+  // Neither GLS nor MLS return the following properties, so set them to NaN
+  // here. nsGeoPositionCoords will convert NaNs to null for optional properties
+  // of the JavaScript Coordinates object.
+  this.altitude = NaN;
+  this.altitudeAccuracy = NaN;
+  this.heading = NaN;
+  this.speed = NaN;
+}
+
+NetworkGeoCoordsObject.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIDOMGeoPositionCoords"]),
+};
+
+function NetworkGeoPositionObject(lat, lng, acc) {
+  this.coords = new NetworkGeoCoordsObject(lat, lng, acc);
+  this.address = null;
+  this.timestamp = Date.now();
+}
+
+NetworkGeoPositionObject.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIDOMGeoPosition"]),
+};
+
+function NetworkGeolocationProvider() {
+  /*
+    The _wifiMonitorTimeout controls how long we wait on receiving an update
+    from the Wifi subsystem.  If this timer fires, we believe the Wifi scan has
+    had a problem and we no longer can use Wifi to position the user this time
+    around (we will continue to be hopeful that Wifi will recover).
+
+    This timeout value is also used when Wifi scanning is disabled (see
+    isWifiScanningEnabled).  In this case, we use this timer to collect cell/ip
+    data and xhr it to the location server.
+  */
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_wifiMonitorTimeout",
+    "geo.provider.network.timeToWaitBeforeSending",
+    5000
+  );
+
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_wifiScanningEnabled",
+    "geo.provider.network.scan",
+    true
+  );
+
+  this.wifiService = null;
+  this.timer = null;
+  this.started = false;
+}
+
+NetworkGeolocationProvider.prototype = {
+  classID: Components.ID("{77DA64D3-7458-4920-9491-86CC9914F904}"),
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIGeolocationProvider",
+    "nsIWifiListener",
+    "nsITimerCallback",
+    "nsIObserver",
+  ]),
+  listener: null,
+
+  get isWifiScanningEnabled() {
+    return Cc["@mozilla.org/wifi/monitor;1"] && this._wifiScanningEnabled;
+  },
+
+  resetTimer() {
+    if (this.timer) {
+      this.timer.cancel();
+      this.timer = null;
+    }
+    // Wifi thread triggers NetworkGeolocationProvider to proceed. With no wifi,
+    // do manual timeout.
+    this.timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    this.timer.initWithCallback(
+      this,
+      this._wifiMonitorTimeout,
+      this.timer.TYPE_REPEATING_SLACK
+    );
+  },
+
+  startup() {
+    if (this.started) {
+      return;
+    }
+
+    this.started = true;
+
+    if (this.isWifiScanningEnabled) {
+      if (this.wifiService) {
+        this.wifiService.stopWatching(this);
+      }
+      this.wifiService = Cc["@mozilla.org/wifi/monitor;1"].getService(
+        Ci.nsIWifiMonitor
+      );
+      this.wifiService.startWatching(this);
+    }
+
+    this.resetTimer();
+    LOG("startup called.");
+  },
+
+  watch(c) {
+    this.listener = c;
+  },
+
+  shutdown() {
+    LOG("shutdown called");
+    if (!this.started) {
+      return;
+    }
+
+    // Without clearing this, we could end up using the cache almost indefinitely
+    // TODO: add logic for cache lifespan, for now just be safe and clear it
+    gCachedRequest = null;
+
+    if (this.timer) {
+      this.timer.cancel();
+      this.timer = null;
+    }
+
+    if (this.wifiService) {
+      this.wifiService.stopWatching(this);
+      this.wifiService = null;
+    }
+
+    this.listener = null;
+    this.started = false;
+  },
+
+  setHighAccuracy(enable) {
+    // Mochitest wants to check this value
+    if (Services.prefs.getBoolPref("geo.provider.testing")) {
+      Services.obs.notifyObservers(
+        null,
+        "testing-geolocation-high-accuracy",
+        enable
+      );
+    }
+  },
+
+  onChange(accessPoints) {
+    // we got some wifi data, rearm the timer.
+    this.resetTimer();
+
+    let wifiData = null;
+    if (accessPoints) {
+      wifiData = lazy.LocationHelper.formatWifiAccessPoints(accessPoints);
+    }
+    this.sendLocationRequest(wifiData);
+  },
+
+  onError(code) {
+    LOG("wifi error: " + code);
+    this.sendLocationRequest(null);
+  },
+
+  onStatus(err, statusMessage) {
+    if (!this.listener) {
+      return;
+    }
+    LOG("onStatus called." + statusMessage);
+
+    if (statusMessage && this.listener.notifyStatus) {
+      this.listener.notifyStatus(statusMessage);
+    }
+
+    if (err && this.listener.notifyError) {
+      this.listener.notifyError(POSITION_UNAVAILABLE, statusMessage);
+    }
+  },
+
+  notify(timer) {
+    this.onStatus(false, "wifi-timeout");
+    this.sendLocationRequest(null);
+  },
+
+  /**
+   * After wifi (and possible cell tower) data has been gathered, this method is
+   * invoked to perform the request to network geolocation provider.
+   * The result of each request is sent to all registered listener (@see watch)
+   * by invoking its respective `update`, `notifyError` or `notifyStatus`
+   * callbacks.
+   * `update` is called upon a successful request with its response data; this will be a `NetworkGeoPositionObject` instance.
+   * `notifyError` is called whenever the request gets an error from the local
+   * network subsystem, the server or simply times out.
+   * `notifyStatus` is called for each status change of the request that may be
+   * of interest to the consumer of this class. Currently the following status
+   * changes are reported: 'xhr-start', 'xhr-timeout', 'xhr-error' and
+   * 'xhr-empty'.
+   *
+   * @param  {Array} wifiData Optional set of publicly available wifi networks
+   *                          in the following structure:
+   *                          <code>
+   *                          [
+   *                            { macAddress: <mac1>, signalStrength: <signal1> },
+   *                            { macAddress: <mac2>, signalStrength: <signal2> }
+   *                          ]
+   *                          </code>
+   */
+  async sendLocationRequest(wifiData) {
+    let data = { cellTowers: undefined, wifiAccessPoints: undefined };
+    if (wifiData && wifiData.length >= 2) {
+      data.wifiAccessPoints = wifiData;
+    }
+
+    let useCached = isCachedRequestMoreAccurateThanServerRequest(
+      data.cellTowers,
+      data.wifiAccessPoints
+    );
+
+    LOG("Use request cache:" + useCached + " reason:" + gDebugCacheReasoning);
+
+    if (useCached) {
+      gCachedRequest.location.timestamp = Date.now();
+      if (this.listener) {
+        this.listener.update(gCachedRequest.location);
+      }
+      return;
+    }
+
+    // From here on, do a network geolocation request //
+    let url = Services.urlFormatter.formatURLPref("geo.provider.network.url");
+    LOG("Sending request");
+
+    let result;
+    try {
+      result = await this.makeRequest(url, wifiData);
+      LOG(
+        `geo provider reported: ${result.location.lng}:${result.location.lat}`
+      );
+      let newLocation = new NetworkGeoPositionObject(
+        result.location.lat,
+        result.location.lng,
+        result.accuracy
+      );
+
+      if (this.listener) {
+        this.listener.update(newLocation);
+      }
+
+      gCachedRequest = new CachedRequest(
+        newLocation,
+        data.cellTowers,
+        data.wifiAccessPoints
+      );
+    } catch (err) {
+      LOG("Location request hit error: " + err.name);
+      console.error(err);
+      if (err.name == "AbortError") {
+        this.onStatus(true, "xhr-timeout");
+      } else {
+        this.onStatus(true, "xhr-error");
+      }
+    }
+  },
+
+  async makeRequest(url, wifiData) {
+    this.onStatus(false, "xhr-start");
+
+    let fetchController = new AbortController();
+    let fetchOpts = {
+      method: "POST",
+      headers: { "Content-Type": "application/json; charset=UTF-8" },
+      credentials: "omit",
+      signal: fetchController.signal,
+    };
+
+    if (wifiData) {
+      fetchOpts.body = JSON.stringify({ wifiAccessPoints: wifiData });
+    }
+
+    let timeoutId = lazy.setTimeout(
+      () => fetchController.abort(),
+      Services.prefs.getIntPref("geo.provider.network.timeout")
+    );
+
+    let req = await fetch(url, fetchOpts);
+    lazy.clearTimeout(timeoutId);
+    let result = req.json();
+    return result;
+  },
+};
+
+var EXPORTED_SYMBOLS = ["NetworkGeolocationProvider"];
diff --git a/dom/system/OSFileConstants.cpp b/dom/system/OSFileConstants.cpp
new file mode 100644
index 0000000000..fa7e9532af
--- /dev/null
+++ b/dom/system/OSFileConstants.cpp
@@ -0,0 +1,983 @@
+/* -*- 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/DebugOnly.h"
+
+#include "fcntl.h"
+#include "errno.h"
+
+#include "prsystem.h"
+
+// Short macro to get the size of a member of a
+// given struct at compile time.
+// t is the type of struct, m the name of the
+// member:
+// DOM_SIZEOF_MEMBER(struct mystruct, myint)
+// will give you the size of the type of myint.
+#define DOM_SIZEOF_MEMBER(t, m) sizeof(((t*)0)->m)
+
+#if defined(XP_UNIX)
+#  include "unistd.h"
+#  include "dirent.h"
+#  include "poll.h"
+#  include "sys/stat.h"
+#  if defined(XP_LINUX)
+#    include <sys/prctl.h>
+#    include <sys/vfs.h>
+#    define statvfs statfs
+#    define f_frsize f_bsize
+#  else
+#    include "sys/statvfs.h"
+#  endif  // defined(XP_LINUX)
+#  if !defined(ANDROID)
+#    include "sys/wait.h"
+#    include <spawn.h>
+#  endif  // !defined(ANDROID)
+#endif    // defined(XP_UNIX)
+
+#if defined(XP_LINUX)
+#  include <linux/fadvise.h>
+#endif  // defined(XP_LINUX)
+
+#if defined(XP_MACOSX)
+#  include "copyfile.h"
+#endif  // defined(XP_MACOSX)
+
+#if defined(XP_WIN)
+#  include <windows.h>
+#  include <accctrl.h>
+
+#  ifndef PATH_MAX
+#    define PATH_MAX MAX_PATH
+#  endif
+
+#endif  // defined(XP_WIN)
+
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/PropertyAndElement.h"  // JS_DefineObject, JS_DefineProperty, JS_GetProperty, JS_SetProperty
+#include "BindingUtils.h"
+
+// Used to provide information on the OS
+
+#include "nsThreadUtils.h"
+#include "nsIObserverService.h"
+#include "nsIObserver.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIXULRuntime.h"
+#include "nsXPCOMCIDInternal.h"
+#include "nsServiceManagerUtils.h"
+#include "nsString.h"
+#include "nsSystemInfo.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsXULAppAPI.h"
+#include "nsAppDirectoryServiceDefs.h"
+#include "mozJSModuleLoader.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/UniquePtr.h"
+
+#include "OSFileConstants.h"
+#include "nsZipArchive.h"
+
+#if defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__) || \
+    defined(__OpenBSD__)
+#  define __dd_fd dd_fd
+#endif
+
+/**
+ * This module defines the basic libc constants (error numbers, open modes,
+ * etc.) used by OS.File and possibly other OS-bound JavaScript libraries.
+ */
+
+namespace mozilla {
+
+namespace {
+
+StaticRefPtr<OSFileConstantsService> gInstance;
+
+}  // anonymous namespace
+
+struct OSFileConstantsService::Paths {
+  /**
+   * The name of the directory holding all the libraries (libxpcom, libnss,
+   * etc.)
+   */
+  nsString libDir;
+  nsString tmpDir;
+  nsString profileDir;
+  nsString localProfileDir;
+
+  Paths() {
+    libDir.SetIsVoid(true);
+    tmpDir.SetIsVoid(true);
+    profileDir.SetIsVoid(true);
+    localProfileDir.SetIsVoid(true);
+  }
+};
+
+/**
+ * Return the path to one of the special directories.
+ *
+ * @param aKey The key to the special directory (e.g. "TmpD", "ProfD", ...)
+ * @param aOutPath The path to the special directory. In case of error,
+ * the string is set to void.
+ */
+nsresult GetPathToSpecialDir(const char* aKey, nsString& aOutPath) {
+  nsCOMPtr<nsIFile> file;
+  nsresult rv = NS_GetSpecialDirectory(aKey, getter_AddRefs(file));
+  if (NS_FAILED(rv) || !file) {
+    return rv;
+  }
+
+  return file->GetPath(aOutPath);
+}
+
+/**
+ * In some cases, OSFileConstants may be instantiated before the
+ * profile is setup. In such cases, |OS.Constants.Path.profileDir| and
+ * |OS.Constants.Path.localProfileDir| are undefined. However, we want
+ * to ensure that this does not break existing code, so that future
+ * workers spawned after the profile is setup have these constants.
+ *
+ * For this purpose, we register an observer to set |mPaths->profileDir|
+ * and |mPaths->localProfileDir| once the profile is setup.
+ */
+NS_IMETHODIMP
+OSFileConstantsService::Observe(nsISupports*, const char* aTopic,
+                                const char16_t*) {
+  if (!mInitialized) {
+    // Initialization has not taken place, something is wrong,
+    // don't make things worse.
+    return NS_OK;
+  }
+
+  nsresult rv =
+      GetPathToSpecialDir(NS_APP_USER_PROFILE_50_DIR, mPaths->profileDir);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  rv = GetPathToSpecialDir(NS_APP_USER_PROFILE_LOCAL_50_DIR,
+                           mPaths->localProfileDir);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+/**
+ * Perform the part of initialization that can only be
+ * executed on the main thread.
+ */
+nsresult OSFileConstantsService::InitOSFileConstants() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (mInitialized) {
+    return NS_OK;
+  }
+
+  UniquePtr<Paths> paths(new Paths);
+
+  // Initialize paths->libDir
+  nsCOMPtr<nsIFile> file;
+  nsresult rv =
+      NS_GetSpecialDirectory(NS_XPCOM_LIBRARY_FILE, getter_AddRefs(file));
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  nsCOMPtr<nsIFile> libDir;
+  rv = file->GetParent(getter_AddRefs(libDir));
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  rv = libDir->GetPath(paths->libDir);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Setup profileDir and localProfileDir immediately if possible (we
+  // assume that NS_APP_USER_PROFILE_50_DIR and
+  // NS_APP_USER_PROFILE_LOCAL_50_DIR are set simultaneously)
+  rv = GetPathToSpecialDir(NS_APP_USER_PROFILE_50_DIR, paths->profileDir);
+  if (NS_SUCCEEDED(rv)) {
+    rv = GetPathToSpecialDir(NS_APP_USER_PROFILE_LOCAL_50_DIR,
+                             paths->localProfileDir);
+  }
+
+  // Otherwise, delay setup of profileDir/localProfileDir until they
+  // become available.
+  if (NS_FAILED(rv)) {
+    nsCOMPtr<nsIObserverService> obsService =
+        do_GetService(NS_OBSERVERSERVICE_CONTRACTID, &rv);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    rv = obsService->AddObserver(this, "profile-do-change", false);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+  }
+
+  GetPathToSpecialDir(NS_OS_TEMP_DIR, paths->tmpDir);
+
+  mPaths = std::move(paths);
+
+  // Get the umask from the system-info service.
+  // The property will always be present, but it will be zero on
+  // non-Unix systems.
+  // nsSystemInfo::gUserUmask is initialized by NS_InitXPCOM so we don't need
+  // to initialize the service.
+  mUserUmask = nsSystemInfo::gUserUmask;
+
+  mInitialized = true;
+  return NS_OK;
+}
+
+/**
+ * Define a simple read-only property holding an integer.
+ *
+ * @param name The name of the constant. Used both as the JS name for the
+ * constant and to access its value. Must be defined.
+ *
+ * Produces a |ConstantSpec|.
+ */
+#define INT_CONSTANT(name) \
+  { #name, JS::Int32Value(name) }
+
+/**
+ * Define a simple read-only property holding an unsigned integer.
+ *
+ * @param name The name of the constant. Used both as the JS name for the
+ * constant and to access its value. Must be defined.
+ *
+ * Produces a |ConstantSpec|.
+ */
+#define UINT_CONSTANT(name) \
+  { #name, JS::NumberValue(name) }
+
+/**
+ * End marker for ConstantSpec
+ */
+#define PROP_END \
+  { nullptr, JS::UndefinedValue() }
+
+// Define missing constants for Android
+#if !defined(S_IRGRP)
+#  define S_IXOTH 0001
+#  define S_IWOTH 0002
+#  define S_IROTH 0004
+#  define S_IRWXO 0007
+#  define S_IXGRP 0010
+#  define S_IWGRP 0020
+#  define S_IRGRP 0040
+#  define S_IRWXG 0070
+#  define S_IXUSR 0100
+#  define S_IWUSR 0200
+#  define S_IRUSR 0400
+#  define S_IRWXU 0700
+#endif  // !defined(S_IRGRP)
+
+/**
+ * The properties defined in libc.
+ *
+ * If you extend this list of properties, please
+ * separate categories ("errors", "open", etc.),
+ * keep properties organized by alphabetical order
+ * and #ifdef-away properties that are not portable.
+ */
+static const dom::ConstantSpec gLibcProperties[] = {
+    // Arguments for open
+    INT_CONSTANT(O_APPEND),
+#if defined(O_CLOEXEC)
+    INT_CONSTANT(O_CLOEXEC),
+#endif  // defined(O_CLOEXEC)
+    INT_CONSTANT(O_CREAT),
+#if defined(O_DIRECTORY)
+    INT_CONSTANT(O_DIRECTORY),
+#endif  // defined(O_DIRECTORY)
+#if defined(O_EVTONLY)
+    INT_CONSTANT(O_EVTONLY),
+#endif  // defined(O_EVTONLY)
+    INT_CONSTANT(O_EXCL),
+#if defined(O_EXLOCK)
+    INT_CONSTANT(O_EXLOCK),
+#endif  // defined(O_EXLOCK)
+#if defined(O_LARGEFILE)
+    INT_CONSTANT(O_LARGEFILE),
+#endif  // defined(O_LARGEFILE)
+#if defined(O_NOFOLLOW)
+    INT_CONSTANT(O_NOFOLLOW),
+#endif  // defined(O_NOFOLLOW)
+#if defined(O_NONBLOCK)
+    INT_CONSTANT(O_NONBLOCK),
+#endif  // defined(O_NONBLOCK)
+    INT_CONSTANT(O_RDONLY),
+    INT_CONSTANT(O_RDWR),
+#if defined(O_RSYNC)
+    INT_CONSTANT(O_RSYNC),
+#endif  // defined(O_RSYNC)
+#if defined(O_SHLOCK)
+    INT_CONSTANT(O_SHLOCK),
+#endif  // defined(O_SHLOCK)
+#if defined(O_SYMLINK)
+    INT_CONSTANT(O_SYMLINK),
+#endif  // defined(O_SYMLINK)
+#if defined(O_SYNC)
+    INT_CONSTANT(O_SYNC),
+#endif  // defined(O_SYNC)
+    INT_CONSTANT(O_TRUNC),
+    INT_CONSTANT(O_WRONLY),
+
+#if defined(FD_CLOEXEC)
+    INT_CONSTANT(FD_CLOEXEC),
+#endif  // defined(FD_CLOEXEC)
+
+#if defined(AT_EACCESS)
+    INT_CONSTANT(AT_EACCESS),
+#endif  // defined(AT_EACCESS)
+#if defined(AT_FDCWD)
+    INT_CONSTANT(AT_FDCWD),
+#endif  // defined(AT_FDCWD)
+#if defined(AT_SYMLINK_NOFOLLOW)
+    INT_CONSTANT(AT_SYMLINK_NOFOLLOW),
+#endif  // defined(AT_SYMLINK_NOFOLLOW)
+
+#if defined(POSIX_FADV_SEQUENTIAL)
+    INT_CONSTANT(POSIX_FADV_SEQUENTIAL),
+#endif  // defined(POSIX_FADV_SEQUENTIAL)
+
+// access
+#if defined(F_OK)
+    INT_CONSTANT(F_OK),
+    INT_CONSTANT(R_OK),
+    INT_CONSTANT(W_OK),
+    INT_CONSTANT(X_OK),
+#endif  // defined(F_OK)
+
+    // modes
+    INT_CONSTANT(S_IRGRP),
+    INT_CONSTANT(S_IROTH),
+    INT_CONSTANT(S_IRUSR),
+    INT_CONSTANT(S_IRWXG),
+    INT_CONSTANT(S_IRWXO),
+    INT_CONSTANT(S_IRWXU),
+    INT_CONSTANT(S_IWGRP),
+    INT_CONSTANT(S_IWOTH),
+    INT_CONSTANT(S_IWUSR),
+    INT_CONSTANT(S_IXOTH),
+    INT_CONSTANT(S_IXGRP),
+    INT_CONSTANT(S_IXUSR),
+
+    // seek
+    INT_CONSTANT(SEEK_CUR),
+    INT_CONSTANT(SEEK_END),
+    INT_CONSTANT(SEEK_SET),
+
+#if defined(XP_UNIX)
+    // poll
+    INT_CONSTANT(POLLERR),
+    INT_CONSTANT(POLLHUP),
+    INT_CONSTANT(POLLIN),
+    INT_CONSTANT(POLLNVAL),
+    INT_CONSTANT(POLLOUT),
+
+// wait
+#  if defined(WNOHANG)
+    INT_CONSTANT(WNOHANG),
+#  endif  // defined(WNOHANG)
+
+    // fcntl command values
+    INT_CONSTANT(F_GETLK),
+    INT_CONSTANT(F_SETFD),
+    INT_CONSTANT(F_SETFL),
+    INT_CONSTANT(F_SETLK),
+    INT_CONSTANT(F_SETLKW),
+
+    // flock type values
+    INT_CONSTANT(F_RDLCK),
+    INT_CONSTANT(F_WRLCK),
+    INT_CONSTANT(F_UNLCK),
+
+// splice
+#  if defined(SPLICE_F_MOVE)
+    INT_CONSTANT(SPLICE_F_MOVE),
+#  endif  // defined(SPLICE_F_MOVE)
+#  if defined(SPLICE_F_NONBLOCK)
+    INT_CONSTANT(SPLICE_F_NONBLOCK),
+#  endif  // defined(SPLICE_F_NONBLOCK)
+#  if defined(SPLICE_F_MORE)
+    INT_CONSTANT(SPLICE_F_MORE),
+#  endif  // defined(SPLICE_F_MORE)
+#  if defined(SPLICE_F_GIFT)
+    INT_CONSTANT(SPLICE_F_GIFT),
+#  endif  // defined(SPLICE_F_GIFT)
+#endif    // defined(XP_UNIX)
+// copyfile
+#if defined(COPYFILE_DATA)
+    INT_CONSTANT(COPYFILE_DATA),
+    INT_CONSTANT(COPYFILE_EXCL),
+    INT_CONSTANT(COPYFILE_XATTR),
+    INT_CONSTANT(COPYFILE_STAT),
+    INT_CONSTANT(COPYFILE_ACL),
+    INT_CONSTANT(COPYFILE_MOVE),
+#endif  // defined(COPYFILE_DATA)
+
+    // error values
+    INT_CONSTANT(EACCES),
+    INT_CONSTANT(EAGAIN),
+    INT_CONSTANT(EBADF),
+    INT_CONSTANT(EEXIST),
+    INT_CONSTANT(EFAULT),
+    INT_CONSTANT(EFBIG),
+    INT_CONSTANT(EINVAL),
+    INT_CONSTANT(EINTR),
+    INT_CONSTANT(EIO),
+    INT_CONSTANT(EISDIR),
+#if defined(ELOOP)  // not defined with VC9
+    INT_CONSTANT(ELOOP),
+#endif  // defined(ELOOP)
+    INT_CONSTANT(EMFILE),
+    INT_CONSTANT(ENAMETOOLONG),
+    INT_CONSTANT(ENFILE),
+    INT_CONSTANT(ENOENT),
+    INT_CONSTANT(ENOMEM),
+    INT_CONSTANT(ENOSPC),
+    INT_CONSTANT(ENOTDIR),
+    INT_CONSTANT(ENXIO),
+#if defined(EOPNOTSUPP)  // not defined with VC 9
+    INT_CONSTANT(EOPNOTSUPP),
+#endif                  // defined(EOPNOTSUPP)
+#if defined(EOVERFLOW)  // not defined with VC 9
+    INT_CONSTANT(EOVERFLOW),
+#endif  // defined(EOVERFLOW)
+    INT_CONSTANT(EPERM),
+    INT_CONSTANT(ERANGE),
+    INT_CONSTANT(ENOSYS),
+#if defined(ETIMEDOUT)  // not defined with VC 9
+    INT_CONSTANT(ETIMEDOUT),
+#endif                    // defined(ETIMEDOUT)
+#if defined(EWOULDBLOCK)  // not defined with VC 9
+    INT_CONSTANT(EWOULDBLOCK),
+#endif  // defined(EWOULDBLOCK)
+    INT_CONSTANT(EXDEV),
+
+#if defined(DT_UNKNOWN)
+    // Constants for |readdir|
+    INT_CONSTANT(DT_UNKNOWN),
+    INT_CONSTANT(DT_FIFO),
+    INT_CONSTANT(DT_CHR),
+    INT_CONSTANT(DT_DIR),
+    INT_CONSTANT(DT_BLK),
+    INT_CONSTANT(DT_REG),
+    INT_CONSTANT(DT_LNK),
+    INT_CONSTANT(DT_SOCK),
+#endif  // defined(DT_UNKNOWN)
+
+#if defined(XP_UNIX)
+    // Constants for |stat|
+    INT_CONSTANT(S_IFMT),
+    INT_CONSTANT(S_IFIFO),
+    INT_CONSTANT(S_IFCHR),
+    INT_CONSTANT(S_IFDIR),
+    INT_CONSTANT(S_IFBLK),
+    INT_CONSTANT(S_IFREG),
+    INT_CONSTANT(S_IFLNK),   // not defined on minGW
+    INT_CONSTANT(S_IFSOCK),  // not defined on minGW
+#endif                       // defined(XP_UNIX)
+
+    INT_CONSTANT(PATH_MAX),
+
+#if defined(XP_LINUX)
+    // prctl options
+    INT_CONSTANT(PR_CAPBSET_READ),
+#endif
+
+// Constants used to define data structures
+//
+// Many data structures have different fields/sizes/etc. on
+// various OSes / versions of the same OS / platforms. For these
+// data structures, we need to compute and export from C the size
+// and, if necessary, the offset of fields, so as to be able to
+// define the structure in JS.
+
+#if defined(XP_UNIX)
+    // The size of |mode_t|.
+    {"OSFILE_SIZEOF_MODE_T", JS::Int32Value(sizeof(mode_t))},
+
+    // The size of |gid_t|.
+    {"OSFILE_SIZEOF_GID_T", JS::Int32Value(sizeof(gid_t))},
+
+    // The size of |uid_t|.
+    {"OSFILE_SIZEOF_UID_T", JS::Int32Value(sizeof(uid_t))},
+
+    // The size of |time_t|.
+    {"OSFILE_SIZEOF_TIME_T", JS::Int32Value(sizeof(time_t))},
+
+    // The size of |fsblkcnt_t|.
+    {"OSFILE_SIZEOF_FSBLKCNT_T", JS::Int32Value(sizeof(fsblkcnt_t))},
+
+#  if !defined(ANDROID)
+    // The size of |posix_spawn_file_actions_t|.
+    {"OSFILE_SIZEOF_POSIX_SPAWN_FILE_ACTIONS_T",
+     JS::Int32Value(sizeof(posix_spawn_file_actions_t))},
+
+    // The size of |posix_spawnattr_t|.
+    {"OSFILE_SIZEOF_POSIX_SPAWNATTR_T",
+     JS::Int32Value(sizeof(posix_spawnattr_t))},
+#  endif  // !defined(ANDROID)
+
+    // Defining |dirent|.
+    // Size
+    {"OSFILE_SIZEOF_DIRENT", JS::Int32Value(sizeof(dirent))},
+
+    // Defining |flock|.
+    {"OSFILE_SIZEOF_FLOCK", JS::Int32Value(sizeof(struct flock))},
+    {"OSFILE_OFFSETOF_FLOCK_L_START",
+     JS::Int32Value(offsetof(struct flock, l_start))},
+    {"OSFILE_OFFSETOF_FLOCK_L_LEN",
+     JS::Int32Value(offsetof(struct flock, l_len))},
+    {"OSFILE_OFFSETOF_FLOCK_L_PID",
+     JS::Int32Value(offsetof(struct flock, l_pid))},
+    {"OSFILE_OFFSETOF_FLOCK_L_TYPE",
+     JS::Int32Value(offsetof(struct flock, l_type))},
+    {"OSFILE_OFFSETOF_FLOCK_L_WHENCE",
+     JS::Int32Value(offsetof(struct flock, l_whence))},
+
+    // Offset of field |d_name|.
+    {"OSFILE_OFFSETOF_DIRENT_D_NAME",
+     JS::Int32Value(offsetof(struct dirent, d_name))},
+    // An upper bound to the length of field |d_name| of struct |dirent|.
+    // (may not be exact, depending on padding).
+    {"OSFILE_SIZEOF_DIRENT_D_NAME",
+     JS::Int32Value(sizeof(struct dirent) - offsetof(struct dirent, d_name))},
+
+    // Defining |timeval|.
+    {"OSFILE_SIZEOF_TIMEVAL", JS::Int32Value(sizeof(struct timeval))},
+    {"OSFILE_OFFSETOF_TIMEVAL_TV_SEC",
+     JS::Int32Value(offsetof(struct timeval, tv_sec))},
+    {"OSFILE_OFFSETOF_TIMEVAL_TV_USEC",
+     JS::Int32Value(offsetof(struct timeval, tv_usec))},
+
+#  if defined(DT_UNKNOWN)
+    // Position of field |d_type| in |dirent|
+    // Not strictly posix, but seems defined on all platforms
+    // except mingw32.
+    {"OSFILE_OFFSETOF_DIRENT_D_TYPE",
+     JS::Int32Value(offsetof(struct dirent, d_type))},
+#  endif  // defined(DT_UNKNOWN)
+
+// Under MacOS X and BSDs, |dirfd| is a macro rather than a
+// function, so we need a little help to get it to work
+#  if defined(dirfd)
+    {"OSFILE_SIZEOF_DIR", JS::Int32Value(sizeof(DIR))},
+
+    {"OSFILE_OFFSETOF_DIR_DD_FD", JS::Int32Value(offsetof(DIR, __dd_fd))},
+#  endif
+
+    // Defining |stat|
+
+    {"OSFILE_SIZEOF_STAT", JS::Int32Value(sizeof(struct stat))},
+
+    {"OSFILE_OFFSETOF_STAT_ST_MODE",
+     JS::Int32Value(offsetof(struct stat, st_mode))},
+    {"OSFILE_OFFSETOF_STAT_ST_UID",
+     JS::Int32Value(offsetof(struct stat, st_uid))},
+    {"OSFILE_OFFSETOF_STAT_ST_GID",
+     JS::Int32Value(offsetof(struct stat, st_gid))},
+    {"OSFILE_OFFSETOF_STAT_ST_SIZE",
+     JS::Int32Value(offsetof(struct stat, st_size))},
+
+#  if defined(HAVE_ST_ATIMESPEC)
+    {"OSFILE_OFFSETOF_STAT_ST_ATIME",
+     JS::Int32Value(offsetof(struct stat, st_atimespec))},
+    {"OSFILE_OFFSETOF_STAT_ST_MTIME",
+     JS::Int32Value(offsetof(struct stat, st_mtimespec))},
+    {"OSFILE_OFFSETOF_STAT_ST_CTIME",
+     JS::Int32Value(offsetof(struct stat, st_ctimespec))},
+#  else
+    {"OSFILE_OFFSETOF_STAT_ST_ATIME",
+     JS::Int32Value(offsetof(struct stat, st_atime))},
+    {"OSFILE_OFFSETOF_STAT_ST_MTIME",
+     JS::Int32Value(offsetof(struct stat, st_mtime))},
+    {"OSFILE_OFFSETOF_STAT_ST_CTIME",
+     JS::Int32Value(offsetof(struct stat, st_ctime))},
+#  endif  // defined(HAVE_ST_ATIME)
+
+// Several OSes have a birthtime field. For the moment, supporting only Darwin.
+#  if defined(_DARWIN_FEATURE_64_BIT_INODE)
+    {"OSFILE_OFFSETOF_STAT_ST_BIRTHTIME",
+     JS::Int32Value(offsetof(struct stat, st_birthtime))},
+#  endif  // defined(_DARWIN_FEATURE_64_BIT_INODE)
+
+    // Defining |statvfs|
+
+    {"OSFILE_SIZEOF_STATVFS", JS::Int32Value(sizeof(struct statvfs))},
+
+    // We have no guarantee how big "f_frsize" is, so we have to calculate that.
+    {"OSFILE_SIZEOF_STATVFS_F_FRSIZE",
+     JS::Int32Value(DOM_SIZEOF_MEMBER(struct statvfs, f_frsize))},
+    {"OSFILE_OFFSETOF_STATVFS_F_FRSIZE",
+     JS::Int32Value(offsetof(struct statvfs, f_frsize))},
+    {"OSFILE_OFFSETOF_STATVFS_F_BAVAIL",
+     JS::Int32Value(offsetof(struct statvfs, f_bavail))},
+
+#endif  // defined(XP_UNIX)
+
+// System configuration
+
+// Under MacOSX, to avoid using deprecated functions that do not
+// match the constants we define in this object (including
+// |sizeof|/|offsetof| stuff, but not only), for a number of
+// functions, we need to use functions with a $INODE64 suffix.
+// That is true on Intel-based mac when the _DARWIN_FEATURE_64_BIT_INODE
+// macro is set. But not on Apple Silicon.
+#if defined(_DARWIN_FEATURE_64_BIT_INODE) && !defined(__aarch64__)
+    {"_DARWIN_INODE64_SYMBOLS", JS::Int32Value(1)},
+#endif  // defined(_DARWIN_FEATURE_64_BIT_INODE)
+
+// Similar feature for Linux
+#if defined(_STAT_VER)
+    INT_CONSTANT(_STAT_VER),
+#endif  // defined(_STAT_VER)
+
+    PROP_END};
+
+#if defined(XP_WIN)
+/**
+ * The properties defined in windows.h.
+ *
+ * If you extend this list of properties, please
+ * separate categories ("errors", "open", etc.),
+ * keep properties organized by alphabetical order
+ * and #ifdef-away properties that are not portable.
+ */
+static const dom::ConstantSpec gWinProperties[] = {
+    // FormatMessage flags
+    INT_CONSTANT(FORMAT_MESSAGE_FROM_SYSTEM),
+    INT_CONSTANT(FORMAT_MESSAGE_IGNORE_INSERTS),
+
+    // The max length of paths
+    INT_CONSTANT(MAX_PATH),
+
+    // CreateFile desired access
+    INT_CONSTANT(GENERIC_ALL),
+    INT_CONSTANT(GENERIC_EXECUTE),
+    INT_CONSTANT(GENERIC_READ),
+    INT_CONSTANT(GENERIC_WRITE),
+
+    // CreateFile share mode
+    INT_CONSTANT(FILE_SHARE_DELETE),
+    INT_CONSTANT(FILE_SHARE_READ),
+    INT_CONSTANT(FILE_SHARE_WRITE),
+
+    // CreateFile creation disposition
+    INT_CONSTANT(CREATE_ALWAYS),
+    INT_CONSTANT(CREATE_NEW),
+    INT_CONSTANT(OPEN_ALWAYS),
+    INT_CONSTANT(OPEN_EXISTING),
+    INT_CONSTANT(TRUNCATE_EXISTING),
+
+    // CreateFile attributes
+    INT_CONSTANT(FILE_ATTRIBUTE_ARCHIVE),
+    INT_CONSTANT(FILE_ATTRIBUTE_DIRECTORY),
+    INT_CONSTANT(FILE_ATTRIBUTE_HIDDEN),
+    INT_CONSTANT(FILE_ATTRIBUTE_NORMAL),
+    INT_CONSTANT(FILE_ATTRIBUTE_READONLY),
+    INT_CONSTANT(FILE_ATTRIBUTE_REPARSE_POINT),
+    INT_CONSTANT(FILE_ATTRIBUTE_SYSTEM),
+    INT_CONSTANT(FILE_ATTRIBUTE_TEMPORARY),
+    INT_CONSTANT(FILE_FLAG_BACKUP_SEMANTICS),
+
+    // CreateFile error constant
+    {"INVALID_HANDLE_VALUE", JS::Int32Value(INT_PTR(INVALID_HANDLE_VALUE))},
+
+    // CreateFile flags
+    INT_CONSTANT(FILE_FLAG_DELETE_ON_CLOSE),
+
+    // SetFilePointer methods
+    INT_CONSTANT(FILE_BEGIN),
+    INT_CONSTANT(FILE_CURRENT),
+    INT_CONSTANT(FILE_END),
+
+    // SetFilePointer error constant
+    UINT_CONSTANT(INVALID_SET_FILE_POINTER),
+
+    // File attributes
+    INT_CONSTANT(FILE_ATTRIBUTE_DIRECTORY),
+
+    // MoveFile flags
+    INT_CONSTANT(MOVEFILE_COPY_ALLOWED),
+    INT_CONSTANT(MOVEFILE_REPLACE_EXISTING),
+
+    // GetFileAttributes error constant
+    INT_CONSTANT(INVALID_FILE_ATTRIBUTES),
+
+    // GetNamedSecurityInfo and SetNamedSecurityInfo constants
+    INT_CONSTANT(UNPROTECTED_DACL_SECURITY_INFORMATION),
+    INT_CONSTANT(SE_FILE_OBJECT),
+    INT_CONSTANT(DACL_SECURITY_INFORMATION),
+
+    // Errors
+    INT_CONSTANT(ERROR_INVALID_HANDLE),
+    INT_CONSTANT(ERROR_ACCESS_DENIED),
+    INT_CONSTANT(ERROR_DIR_NOT_EMPTY),
+    INT_CONSTANT(ERROR_FILE_EXISTS),
+    INT_CONSTANT(ERROR_ALREADY_EXISTS),
+    INT_CONSTANT(ERROR_FILE_NOT_FOUND),
+    INT_CONSTANT(ERROR_NO_MORE_FILES),
+    INT_CONSTANT(ERROR_PATH_NOT_FOUND),
+    INT_CONSTANT(ERROR_BAD_ARGUMENTS),
+    INT_CONSTANT(ERROR_SHARING_VIOLATION),
+    INT_CONSTANT(ERROR_NOT_SUPPORTED),
+
+    PROP_END};
+#endif  // defined(XP_WIN)
+
+/**
+ * Get a field of an object as an object.
+ *
+ * If the field does not exist, create it. If it exists but is not an
+ * object, throw a JS error.
+ */
+JSObject* GetOrCreateObjectProperty(JSContext* cx,
+                                    JS::Handle<JSObject*> aObject,
+                                    const char* aProperty) {
+  JS::Rooted<JS::Value> val(cx);
+  if (!JS_GetProperty(cx, aObject, aProperty, &val)) {
+    return nullptr;
+  }
+  if (!val.isUndefined()) {
+    if (val.isObject()) {
+      return &val.toObject();
+    }
+
+    JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                              JSMSG_UNEXPECTED_TYPE, aProperty,
+                              "not an object");
+    return nullptr;
+  }
+  return JS_DefineObject(cx, aObject, aProperty, nullptr, JSPROP_ENUMERATE);
+}
+
+/**
+ * Set a property of an object from a nsString.
+ *
+ * If the nsString is void (i.e. IsVoid is true), do nothing.
+ */
+bool SetStringProperty(JSContext* cx, JS::Handle<JSObject*> aObject,
+                       const char* aProperty, const nsString aValue) {
+  if (aValue.IsVoid()) {
+    return true;
+  }
+  JSString* strValue = JS_NewUCStringCopyZ(cx, aValue.get());
+  NS_ENSURE_TRUE(strValue, false);
+  JS::Rooted<JS::Value> valValue(cx, JS::StringValue(strValue));
+  return JS_SetProperty(cx, aObject, aProperty, valValue);
+}
+
+/**
+ * Define OS-specific constants.
+ *
+ * This function creates or uses JS object |OS.Constants| to store
+ * all its constants.
+ */
+bool OSFileConstantsService::DefineOSFileConstants(
+    JSContext* aCx, JS::Handle<JSObject*> aGlobal) {
+  if (!mInitialized) {
+    JS_ReportErrorNumberASCII(aCx, js::GetErrorMessage, nullptr,
+                              JSMSG_CANT_OPEN, "OSFileConstants",
+                              "initialization has failed");
+    return false;
+  }
+
+  JS::Rooted<JSObject*> objOS(aCx);
+  if (!(objOS = GetOrCreateObjectProperty(aCx, aGlobal, "OS"))) {
+    return false;
+  }
+  JS::Rooted<JSObject*> objConstants(aCx);
+  if (!(objConstants = GetOrCreateObjectProperty(aCx, objOS, "Constants"))) {
+    return false;
+  }
+
+  // Build OS.Constants.libc
+
+  JS::Rooted<JSObject*> objLibc(aCx);
+  if (!(objLibc = GetOrCreateObjectProperty(aCx, objConstants, "libc"))) {
+    return false;
+  }
+  if (!dom::DefineConstants(aCx, objLibc, gLibcProperties)) {
+    return false;
+  }
+
+#if defined(XP_WIN)
+  // Build OS.Constants.Win
+
+  JS::Rooted<JSObject*> objWin(aCx);
+  if (!(objWin = GetOrCreateObjectProperty(aCx, objConstants, "Win"))) {
+    return false;
+  }
+  if (!dom::DefineConstants(aCx, objWin, gWinProperties)) {
+    return false;
+  }
+#endif  // defined(XP_WIN)
+
+  // Build OS.Constants.Sys
+
+  JS::Rooted<JSObject*> objSys(aCx);
+  if (!(objSys = GetOrCreateObjectProperty(aCx, objConstants, "Sys"))) {
+    return false;
+  }
+
+  nsCOMPtr<nsIXULRuntime> runtime =
+      do_GetService(XULRUNTIME_SERVICE_CONTRACTID);
+  if (runtime) {
+    nsAutoCString os;
+    DebugOnly<nsresult> rv = runtime->GetOS(os);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    JSString* strVersion = JS_NewStringCopyZ(aCx, os.get());
+    if (!strVersion) {
+      return false;
+    }
+
+    JS::Rooted<JS::Value> valVersion(aCx, JS::StringValue(strVersion));
+    if (!JS_SetProperty(aCx, objSys, "Name", valVersion)) {
+      return false;
+    }
+  }
+
+#if defined(DEBUG)
+  JS::Rooted<JS::Value> valDebug(aCx, JS::TrueValue());
+  if (!JS_SetProperty(aCx, objSys, "DEBUG", valDebug)) {
+    return false;
+  }
+#endif
+
+#if defined(HAVE_64BIT_BUILD)
+  JS::Rooted<JS::Value> valBits(aCx, JS::Int32Value(64));
+#else
+  JS::Rooted<JS::Value> valBits(aCx, JS::Int32Value(32));
+#endif  // defined (HAVE_64BIT_BUILD)
+  if (!JS_SetProperty(aCx, objSys, "bits", valBits)) {
+    return false;
+  }
+
+  if (!JS_DefineProperty(
+          aCx, objSys, "umask", mUserUmask,
+          JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT)) {
+    return false;
+  }
+
+  // Build OS.Constants.Path
+
+  JS::Rooted<JSObject*> objPath(aCx);
+  if (!(objPath = GetOrCreateObjectProperty(aCx, objConstants, "Path"))) {
+    return false;
+  }
+
+  // Locate libxul
+  // Note that we don't actually provide the full path, only the name of the
+  // library, which is sufficient to link to the library using js-ctypes.
+
+#if defined(XP_MACOSX)
+  // Under MacOS X, for some reason, libxul is called simply "XUL",
+  // and we need to provide the full path.
+  nsAutoString libxul;
+  libxul.Append(mPaths->libDir);
+  libxul.AppendLiteral("/XUL");
+#else
+  // On other platforms, libxul is a library "xul" with regular
+  // library prefix/suffix.
+  nsAutoString libxul;
+  libxul.AppendLiteral(MOZ_DLL_PREFIX);
+  libxul.AppendLiteral("xul");
+  libxul.AppendLiteral(MOZ_DLL_SUFFIX);
+#endif  // defined(XP_MACOSX)
+
+  if (!SetStringProperty(aCx, objPath, "libxul", libxul)) {
+    return false;
+  }
+
+  if (!SetStringProperty(aCx, objPath, "libDir", mPaths->libDir)) {
+    return false;
+  }
+
+  if (!SetStringProperty(aCx, objPath, "tmpDir", mPaths->tmpDir)) {
+    return false;
+  }
+
+  // Configure profileDir only if it is available at this stage
+  if (!mPaths->profileDir.IsVoid() &&
+      !SetStringProperty(aCx, objPath, "profileDir", mPaths->profileDir)) {
+    return false;
+  }
+
+  // Configure localProfileDir only if it is available at this stage
+  if (!mPaths->localProfileDir.IsVoid() &&
+      !SetStringProperty(aCx, objPath, "localProfileDir",
+                         mPaths->localProfileDir)) {
+    return false;
+  }
+
+  return true;
+}
+
+NS_IMPL_ISUPPORTS(OSFileConstantsService, nsIOSFileConstantsService,
+                  nsIObserver)
+
+/* static */
+already_AddRefed<OSFileConstantsService> OSFileConstantsService::GetOrCreate() {
+  if (!gInstance) {
+    MOZ_ASSERT(NS_IsMainThread());
+
+    RefPtr<OSFileConstantsService> service = new OSFileConstantsService();
+    nsresult rv = service->InitOSFileConstants();
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return nullptr;
+    }
+
+    gInstance = std::move(service);
+    ClearOnShutdown(&gInstance);
+  }
+
+  RefPtr<OSFileConstantsService> copy = gInstance;
+  return copy.forget();
+}
+
+OSFileConstantsService::OSFileConstantsService()
+    : mInitialized(false), mUserUmask(0) {
+  MOZ_ASSERT(NS_IsMainThread());
+}
+
+OSFileConstantsService::~OSFileConstantsService() {
+  MOZ_ASSERT(NS_IsMainThread());
+}
+
+NS_IMETHODIMP
+OSFileConstantsService::Init(JSContext* aCx) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  nsresult rv = InitOSFileConstants();
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  mozJSModuleLoader* loader = mozJSModuleLoader::Get();
+  JS::Rooted<JSObject*> targetObj(aCx);
+  loader->FindTargetObject(aCx, &targetObj);
+
+  if (!DefineOSFileConstants(aCx, targetObj)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla
diff --git a/dom/system/OSFileConstants.h b/dom/system/OSFileConstants.h
new file mode 100644
index 0000000000..73b5911579
--- /dev/null
+++ b/dom/system/OSFileConstants.h
@@ -0,0 +1,53 @@
+/* -*- 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_osfileconstants_h__
+#define mozilla_osfileconstants_h__
+
+#include "nsIObserver.h"
+#include "nsIOSFileConstantsService.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+/**
+ * XPConnect initializer, for use in the main thread.
+ * This class is thread-safe but it must be first be initialized on the
+ * main-thread.
+ */
+class OSFileConstantsService final : public nsIOSFileConstantsService,
+                                     public nsIObserver {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOSFILECONSTANTSSERVICE
+  NS_DECL_NSIOBSERVER
+
+  static already_AddRefed<OSFileConstantsService> GetOrCreate();
+
+  bool DefineOSFileConstants(JSContext* aCx, JS::Handle<JSObject*> aGlobal);
+
+ private:
+  nsresult InitOSFileConstants();
+
+  OSFileConstantsService();
+  ~OSFileConstantsService();
+
+  bool mInitialized;
+
+  struct Paths;
+  UniquePtr<Paths> mPaths;
+
+  /**
+   * (Unix) the umask, which goes in OS.Constants.Sys but
+   * can only be looked up (via the system-info service)
+   * on the main thread.
+   */
+  uint32_t mUserUmask;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_osfileconstants_h__
diff --git a/dom/system/PathUtils.cpp b/dom/system/PathUtils.cpp
new file mode 100644
index 0000000000..eb11b46269
--- /dev/null
+++ b/dom/system/PathUtils.cpp
@@ -0,0 +1,657 @@
+/* -*- Mode: IDL; 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/.
+ */
+
+#include "PathUtils.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/DataMutex.h"
+#include "mozilla/ErrorNames.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Result.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Span.h"
+#include "mozilla/dom/DOMParser.h"
+#include "mozilla/dom/PathUtilsBinding.h"
+#include "mozilla/dom/Promise.h"
+#include "nsAppDirectoryServiceDefs.h"
+#include "nsCOMPtr.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIFile.h"
+#include "nsIGlobalObject.h"
+#include "nsLocalFile.h"
+#include "nsNetUtil.h"
+#include "nsString.h"
+#include "xpcpublic.h"
+
+namespace mozilla::dom {
+
+static constexpr auto ERROR_EMPTY_PATH =
+    "PathUtils does not support empty paths"_ns;
+static constexpr auto ERROR_INITIALIZE_PATH = "Could not initialize path"_ns;
+static constexpr auto ERROR_GET_PARENT = "Could not get parent path"_ns;
+static constexpr auto ERROR_JOIN = "Could not append to path"_ns;
+
+static constexpr auto COLON = ": "_ns;
+
+static void ThrowError(ErrorResult& aErr, const nsresult aResult,
+                       const nsCString& aMessage) {
+  nsAutoCStringN<32> errName;
+  GetErrorName(aResult, errName);
+
+  nsAutoCStringN<256> formattedMsg;
+  formattedMsg.Append(aMessage);
+  formattedMsg.Append(COLON);
+  formattedMsg.Append(errName);
+
+  switch (aResult) {
+    case NS_ERROR_FILE_UNRECOGNIZED_PATH:
+      aErr.ThrowOperationError(formattedMsg);
+      break;
+
+    case NS_ERROR_FILE_ACCESS_DENIED:
+      aErr.ThrowInvalidAccessError(formattedMsg);
+      break;
+
+    case NS_ERROR_FAILURE:
+    default:
+      aErr.ThrowUnknownError(formattedMsg);
+      break;
+  }
+}
+
+static bool DoWindowsPathCheck() {
+#ifdef XP_WIN
+#  ifdef DEBUG
+  return true;
+#  else   // DEBUG
+  return xpc::IsInAutomation();
+#  endif  // DEBUG
+#else     // XP_WIN
+  return false;
+#endif    // XP_WIN
+}
+
+/* static */
+nsresult PathUtils::InitFileWithPath(nsIFile* aFile, const nsAString& aPath) {
+  if (DoWindowsPathCheck()) {
+    MOZ_RELEASE_ASSERT(!aPath.Contains(u'/'),
+                       "Windows paths cannot include forward slashes");
+  }
+
+  MOZ_ASSERT(aFile);
+  return aFile->InitWithPath(aPath);
+}
+
+StaticDataMutex<Maybe<PathUtils::DirectoryCache>> PathUtils::sDirCache{
+    "sDirCache"};
+
+/**
+ * Return the leaf name, including leading path separators in the case of
+ * Windows UNC drive paths.
+ *
+ * @param aFile The file whose leaf name is to be returned.
+ * @param aResult The string to hold the resulting leaf name.
+ * @param aParent The pre-computed parent of |aFile|. If not provided, it will
+ *                be computed.
+ */
+static nsresult GetLeafNamePreservingRoot(nsIFile* aFile, nsString& aResult,
+                                          nsIFile* aParent = nullptr) {
+  MOZ_ASSERT(aFile);
+
+  nsCOMPtr<nsIFile> parent = aParent;
+  if (!parent) {
+    MOZ_TRY(aFile->GetParent(getter_AddRefs(parent)));
+  }
+
+  if (parent) {
+    return aFile->GetLeafName(aResult);
+  }
+
+  // We have reached the root path. On Windows, the leafname for a UNC path
+  // will not have the leading backslashes, so we need to use the entire path
+  // here:
+  //
+  // * for a UNIX root path (/) this will be /;
+  // * for a Windows drive path (e.g., C:), this will be the drive path (C:);
+  //   and
+  // * for a Windows UNC server path (e.g., \\\\server), this will be the full
+  //   server path (\\\\server).
+  return aFile->GetPath(aResult);
+}
+
+void PathUtils::Filename(const GlobalObject&, const nsAString& aPath,
+                         nsString& aResult, ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aPath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  if (nsresult rv = GetLeafNamePreservingRoot(path, aResult); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, "Could not get leaf name of path"_ns);
+    return;
+  }
+}
+
+void PathUtils::Parent(const GlobalObject&, const nsAString& aPath,
+                       const int32_t aDepth, nsString& aResult,
+                       ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aPath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  if (aDepth <= 0) {
+    aErr.ThrowNotSupportedError("A depth of at least 1 is required");
+    return;
+  }
+
+  nsCOMPtr<nsIFile> parent;
+  for (int32_t i = 0; path && i < aDepth; i++) {
+    if (nsresult rv = path->GetParent(getter_AddRefs(parent)); NS_FAILED(rv)) {
+      ThrowError(aErr, rv, ERROR_GET_PARENT);
+      return;
+    }
+    path = parent;
+  }
+
+  if (parent) {
+    MOZ_ALWAYS_SUCCEEDS(parent->GetPath(aResult));
+  } else {
+    aResult = VoidString();
+  }
+}
+
+void PathUtils::Join(const GlobalObject&, const Sequence<nsString>& aComponents,
+                     nsString& aResult, ErrorResult& aErr) {
+  nsCOMPtr<nsIFile> path = Join(Span(aComponents), aErr);
+  if (aErr.Failed()) {
+    return;
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(path->GetPath(aResult));
+}
+
+already_AddRefed<nsIFile> PathUtils::Join(
+    const Span<const nsString>& aComponents, ErrorResult& aErr) {
+  if (aComponents.IsEmpty() || aComponents[0].IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return nullptr;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aComponents[0]); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return nullptr;
+  }
+
+  const auto components = Span<const nsString>(aComponents).Subspan(1);
+  for (const auto& component : components) {
+    if (nsresult rv = path->Append(component); NS_FAILED(rv)) {
+      ThrowError(aErr, rv, ERROR_JOIN);
+      return nullptr;
+    }
+  }
+
+  return path.forget();
+}
+
+void PathUtils::JoinRelative(const GlobalObject&, const nsAString& aBasePath,
+                             const nsAString& aRelativePath, nsString& aResult,
+                             ErrorResult& aErr) {
+  if (aRelativePath.IsEmpty()) {
+    aResult = aBasePath;
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aBasePath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  if (nsresult rv = path->AppendRelativePath(aRelativePath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_JOIN);
+    return;
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(path->GetPath(aResult));
+}
+
+void PathUtils::ToExtendedWindowsPath(const GlobalObject&,
+                                      const nsAString& aPath, nsString& aResult,
+                                      ErrorResult& aErr) {
+#ifndef XP_WIN
+  aErr.ThrowNotAllowedError("Operation is windows specific"_ns);
+  return;
+#else
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  const nsAString& str1 = Substring(aPath, 1, 1);
+  const nsAString& str2 = Substring(aPath, 2, aPath.Length() - 2);
+
+  bool isUNC = aPath.Length() >= 2 &&
+               (aPath.First() == '\\' || aPath.First() == '/') &&
+               (str1.EqualsLiteral("\\") || str1.EqualsLiteral("/"));
+
+  constexpr auto pathPrefix = u"\\\\?\\"_ns;
+  const nsAString& uncPath = pathPrefix + u"UNC\\"_ns + str2;
+  const nsAString& normalPath = pathPrefix + aPath;
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, isUNC ? uncPath : normalPath);
+      NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(path->GetPath(aResult));
+#endif
+}
+
+void PathUtils::Normalize(const GlobalObject&, const nsAString& aPath,
+                          nsString& aResult, ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aPath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  if (nsresult rv = path->Normalize(); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, "Could not normalize path"_ns);
+    return;
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(path->GetPath(aResult));
+}
+
+void PathUtils::Split(const GlobalObject&, const nsAString& aPath,
+                      nsTArray<nsString>& aResult, ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aPath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  while (path) {
+    auto* component = aResult.EmplaceBack(fallible);
+    if (!component) {
+      aErr.Throw(NS_ERROR_OUT_OF_MEMORY);
+      return;
+    }
+
+    nsCOMPtr<nsIFile> parent;
+    if (nsresult rv = path->GetParent(getter_AddRefs(parent)); NS_FAILED(rv)) {
+      ThrowError(aErr, rv, ERROR_GET_PARENT);
+      return;
+    }
+
+    // GetLeafPreservingRoot cannot fail if we pass it a parent path.
+    MOZ_ALWAYS_SUCCEEDS(GetLeafNamePreservingRoot(path, *component, parent));
+
+    path = parent;
+  }
+
+  aResult.Reverse();
+}
+
+void PathUtils::SplitRelative(const GlobalObject& aGlobal,
+                              const nsAString& aPath,
+                              const SplitRelativeOptions& aOptions,
+                              nsTArray<nsString>& aResult, ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  if (DoWindowsPathCheck()) {
+    MOZ_RELEASE_ASSERT(!aPath.Contains(u'/'),
+                       "Windows paths cannot include forward slashes");
+  }
+
+  if (IsAbsolute(aGlobal, aPath)) {
+    aErr.ThrowNotAllowedError(
+        "PathUtils.splitRelative requires a relative path"_ns);
+    return;
+  }
+
+#ifdef XP_WIN
+  constexpr auto SEPARATOR = u'\\';
+#else
+  constexpr auto SEPARATOR = u'/';
+#endif
+
+  constexpr auto PARENT = u".."_ns;
+  constexpr auto CURRENT = u"."_ns;
+
+  for (const nsAString& pathComponent :
+       nsCharSeparatedTokenizerTemplate<NS_TokenizerIgnoreNothing>{aPath,
+                                                                   SEPARATOR}
+           .ToRange()) {
+    if (!aOptions.mAllowEmpty && pathComponent.IsEmpty()) {
+      aErr.ThrowNotAllowedError(
+          "PathUtils.splitRelative: Empty directory components (\"\") not "
+          "allowed by options");
+      return;
+    }
+
+    if (!aOptions.mAllowParentDir && pathComponent == PARENT) {
+      aErr.ThrowNotAllowedError(
+          "PathUtils.splitRelative: Parent directory components (\"..\") not "
+          "allowed by options");
+      return;
+    }
+
+    if (!aOptions.mAllowCurrentDir && pathComponent == CURRENT) {
+      aErr.ThrowNotAllowedError(
+          "PathUtils.splitRelative: Current directory components (\".\") not "
+          "allowed by options");
+      return;
+    }
+
+    aResult.AppendElement(pathComponent);
+  }
+}
+
+void PathUtils::ToFileURI(const GlobalObject&, const nsAString& aPath,
+                          nsCString& aResult, ErrorResult& aErr) {
+  if (aPath.IsEmpty()) {
+    aErr.ThrowNotAllowedError(ERROR_EMPTY_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  if (nsresult rv = InitFileWithPath(path, aPath); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, ERROR_INITIALIZE_PATH);
+    return;
+  }
+
+  nsCOMPtr<nsIURI> uri;
+  if (nsresult rv = NS_NewFileURI(getter_AddRefs(uri), path); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, "Could not initialize File URI"_ns);
+    return;
+  }
+
+  if (nsresult rv = uri->GetSpec(aResult); NS_FAILED(rv)) {
+    ThrowError(aErr, rv, "Could not retrieve URI spec"_ns);
+    return;
+  }
+}
+
+bool PathUtils::IsAbsolute(const GlobalObject&, const nsAString& aPath) {
+  nsCOMPtr<nsIFile> path = new nsLocalFile();
+  nsresult rv = InitFileWithPath(path, aPath);
+  return NS_SUCCEEDED(rv);
+}
+
+void PathUtils::GetProfileDirSync(const GlobalObject&, nsString& aResult,
+                                  ErrorResult& aErr) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  DirectoryCache::Ensure(guard.ref())
+      .GetDirectorySync(aResult, aErr, DirectoryCache::Directory::Profile);
+}
+void PathUtils::GetLocalProfileDirSync(const GlobalObject&, nsString& aResult,
+                                       ErrorResult& aErr) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  DirectoryCache::Ensure(guard.ref())
+      .GetDirectorySync(aResult, aErr, DirectoryCache::Directory::LocalProfile);
+}
+void PathUtils::GetTempDirSync(const GlobalObject&, nsString& aResult,
+                               ErrorResult& aErr) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  DirectoryCache::Ensure(guard.ref())
+      .GetDirectorySync(aResult, aErr, DirectoryCache::Directory::Temp);
+}
+
+void PathUtils::GetOSTempDirSync(const GlobalObject&, nsString& aResult,
+                                 ErrorResult& aErr) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  DirectoryCache::Ensure(guard.ref())
+      .GetDirectorySync(aResult, aErr, DirectoryCache::Directory::OSTemp);
+}
+
+void PathUtils::GetXulLibraryPathSync(const GlobalObject&, nsString& aResult,
+                                      ErrorResult& aErr) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  DirectoryCache::Ensure(guard.ref())
+      .GetDirectorySync(aResult, aErr, DirectoryCache::Directory::XulLibrary);
+}
+
+already_AddRefed<Promise> PathUtils::GetProfileDirAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  return DirectoryCache::Ensure(guard.ref())
+      .GetDirectoryAsync(aGlobal, aErr, DirectoryCache::Directory::Profile);
+}
+
+already_AddRefed<Promise> PathUtils::GetLocalProfileDirAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  return DirectoryCache::Ensure(guard.ref())
+      .GetDirectoryAsync(aGlobal, aErr,
+                         DirectoryCache::Directory::LocalProfile);
+}
+
+already_AddRefed<Promise> PathUtils::GetTempDirAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  return DirectoryCache::Ensure(guard.ref())
+      .GetDirectoryAsync(aGlobal, aErr, DirectoryCache::Directory::Temp);
+}
+
+already_AddRefed<Promise> PathUtils::GetOSTempDirAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  return DirectoryCache::Ensure(guard.ref())
+      .GetDirectoryAsync(aGlobal, aErr, DirectoryCache::Directory::OSTemp);
+}
+
+already_AddRefed<Promise> PathUtils::GetXulLibraryPathAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr) {
+  MOZ_ASSERT(!NS_IsMainThread());
+
+  auto guard = sDirCache.Lock();
+  return DirectoryCache::Ensure(guard.ref())
+      .GetDirectoryAsync(aGlobal, aErr, DirectoryCache::Directory::XulLibrary);
+}
+
+PathUtils::DirectoryCache::DirectoryCache() {
+  for (auto& dir : mDirectories) {
+    dir.SetIsVoid(true);
+  }
+}
+
+PathUtils::DirectoryCache& PathUtils::DirectoryCache::Ensure(
+    Maybe<PathUtils::DirectoryCache>& aCache) {
+  if (aCache.isNothing()) {
+    aCache.emplace();
+
+    auto clearAtShutdown = []() {
+      RunOnShutdown([]() {
+        auto cache = PathUtils::sDirCache.Lock();
+        cache->reset();
+      });
+    };
+
+    if (NS_IsMainThread()) {
+      clearAtShutdown();
+    } else {
+      NS_DispatchToMainThread(
+          NS_NewRunnableFunction(__func__, std::move(clearAtShutdown)));
+    }
+  }
+
+  return aCache.ref();
+}
+
+void PathUtils::DirectoryCache::GetDirectorySync(
+    nsString& aResult, ErrorResult& aErr, const Directory aRequestedDir) {
+  MOZ_RELEASE_ASSERT(aRequestedDir < Directory::Count);
+
+  if (nsresult rv = PopulateDirectoriesImpl(aRequestedDir); NS_FAILED(rv)) {
+    nsAutoCStringN<32> errorName;
+    GetErrorName(rv, errorName);
+
+    nsAutoCStringN<256> msg;
+    msg.Append("Could not retrieve directory "_ns);
+    msg.Append(kDirectoryNames[aRequestedDir]);
+    msg.Append(COLON);
+    msg.Append(errorName);
+
+    aErr.ThrowUnknownError(msg);
+    return;
+  }
+
+  aResult = mDirectories[aRequestedDir];
+}
+
+already_AddRefed<Promise> PathUtils::DirectoryCache::GetDirectoryAsync(
+    const GlobalObject& aGlobal, ErrorResult& aErr,
+    const Directory aRequestedDir) {
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<Promise> promise = Promise::Create(global, aErr);
+  if (aErr.Failed()) {
+    return nullptr;
+  }
+
+  if (RefPtr<PopulateDirectoriesPromise> p =
+          PopulateDirectories(aRequestedDir)) {
+    p->Then(
+        GetCurrentSerialEventTarget(), __func__,
+        [promise, aRequestedDir](const Ok&) {
+          auto cache = PathUtils::sDirCache.Lock();
+          cache.ref()->ResolveWithDirectory(promise, aRequestedDir);
+        },
+        [promise](const nsresult& aRv) { promise->MaybeReject(aRv); });
+  } else {
+    ResolveWithDirectory(promise, aRequestedDir);
+  }
+
+  return promise.forget();
+}
+
+void PathUtils::DirectoryCache::ResolveWithDirectory(
+    Promise* aPromise, const Directory aRequestedDir) {
+  MOZ_RELEASE_ASSERT(aRequestedDir < Directory::Count);
+  MOZ_RELEASE_ASSERT(!mDirectories[aRequestedDir].IsVoid());
+  aPromise->MaybeResolve(mDirectories[aRequestedDir]);
+}
+
+already_AddRefed<PathUtils::DirectoryCache::PopulateDirectoriesPromise>
+PathUtils::DirectoryCache::PopulateDirectories(
+    const PathUtils::DirectoryCache::Directory aRequestedDir) {
+  MOZ_RELEASE_ASSERT(aRequestedDir < Directory::Count);
+
+  // If we have already resolved the requested directory, we can return
+  // immediately.
+  // Otherwise, if we have already fired off a request to populate the entry,
+  // so we can return the corresponding promise immediately. caller will queue
+  // a Thenable onto that promise to resolve/reject the request.
+  if (!mDirectories[aRequestedDir].IsVoid()) {
+    return nullptr;
+  }
+  if (!mPromises[aRequestedDir].IsEmpty()) {
+    return mPromises[aRequestedDir].Ensure(__func__);
+  }
+
+  RefPtr<PopulateDirectoriesPromise> promise =
+      mPromises[aRequestedDir].Ensure(__func__);
+
+  if (NS_IsMainThread()) {
+    nsresult rv = PopulateDirectoriesImpl(aRequestedDir);
+    ResolvePopulateDirectoriesPromise(rv, aRequestedDir);
+  } else {
+    nsCOMPtr<nsIRunnable> runnable =
+        NS_NewRunnableFunction(__func__, [aRequestedDir]() {
+          auto cache = PathUtils::sDirCache.Lock();
+          nsresult rv = cache.ref()->PopulateDirectoriesImpl(aRequestedDir);
+          cache.ref()->ResolvePopulateDirectoriesPromise(rv, aRequestedDir);
+        });
+    NS_DispatchToMainThread(runnable.forget());
+  }
+
+  return promise.forget();
+}
+
+void PathUtils::DirectoryCache::ResolvePopulateDirectoriesPromise(
+    nsresult aRv, const PathUtils::DirectoryCache::Directory aRequestedDir) {
+  MOZ_RELEASE_ASSERT(aRequestedDir < Directory::Count);
+
+  if (NS_SUCCEEDED(aRv)) {
+    mPromises[aRequestedDir].Resolve(Ok{}, __func__);
+  } else {
+    mPromises[aRequestedDir].Reject(aRv, __func__);
+  }
+}
+
+nsresult PathUtils::DirectoryCache::PopulateDirectoriesImpl(
+    const PathUtils::DirectoryCache::Directory aRequestedDir) {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+  MOZ_RELEASE_ASSERT(aRequestedDir < Directory::Count);
+
+  if (!mDirectories[aRequestedDir].IsVoid()) {
+    // In between when this promise was dispatched to the main thread and now,
+    // the directory cache has had this entry populated (via the
+    // on-main-thread sync method).
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIFile> path;
+
+  MOZ_TRY(NS_GetSpecialDirectory(kDirectoryNames[aRequestedDir],
+                                 getter_AddRefs(path)));
+  MOZ_TRY(path->GetPath(mDirectories[aRequestedDir]));
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/system/PathUtils.h b/dom/system/PathUtils.h
new file mode 100644
index 0000000000..cbfceef269
--- /dev/null
+++ b/dom/system/PathUtils.h
@@ -0,0 +1,267 @@
+/* -*- 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_dom_PathUtils__
+#define mozilla_dom_PathUtils__
+
+#include "mozilla/DataMutex.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Result.h"
+#include "mozilla/dom/PathUtilsBinding.h"
+#include "mozilla/dom/Promise.h"
+#include "nsAppDirectoryServiceDefs.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsString.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+class ErrorResult;
+
+namespace dom {
+
+class PathUtils final {
+ public:
+  /**
+   * Initialize the given nsIFile with the given path.
+   *
+   * This is equivalent to calling nsIFile::InitWithPath() with the caveat that
+   * on Windows debug or during Windows CI tests, we will crash if the path
+   * contains a forward slash.
+   *
+   * @param aFile The file to initialize.
+   * @param aPath The path to initialize the file with.
+   *
+   * @return The result of calling nsIFile::InitWithPath.
+   */
+  static nsresult InitFileWithPath(nsIFile* aFile, const nsAString& aPath);
+
+  static void Filename(const GlobalObject&, const nsAString& aPath,
+                       nsString& aResult, ErrorResult& aErr);
+
+  static void Parent(const GlobalObject&, const nsAString& aPath,
+                     const int32_t aDepth, nsString& aResult,
+                     ErrorResult& aErr);
+
+  static void Join(const GlobalObject&, const Sequence<nsString>& aComponents,
+                   nsString& aResult, ErrorResult& aErr);
+
+  /**
+   * Join a sequence of path components and return an nsIFile with the resulting
+   * path.
+   *
+   * @param aComponents  A sequence of path components. The first component must
+   *                     be an absolute path.
+   * @param aErr  The error result, if any.
+   *
+   * @return An nsIFile with the resulting path, if there were no errors.
+   * Otherwise, nullptr is returned.
+   */
+  static already_AddRefed<nsIFile> Join(const Span<const nsString>& aComponents,
+                                        ErrorResult& aErr);
+
+  static void JoinRelative(const GlobalObject&, const nsAString& aBasePath,
+                           const nsAString& aRelativePath, nsString& aResult,
+                           ErrorResult& aErr);
+
+  static void ToExtendedWindowsPath(const GlobalObject&, const nsAString& aPath,
+                                    nsString& aResult, ErrorResult& aErr);
+
+  static void Normalize(const GlobalObject&, const nsAString& aPath,
+                        nsString& aResult, ErrorResult& aErr);
+
+  static void Split(const GlobalObject&, const nsAString& aPath,
+                    nsTArray<nsString>& aResult, ErrorResult& aErr);
+
+  static void SplitRelative(const GlobalObject& aGlobal, const nsAString& aPath,
+                            const SplitRelativeOptions& aOptions,
+                            nsTArray<nsString>& aResult, ErrorResult& aErr);
+
+  static void ToFileURI(const GlobalObject&, const nsAString& aPath,
+                        nsCString& aResult, ErrorResult& aErr);
+
+  static bool IsAbsolute(const GlobalObject&, const nsAString& aPath);
+
+  static void GetProfileDirSync(const GlobalObject&, nsString& aResult,
+                                ErrorResult& aErr);
+  static void GetLocalProfileDirSync(const GlobalObject&, nsString& aResult,
+                                     ErrorResult& aErr);
+  static void GetTempDirSync(const GlobalObject&, nsString& aResult,
+                             ErrorResult& aErr);
+  static void GetOSTempDirSync(const GlobalObject&, nsString& aResult,
+                               ErrorResult& aErr);
+  static void GetXulLibraryPathSync(const GlobalObject&, nsString& aResult,
+                                    ErrorResult& aErr);
+
+  static already_AddRefed<Promise> GetProfileDirAsync(
+      const GlobalObject& aGlobal, ErrorResult& aErr);
+  static already_AddRefed<Promise> GetLocalProfileDirAsync(
+      const GlobalObject& aGlobal, ErrorResult& aErr);
+  static already_AddRefed<Promise> GetTempDirAsync(const GlobalObject& aGlobal,
+                                                   ErrorResult& aErr);
+  static already_AddRefed<Promise> GetOSTempDirAsync(
+      const GlobalObject& aGlobal, ErrorResult& aErr);
+  static already_AddRefed<Promise> GetXulLibraryPathAsync(
+      const GlobalObject& aGlobal, ErrorResult& aErr);
+
+ private:
+  class DirectoryCache;
+  friend class DirectoryCache;
+
+  static StaticDataMutex<Maybe<DirectoryCache>> sDirCache;
+};
+
+/**
+ * A cache of commonly used directories
+ */
+class PathUtils::DirectoryCache final {
+ public:
+  /**
+   * A directory that can be requested via |GetDirectorySync| or
+   * |GetDirectoryAsync|.
+   */
+  enum class Directory {
+    /**
+     * The user's profile directory.
+     */
+    Profile,
+    /**
+     * The user's local profile directory.
+     */
+    LocalProfile,
+    /**
+     * The temporary directory for the process.
+     */
+    Temp,
+    /**
+     * The OS temporary directory.
+     */
+    OSTemp,
+    /**
+     * The libxul path.
+     */
+    XulLibrary,
+    /**
+     * The number of Directory entries.
+     */
+    Count,
+  };
+
+  DirectoryCache();
+  DirectoryCache(const DirectoryCache&) = delete;
+  DirectoryCache(DirectoryCache&&) = delete;
+  DirectoryCache& operator=(const DirectoryCache&) = delete;
+  DirectoryCache& operator=(DirectoryCache&&) = delete;
+
+  /**
+   * Ensure the cache is instantiated and schedule its destructor to run at
+   * shutdown.
+   *
+   * If the cache is already instantiated, this is a no-op.
+   *
+   * @param aCache The cache to ensure is instantiated.
+   */
+  static DirectoryCache& Ensure(Maybe<DirectoryCache>& aCache);
+
+  void GetDirectorySync(nsString& aResult, ErrorResult& aErr,
+                        const Directory aRequestedDir);
+
+  /**
+   * Request the path of a specific directory.
+   *
+   * If the directory has not been requested before, this may require a trip to
+   * the main thread to retrieve its path.
+   *
+   * @param aGlobalObject The JavaScript global.
+   * @param aErr The error result.
+   * @param aRequestedDir The directory for which the path is to be retrieved.
+   *
+   * @return A promise that resolves to the path of the requested directory.
+   */
+  already_AddRefed<Promise> GetDirectoryAsync(const GlobalObject& aGlobalObject,
+                                              ErrorResult& aErr,
+                                              const Directory aRequestedDir);
+
+ private:
+  using PopulateDirectoriesPromise = MozPromise<Ok, nsresult, false>;
+
+  /**
+   * Populate the directory cache entry for the requested directory.
+   *
+   * @param aRequestedDir The directory cache entry that was requested via
+   *                      |GetDirectory|.
+   *
+   * @return If the requested directory has not been populated, this returns a
+   *         promise that resolves when the population is complete.
+   *
+   *         If the requested directory has already been populated, it returns
+   *         nullptr instead.
+   */
+  already_AddRefed<PopulateDirectoriesPromise> PopulateDirectories(
+      const Directory aRequestedDir);
+
+  /**
+   * Initialize the requested directory cache entry.
+   *
+   * If |Directory::Temp| is requested, all cache entries will be populated.
+   * Otherwise, only the profile and local profile cache entries will be
+   * populated. The profile and local profile cache entries have no additional
+   * overhead for populating them, but the temp directory requires creating a
+   * directory on the main thread if it has not already happened.
+   *
+   * Must be called on the main thread.
+   *
+   * @param aRequestedDir The requested directory.
+   *
+   * @return The result of initializing directories.
+   */
+  nsresult PopulateDirectoriesImpl(const Directory aRequestedDir);
+
+  /**
+   * Resolve the internal PopulateDirectoriesPromise corresponding to
+   * |aRequestedDir| with the given result.
+   *
+   * This will allow all pending queries for the requested directory to resolve
+   * or be rejected.
+   *
+   * @param aRv The return value from PopulateDirectoriesImpl.
+   * @param aRequestedDir The requested directory cache entry. This is used to
+   *                      determine which internal MozPromiseHolder we are
+   * resolving.
+   */
+  void ResolvePopulateDirectoriesPromise(nsresult aRv,
+                                         const Directory aRequestedDir);
+
+  /**
+   * Resolve the given JS promise with the path of the requested directory
+   *
+   * Can only be called once the cache entry for the requested directory is
+   * populated.
+   *
+   * @param aPromise The JS promise to resolve.
+   * @param aRequestedDir The requested directory cache entry.
+   */
+  void ResolveWithDirectory(Promise* aPromise, const Directory aRequestedDir);
+
+  template <typename T>
+  using DirectoryArray = EnumeratedArray<Directory, Directory::Count, T>;
+
+  DirectoryArray<nsString> mDirectories;
+  DirectoryArray<MozPromiseHolder<PopulateDirectoriesPromise>> mPromises;
+
+  static constexpr DirectoryArray<const char*> kDirectoryNames{
+      NS_APP_USER_PROFILE_50_DIR,      NS_APP_USER_PROFILE_LOCAL_50_DIR,
+      NS_APP_CONTENT_PROCESS_TEMP_DIR, NS_OS_TEMP_DIR,
+      NS_XPCOM_LIBRARY_FILE,
+  };
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif
diff --git a/dom/system/android/AndroidLocationProvider.cpp b/dom/system/android/AndroidLocationProvider.cpp
new file mode 100644
index 0000000000..8861c30298
--- /dev/null
+++ b/dom/system/android/AndroidLocationProvider.cpp
@@ -0,0 +1,52 @@
+/* -*- 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 "Geolocation.h"
+#include "GeolocationPosition.h"
+#include "AndroidLocationProvider.h"
+#include "mozilla/java/GeckoAppShellWrappers.h"
+
+using namespace mozilla;
+
+extern nsIGeolocationUpdate* gLocationCallback;
+
+NS_IMPL_ISUPPORTS(AndroidLocationProvider, nsIGeolocationProvider)
+
+AndroidLocationProvider::AndroidLocationProvider() {}
+
+AndroidLocationProvider::~AndroidLocationProvider() {
+  NS_IF_RELEASE(gLocationCallback);
+}
+
+NS_IMETHODIMP
+AndroidLocationProvider::Startup() {
+  if (java::GeckoAppShell::EnableLocationUpdates(true)) {
+    return NS_OK;
+  }
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+AndroidLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  NS_IF_RELEASE(gLocationCallback);
+  gLocationCallback = aCallback;
+  NS_IF_ADDREF(gLocationCallback);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+AndroidLocationProvider::Shutdown() {
+  if (java::GeckoAppShell::EnableLocationUpdates(false)) {
+    return NS_OK;
+  }
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+AndroidLocationProvider::SetHighAccuracy(bool enable) {
+  java::GeckoAppShell::EnableLocationHighAccuracy(enable);
+  return NS_OK;
+}
diff --git a/dom/system/android/AndroidLocationProvider.h b/dom/system/android/AndroidLocationProvider.h
new file mode 100644
index 0000000000..e0d38f6c8f
--- /dev/null
+++ b/dom/system/android/AndroidLocationProvider.h
@@ -0,0 +1,23 @@
+/* -*- 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 AndroidLocationProvider_h
+#define AndroidLocationProvider_h
+
+#include "nsIGeolocationProvider.h"
+
+class AndroidLocationProvider final : public nsIGeolocationProvider {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  AndroidLocationProvider();
+
+ private:
+  ~AndroidLocationProvider();
+};
+
+#endif /* AndroidLocationProvider_h */
diff --git a/dom/system/android/moz.build b/dom/system/android/moz.build
new file mode 100644
index 0000000000..04dffba024
--- /dev/null
+++ b/dom/system/android/moz.build
@@ -0,0 +1,17 @@
+# -*- 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/.
+
+SOURCES += [
+    "AndroidLocationProvider.cpp",
+    "nsHapticFeedback.cpp",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+LOCAL_INCLUDES += [
+    "/dom/geolocation",
+]
diff --git a/dom/system/android/nsHapticFeedback.cpp b/dom/system/android/nsHapticFeedback.cpp
new file mode 100644
index 0000000000..87c77d8334
--- /dev/null
+++ b/dom/system/android/nsHapticFeedback.cpp
@@ -0,0 +1,18 @@
+/* -*- 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 "nsHapticFeedback.h"
+#include "mozilla/java/GeckoAppShellWrappers.h"
+
+using namespace mozilla;
+
+NS_IMPL_ISUPPORTS(nsHapticFeedback, nsIHapticFeedback)
+
+NS_IMETHODIMP
+nsHapticFeedback::PerformSimpleAction(int32_t aType) {
+  java::GeckoAppShell::PerformHapticFeedback(aType == LongPress);
+  return NS_OK;
+}
diff --git a/dom/system/android/nsHapticFeedback.h b/dom/system/android/nsHapticFeedback.h
new file mode 100644
index 0000000000..e55062058b
--- /dev/null
+++ b/dom/system/android/nsHapticFeedback.h
@@ -0,0 +1,16 @@
+/* -*- 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 "nsIHapticFeedback.h"
+
+class nsHapticFeedback final : public nsIHapticFeedback {
+ private:
+  ~nsHapticFeedback() {}
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIHAPTICFEEDBACK
+};
diff --git a/dom/system/components.conf b/dom/system/components.conf
new file mode 100644
index 0000000000..3523c02147
--- /dev/null
+++ b/dom/system/components.conf
@@ -0,0 +1,17 @@
+# -*- 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/.
+
+Classes = [
+    {
+        'cid': '{77DA64D3-7458-4920-9491-86CC9914F904}',
+        'contract_ids': [
+            '@mozilla.org/geolocation/provider;1',
+            '@mozilla.org/geolocation/mls-provider;1',
+        ],
+        'jsm': 'resource://gre/modules/NetworkGeolocationProvider.jsm',
+        'constructor': 'NetworkGeolocationProvider',
+    },
+]
diff --git a/dom/system/linux/GeoclueLocationProvider.cpp b/dom/system/linux/GeoclueLocationProvider.cpp
new file mode 100644
index 0000000000..7e5006141f
--- /dev/null
+++ b/dom/system/linux/GeoclueLocationProvider.cpp
@@ -0,0 +1,1060 @@
+/*
+ * 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/.
+ *
+ * Author: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
+ */
+
+#include "GeoclueLocationProvider.h"
+
+#include <gio/gio.h>
+#include <glib.h>
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/GRefPtr.h"
+#include "mozilla/GUniquePtr.h"
+#include "mozilla/Logging.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/StaticPrefs_geo.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/WeakPtr.h"
+#include "mozilla/XREAppData.h"
+#include "mozilla/dom/GeolocationPosition.h"
+#include "mozilla/dom/GeolocationPositionErrorBinding.h"
+#include "MLSFallback.h"
+#include "nsAppRunner.h"
+#include "nsCOMPtr.h"
+#include "nsIDOMGeoPosition.h"
+#include "nsINamed.h"
+#include "nsITimer.h"
+#include "nsStringFwd.h"
+#include "prtime.h"
+
+namespace mozilla::dom {
+
+static LazyLogModule gGCLocationLog("GeoclueLocation");
+
+#define GCL_LOG(level, ...) \
+  MOZ_LOG(gGCLocationLog, mozilla::LogLevel::level, (__VA_ARGS__))
+
+static const char* const kGeoclueBusName = "org.freedesktop.GeoClue2";
+static const char* const kGCManagerPath = "/org/freedesktop/GeoClue2/Manager";
+static const char* const kGCManagerInterface =
+    "org.freedesktop.GeoClue2.Manager";
+static const char* const kGCClientInterface = "org.freedesktop.GeoClue2.Client";
+static const char* const kGCLocationInterface =
+    "org.freedesktop.GeoClue2.Location";
+static const char* const kDBPropertySetMethod =
+    "org.freedesktop.DBus.Properties.Set";
+
+/*
+ * Minimum altitude reported as valid (in meters),
+ * https://en.wikipedia.org/wiki/List_of_places_on_land_with_elevations_below_sea_level
+ * says that lowest land in the world is at -430 m, so let's use -500 m here.
+ */
+static const double kGCMinAlt = -500;
+
+/*
+ * Matches "enum GClueAccuracyLevel" values, see:
+ * https://www.freedesktop.org/software/geoclue/docs/geoclue-gclue-enums.html#GClueAccuracyLevel
+ */
+enum class GCAccuracyLevel {
+  None = 0,
+  Country = 1,
+  City = 4,
+  Neighborhood = 5,
+  Street = 6,
+  Exact = 8,
+};
+
+/*
+ * Whether to reuse D-Bus proxies between uses of this provider.
+ * Usually a good thing, can be disabled for debug purposes.
+ */
+static const bool kGCReuseDBusProxy = true;
+
+class GCLocProviderPriv final : public nsIGeolocationProvider,
+                                public SupportsWeakPtr {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  GCLocProviderPriv();
+
+  void UpdateLastPosition();
+
+ private:
+  class LocationTimerCallback final : public nsITimerCallback, public nsINamed {
+   public:
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSITIMERCALLBACK
+
+    explicit LocationTimerCallback(GCLocProviderPriv* aParent)
+        : mParent(aParent) {}
+
+    NS_IMETHOD GetName(nsACString& aName) override {
+      aName.AssignLiteral("GCLocProvider::LocationTimerCallback");
+      return NS_OK;
+    }
+
+   private:
+    ~LocationTimerCallback() = default;
+    WeakPtr<GCLocProviderPriv> mParent;
+  };
+
+  enum class Accuracy { Unset, Low, High };
+  // States:
+  //   Uninit: The default / initial state, with no client proxy yet.
+  //   Initing: Takes care of establishing the client connection (GetClient /
+  //            ConnectClient / SetDesktopID).
+  //   SettingAccuracy: Does SetAccuracy operation, knows it should just go idle
+  //                    after finishing it.
+  //   SettingAccuracyForStart: Does SetAccuracy operation, knows it then needs
+  //                            to do a Start operation after finishing it.
+  //   Idle: Fully initialized, but not running state (quiescent).
+  //   Starting: Starts the client by calling the Start D-Bus method.
+  //   Started: Normal running state.
+  //   Stopping: Stops the client by calling the Stop D-Bus method, knows it
+  //             should just go idle after finishing it.
+  //   StoppingForRestart: Stops the client by calling the Stop D-Bus method as
+  //                       a part of a Stop -> Start sequence (with possibly
+  //                       an accuracy update between these method calls).
+  //
+  // Valid state transitions are:
+  //  (any state) -> Uninit: Transition when a D-Bus call failed or
+  //                         provided invalid data.
+  //
+  //   Watch() startup path:
+  //   Uninit -> Initing: Transition after getting the very first Watch()
+  //   request
+  //                      or any such request while not having the client proxy.
+  //   Initing -> SettingAccuracyForStart: Transition after getting a successful
+  //                                       SetDesktopID response.
+  //   SettingAccuracyForStart -> Starting: Transition after getting a
+  //   successful
+  //                                        SetAccuracy response.
+  //   Idle -> Starting: Transition after getting a Watch() request while in
+  //   fully
+  //                     initialized, but not running state.
+  //   SettingAccuracy -> SettingAccuracyForStart: Transition after getting a
+  //   Watch()
+  //                                               request in the middle of
+  //                                               setting accuracy during idle
+  //                                               status.
+  //   Stopping -> StoppingForRestart: Transition after getting a Watch()
+  //   request
+  //                                   in the middle of doing a Stop D-Bus call
+  //                                   for idle status.
+  //   StoppingForRestart -> Starting: Transition after getting a successful
+  //                                   Stop response as a part of a Stop ->
+  //                                   Start sequence while the previously set
+  //                                   accuracy is still correct.
+  //   StoppingForRestart -> SettingAccuracyForStart: Transition after getting
+  //                                                  a successful Stop response
+  //                                                  as a part of a Stop ->
+  //                                                  Start sequence but the set
+  //                                                  accuracy needs updating.
+  //   Starting -> Started: Transition after getting a successful Start
+  //   response.
+  //
+  //   Shutdown() path:
+  //   (any state) -> Uninit: Transition when not reusing the client proxy for
+  //                          any reason.
+  //   Started -> Stopping: Transition from normal running state when reusing
+  //                        the client proxy.
+  //   SettingAccuracyForStart -> SettingAccuracy: Transition when doing
+  //                                               a shutdown in the middle of
+  //                                               setting accuracy for a start
+  //                                               when reusing the client
+  //                                               proxy.
+  //   SettingAccuracy -> Idle: Transition after getting a successful
+  //   SetAccuracy
+  //                            response.
+  //   StoppingForRestart -> Stopping: Transition when doing shutdown
+  //                                   in the middle of a Stop -> Start sequence
+  //                                   when reusing the client proxy.
+  //   Stopping -> Idle: Transition after getting a successful Stop response.
+  //
+  //   SetHighAccuracy() path:
+  //   Started -> StoppingForRestart: Transition when accuracy needs updating
+  //                                  on a running client.
+  //   (the rest of the flow in StoppingForRestart state is the same as when
+  //    being in this state in the Watch() startup path)
+  enum class ClientState {
+    Uninit,
+    Initing,
+    SettingAccuracy,
+    SettingAccuracyForStart,
+    Idle,
+    Starting,
+    Started,
+    Stopping,
+    StoppingForRestart
+  };
+
+  ~GCLocProviderPriv();
+
+  static bool AlwaysHighAccuracy();
+
+  void SetState(ClientState aNewState, const char* aNewStateStr);
+
+  void Update(nsIDOMGeoPosition* aPosition);
+  MOZ_CAN_RUN_SCRIPT void NotifyError(int aError);
+  MOZ_CAN_RUN_SCRIPT void DBusProxyError(const GError* aGError,
+                                         bool aResetManager = false);
+
+  MOZ_CAN_RUN_SCRIPT static void GetClientResponse(GDBusProxy* aProxy,
+                                                   GAsyncResult* aResult,
+                                                   gpointer aUserData);
+  void ConnectClient(const gchar* aClientPath);
+  MOZ_CAN_RUN_SCRIPT static void ConnectClientResponse(GObject* aObject,
+                                                       GAsyncResult* aResult,
+                                                       gpointer aUserData);
+  void SetDesktopID();
+  MOZ_CAN_RUN_SCRIPT static void SetDesktopIDResponse(GDBusProxy* aProxy,
+                                                      GAsyncResult* aResult,
+                                                      gpointer aUserData);
+  void SetAccuracy();
+  MOZ_CAN_RUN_SCRIPT static void SetAccuracyResponse(GDBusProxy* aProxy,
+                                                     GAsyncResult* aResult,
+                                                     gpointer aUserData);
+  void StartClient();
+  MOZ_CAN_RUN_SCRIPT static void StartClientResponse(GDBusProxy* aProxy,
+                                                     GAsyncResult* aResult,
+                                                     gpointer aUserData);
+  void StopClient(bool aForRestart);
+  MOZ_CAN_RUN_SCRIPT static void StopClientResponse(GDBusProxy* aProxy,
+                                                    GAsyncResult* aResult,
+                                                    gpointer aUserData);
+  void StopClientNoWait();
+  void MaybeRestartForAccuracy();
+
+  MOZ_CAN_RUN_SCRIPT static void GCManagerOwnerNotify(GObject* aObject,
+                                                      GParamSpec* aPSpec,
+                                                      gpointer aUserData);
+
+  static void GCClientSignal(GDBusProxy* aProxy, gchar* aSenderName,
+                             gchar* aSignalName, GVariant* aParameters,
+                             gpointer aUserData);
+  void ConnectLocation(const gchar* aLocationPath);
+  static bool GetLocationProperty(GDBusProxy* aProxyLocation,
+                                  const gchar* aName, double* aOut);
+  static void ConnectLocationResponse(GObject* aObject, GAsyncResult* aResult,
+                                      gpointer aUserData);
+
+  void SetLocationTimer();
+  void StopLocationTimer();
+
+  bool InDBusCall();
+  bool InDBusStoppingCall();
+  bool InDBusStoppedCall();
+
+  void DeleteManager();
+  void DoShutdown(bool aDeleteClient, bool aDeleteManager);
+  void DoShutdownClearCallback(bool aDestroying);
+
+  nsresult FallbackToMLS();
+  void StopMLSFallback();
+
+  void WatchStart();
+
+  Accuracy mAccuracyWanted = Accuracy::Unset;
+  Accuracy mAccuracySet = Accuracy::Unset;
+  RefPtr<GDBusProxy> mProxyManager;
+  RefPtr<GDBusProxy> mProxyClient;
+  RefPtr<GCancellable> mCancellable;
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+  ClientState mClientState = ClientState::Uninit;
+  RefPtr<nsIDOMGeoPosition> mLastPosition;
+  RefPtr<nsITimer> mLocationTimer;
+  RefPtr<MLSFallback> mMLSFallback;
+};
+
+//
+// GCLocProviderPriv
+//
+
+#define GCLP_SETSTATE(this, state) this->SetState(ClientState::state, #state)
+
+GCLocProviderPriv::GCLocProviderPriv() {
+  if (AlwaysHighAccuracy()) {
+    mAccuracyWanted = Accuracy::High;
+  } else {
+    mAccuracyWanted = Accuracy::Low;
+  }
+}
+
+GCLocProviderPriv::~GCLocProviderPriv() { DoShutdownClearCallback(true); }
+
+bool GCLocProviderPriv::AlwaysHighAccuracy() {
+  return StaticPrefs::geo_provider_geoclue_always_high_accuracy();
+}
+
+void GCLocProviderPriv::SetState(ClientState aNewState,
+                                 const char* aNewStateStr) {
+  if (mClientState == aNewState) {
+    return;
+  }
+
+  GCL_LOG(Debug, "changing state to %s", aNewStateStr);
+  mClientState = aNewState;
+}
+
+void GCLocProviderPriv::Update(nsIDOMGeoPosition* aPosition) {
+  if (!mCallback) {
+    return;
+  }
+
+  mCallback->Update(aPosition);
+}
+
+void GCLocProviderPriv::UpdateLastPosition() {
+  MOZ_DIAGNOSTIC_ASSERT(mLastPosition, "No last position to update");
+  StopLocationTimer();
+  Update(mLastPosition);
+}
+
+nsresult GCLocProviderPriv::FallbackToMLS() {
+  GCL_LOG(Debug, "trying to fall back to MLS");
+  StopMLSFallback();
+
+  RefPtr fallback = new MLSFallback(0);
+  MOZ_TRY(fallback->Startup(mCallback));
+
+  GCL_LOG(Debug, "Started up MLS fallback");
+  mMLSFallback = std::move(fallback);
+  return NS_OK;
+}
+
+void GCLocProviderPriv::StopMLSFallback() {
+  if (!mMLSFallback) {
+    return;
+  }
+  GCL_LOG(Debug, "Clearing MLS fallback");
+  if (mMLSFallback) {
+    mMLSFallback->Shutdown();
+    mMLSFallback = nullptr;
+  }
+}
+
+void GCLocProviderPriv::NotifyError(int aError) {
+  if (!mCallback) {
+    return;
+  }
+
+  // We errored out, try to fall back to MLS.
+  if (NS_SUCCEEDED(FallbackToMLS())) {
+    return;
+  }
+
+  nsCOMPtr callback = mCallback;
+  callback->NotifyError(aError);
+}
+
+void GCLocProviderPriv::DBusProxyError(const GError* aGError,
+                                       bool aResetManager) {
+  // that G_DBUS_ERROR below is actually a function call, not a constant
+  GQuark gdbusDomain = G_DBUS_ERROR;
+  int error = GeolocationPositionError_Binding::POSITION_UNAVAILABLE;
+  if (aGError) {
+    if (g_error_matches(aGError, gdbusDomain, G_DBUS_ERROR_TIMEOUT) ||
+        g_error_matches(aGError, gdbusDomain, G_DBUS_ERROR_TIMED_OUT)) {
+      error = GeolocationPositionError_Binding::TIMEOUT;
+    } else if (g_error_matches(aGError, gdbusDomain,
+                               G_DBUS_ERROR_LIMITS_EXCEEDED) ||
+               g_error_matches(aGError, gdbusDomain,
+                               G_DBUS_ERROR_ACCESS_DENIED) ||
+               g_error_matches(aGError, gdbusDomain,
+                               G_DBUS_ERROR_AUTH_FAILED)) {
+      error = GeolocationPositionError_Binding::PERMISSION_DENIED;
+    }
+  }
+
+  DoShutdown(true, aResetManager);
+  NotifyError(error);
+}
+
+void GCLocProviderPriv::GetClientResponse(GDBusProxy* aProxy,
+                                          GAsyncResult* aResult,
+                                          gpointer aUserData) {
+  GUniquePtr<GError> error;
+  RefPtr<GVariant> variant = dont_AddRef(
+      g_dbus_proxy_call_finish(aProxy, aResult, getter_Transfers(error)));
+  if (!variant) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to get client: %s\n", error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      self->DBusProxyError(error.get(), true);
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  MOZ_DIAGNOSTIC_ASSERT(self->mClientState == ClientState::Initing,
+                        "Client in a wrong state");
+
+  auto signalError = MakeScopeExit([&]() MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+    self->DBusProxyError(nullptr, true);
+  });
+
+  if (!g_variant_is_of_type(variant, G_VARIANT_TYPE_TUPLE)) {
+    GCL_LOG(Error, "Unexpected get client call return type: %s\n",
+            g_variant_get_type_string(variant));
+    return;
+  }
+
+  if (g_variant_n_children(variant) < 1) {
+    GCL_LOG(Error,
+            "Not enough params in get client call return: %" G_GSIZE_FORMAT
+            "\n",
+            g_variant_n_children(variant));
+    return;
+  }
+
+  variant = dont_AddRef(g_variant_get_child_value(variant, 0));
+  if (!g_variant_is_of_type(variant, G_VARIANT_TYPE_OBJECT_PATH)) {
+    GCL_LOG(Error, "Unexpected get client call return type inside tuple: %s\n",
+            g_variant_get_type_string(variant));
+    return;
+  }
+
+  const gchar* clientPath = g_variant_get_string(variant, nullptr);
+  GCL_LOG(Debug, "Client path: %s\n", clientPath);
+
+  signalError.release();
+  self->ConnectClient(clientPath);
+}
+
+void GCLocProviderPriv::ConnectClient(const gchar* aClientPath) {
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Initing,
+                        "Client in a wrong state");
+  MOZ_ASSERT(mCancellable, "Watch() wasn't successfully called");
+  g_dbus_proxy_new_for_bus(
+      G_BUS_TYPE_SYSTEM, G_DBUS_PROXY_FLAGS_NONE, nullptr, kGeoclueBusName,
+      aClientPath, kGCClientInterface, mCancellable,
+      reinterpret_cast<GAsyncReadyCallback>(ConnectClientResponse), this);
+}
+
+void GCLocProviderPriv::ConnectClientResponse(GObject* aObject,
+                                              GAsyncResult* aResult,
+                                              gpointer aUserData) {
+  GUniquePtr<GError> error;
+  RefPtr<GDBusProxy> proxyClient =
+      dont_AddRef(g_dbus_proxy_new_finish(aResult, getter_Transfers(error)));
+  if (!proxyClient) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to connect to client: %s\n", error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      self->DBusProxyError(error.get());
+    }
+    return;
+  }
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  self->mProxyClient = std::move(proxyClient);
+
+  MOZ_DIAGNOSTIC_ASSERT(self->mClientState == ClientState::Initing,
+                        "Client in a wrong state");
+
+  GCL_LOG(Info, "Client interface connected\n");
+
+  g_signal_connect(self->mProxyClient, "g-signal", G_CALLBACK(GCClientSignal),
+                   self);
+  self->SetDesktopID();
+}
+
+void GCLocProviderPriv::SetDesktopID() {
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Initing,
+                        "Client in a wrong state");
+  MOZ_DIAGNOSTIC_ASSERT(mProxyClient && mCancellable,
+                        "Watch() wasn't successfully called");
+
+  nsAutoCString appName;
+  gAppData->GetDBusAppName(appName);
+  g_dbus_proxy_call(mProxyClient, kDBPropertySetMethod,
+                    g_variant_new("(ssv)", kGCClientInterface, "DesktopId",
+                                  g_variant_new_string(appName.get())),
+                    G_DBUS_CALL_FLAGS_NONE, -1, mCancellable,
+                    reinterpret_cast<GAsyncReadyCallback>(SetDesktopIDResponse),
+                    this);
+}
+
+void GCLocProviderPriv::SetDesktopIDResponse(GDBusProxy* aProxy,
+                                             GAsyncResult* aResult,
+                                             gpointer aUserData) {
+  GUniquePtr<GError> error;
+
+  RefPtr<GVariant> variant = dont_AddRef(
+      g_dbus_proxy_call_finish(aProxy, aResult, getter_Transfers(error)));
+  if (!variant) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to set DesktopId: %s\n", error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      self->DBusProxyError(error.get());
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  MOZ_DIAGNOSTIC_ASSERT(self->mClientState == ClientState::Initing,
+                        "Client in a wrong state");
+
+  GCLP_SETSTATE(self, Idle);
+  self->SetAccuracy();
+}
+
+void GCLocProviderPriv::SetAccuracy() {
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Idle,
+                        "Client in a wrong state");
+  MOZ_DIAGNOSTIC_ASSERT(mProxyClient && mCancellable,
+                        "Watch() wasn't successfully called");
+  MOZ_ASSERT(mAccuracyWanted != Accuracy::Unset, "Invalid accuracy");
+
+  guint32 accuracy;
+  if (mAccuracyWanted == Accuracy::High) {
+    accuracy = (guint32)GCAccuracyLevel::Exact;
+  } else {
+    accuracy = (guint32)GCAccuracyLevel::City;
+  }
+
+  mAccuracySet = mAccuracyWanted;
+  GCLP_SETSTATE(this, SettingAccuracyForStart);
+  g_dbus_proxy_call(
+      mProxyClient, kDBPropertySetMethod,
+      g_variant_new("(ssv)", kGCClientInterface, "RequestedAccuracyLevel",
+                    g_variant_new_uint32(accuracy)),
+      G_DBUS_CALL_FLAGS_NONE, -1, mCancellable,
+      reinterpret_cast<GAsyncReadyCallback>(SetAccuracyResponse), this);
+}
+
+void GCLocProviderPriv::SetAccuracyResponse(GDBusProxy* aProxy,
+                                            GAsyncResult* aResult,
+                                            gpointer aUserData) {
+  GUniquePtr<GError> error;
+  RefPtr<GVariant> variant = dont_AddRef(
+      g_dbus_proxy_call_finish(aProxy, aResult, getter_Transfers(error)));
+  if (!variant) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to set requested accuracy level: %s\n",
+              error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      self->DBusProxyError(error.get());
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  MOZ_DIAGNOSTIC_ASSERT(
+      self->mClientState == ClientState::SettingAccuracyForStart ||
+          self->mClientState == ClientState::SettingAccuracy,
+      "Client in a wrong state");
+  bool wantStart = self->mClientState == ClientState::SettingAccuracyForStart;
+  GCLP_SETSTATE(self, Idle);
+
+  if (wantStart) {
+    self->StartClient();
+  }
+}
+
+void GCLocProviderPriv::StartClient() {
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Idle,
+                        "Client in a wrong state");
+  MOZ_DIAGNOSTIC_ASSERT(mProxyClient && mCancellable,
+                        "Watch() wasn't successfully called");
+  GCLP_SETSTATE(this, Starting);
+  g_dbus_proxy_call(
+      mProxyClient, "Start", nullptr, G_DBUS_CALL_FLAGS_NONE, -1, mCancellable,
+      reinterpret_cast<GAsyncReadyCallback>(StartClientResponse), this);
+}
+
+void GCLocProviderPriv::StartClientResponse(GDBusProxy* aProxy,
+                                            GAsyncResult* aResult,
+                                            gpointer aUserData) {
+  GUniquePtr<GError> error;
+
+  RefPtr<GVariant> variant = dont_AddRef(
+      g_dbus_proxy_call_finish(aProxy, aResult, getter_Transfers(error)));
+  if (!variant) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to start client: %s\n", error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      /*
+       * A workaround for
+       * https://gitlab.freedesktop.org/geoclue/geoclue/-/issues/143 We need to
+       * get a new client instance once the agent finally connects to the
+       * Geoclue service, otherwise every Start request on the old client
+       * interface will be denied. We need to reconnect to the Manager interface
+       * to achieve this since otherwise GetClient call will simply return the
+       * old client instance.
+       */
+      bool resetManager = g_error_matches(error.get(), G_DBUS_ERROR,
+                                          G_DBUS_ERROR_ACCESS_DENIED);
+      self->DBusProxyError(error.get(), resetManager);
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  MOZ_DIAGNOSTIC_ASSERT(self->mClientState == ClientState::Starting,
+                        "Client in a wrong state");
+  GCLP_SETSTATE(self, Started);
+  self->MaybeRestartForAccuracy();
+}
+
+void GCLocProviderPriv::StopClient(bool aForRestart) {
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Started,
+                        "Client in a wrong state");
+  MOZ_DIAGNOSTIC_ASSERT(mProxyClient && mCancellable,
+                        "Watch() wasn't successfully called");
+
+  if (aForRestart) {
+    GCLP_SETSTATE(this, StoppingForRestart);
+  } else {
+    GCLP_SETSTATE(this, Stopping);
+  }
+
+  g_dbus_proxy_call(
+      mProxyClient, "Stop", nullptr, G_DBUS_CALL_FLAGS_NONE, -1, mCancellable,
+      reinterpret_cast<GAsyncReadyCallback>(StopClientResponse), this);
+}
+
+void GCLocProviderPriv::StopClientResponse(GDBusProxy* aProxy,
+                                           GAsyncResult* aResult,
+                                           gpointer aUserData) {
+  GUniquePtr<GError> error;
+  RefPtr<GVariant> variant = dont_AddRef(
+      g_dbus_proxy_call_finish(aProxy, aResult, getter_Transfers(error)));
+  if (!variant) {
+    // if cancelled |self| might no longer be there
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Error, "Failed to stop client: %s\n", error->message);
+      RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+      self->DBusProxyError(error.get());
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  MOZ_DIAGNOSTIC_ASSERT(self->InDBusStoppingCall(), "Client in a wrong state");
+  bool wantRestart = self->mClientState == ClientState::StoppingForRestart;
+  GCLP_SETSTATE(self, Idle);
+
+  if (!wantRestart) {
+    return;
+  }
+
+  if (self->mAccuracyWanted != self->mAccuracySet) {
+    self->SetAccuracy();
+  } else {
+    self->StartClient();
+  }
+}
+
+void GCLocProviderPriv::StopClientNoWait() {
+  MOZ_DIAGNOSTIC_ASSERT(mProxyClient, "Watch() wasn't successfully called");
+  g_dbus_proxy_call(mProxyClient, "Stop", nullptr, G_DBUS_CALL_FLAGS_NONE, -1,
+                    nullptr, nullptr, nullptr);
+}
+
+void GCLocProviderPriv::MaybeRestartForAccuracy() {
+  if (mAccuracyWanted == mAccuracySet) {
+    return;
+  }
+
+  if (mClientState != ClientState::Started) {
+    return;
+  }
+
+  // Setting a new accuracy requires restarting the client
+  StopClient(true);
+}
+
+void GCLocProviderPriv::GCManagerOwnerNotify(GObject* aObject,
+                                             GParamSpec* aPSpec,
+                                             gpointer aUserData) {
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  GUniquePtr<gchar> managerOwner(
+      g_dbus_proxy_get_name_owner(self->mProxyManager));
+  if (!managerOwner) {
+    GCL_LOG(Info, "The Manager interface has lost its owner\n");
+    self->DBusProxyError(nullptr, true);
+  }
+}
+
+void GCLocProviderPriv::GCClientSignal(GDBusProxy* aProxy, gchar* aSenderName,
+                                       gchar* aSignalName,
+                                       GVariant* aParameters,
+                                       gpointer aUserData) {
+  if (g_strcmp0(aSignalName, "LocationUpdated")) {
+    return;
+  }
+
+  if (!g_variant_is_of_type(aParameters, G_VARIANT_TYPE_TUPLE)) {
+    GCL_LOG(Error, "Unexpected location updated signal params type: %s\n",
+            g_variant_get_type_string(aParameters));
+    return;
+  }
+
+  if (g_variant_n_children(aParameters) < 2) {
+    GCL_LOG(Error,
+            "Not enough params in location updated signal: %" G_GSIZE_FORMAT
+            "\n",
+            g_variant_n_children(aParameters));
+    return;
+  }
+
+  RefPtr<GVariant> variant =
+      dont_AddRef(g_variant_get_child_value(aParameters, 1));
+  if (!g_variant_is_of_type(variant, G_VARIANT_TYPE_OBJECT_PATH)) {
+    GCL_LOG(Error,
+            "Unexpected location updated signal new location path type: %s\n",
+            g_variant_get_type_string(variant));
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  const gchar* locationPath = g_variant_get_string(variant, nullptr);
+  GCL_LOG(Verbose, "New location path: %s\n", locationPath);
+  self->ConnectLocation(locationPath);
+}
+
+void GCLocProviderPriv::ConnectLocation(const gchar* aLocationPath) {
+  MOZ_ASSERT(mCancellable, "Startup() wasn't successfully called");
+  g_dbus_proxy_new_for_bus(
+      G_BUS_TYPE_SYSTEM, G_DBUS_PROXY_FLAGS_NONE, nullptr, kGeoclueBusName,
+      aLocationPath, kGCLocationInterface, mCancellable,
+      reinterpret_cast<GAsyncReadyCallback>(ConnectLocationResponse), this);
+}
+
+bool GCLocProviderPriv::GetLocationProperty(GDBusProxy* aProxyLocation,
+                                            const gchar* aName, double* aOut) {
+  RefPtr<GVariant> property =
+      dont_AddRef(g_dbus_proxy_get_cached_property(aProxyLocation, aName));
+  if (!g_variant_is_of_type(property, G_VARIANT_TYPE_DOUBLE)) {
+    GCL_LOG(Error, "Unexpected location property %s type: %s\n", aName,
+            g_variant_get_type_string(property));
+    return false;
+  }
+
+  *aOut = g_variant_get_double(property);
+  return true;
+}
+
+void GCLocProviderPriv::ConnectLocationResponse(GObject* aObject,
+                                                GAsyncResult* aResult,
+                                                gpointer aUserData) {
+  GUniquePtr<GError> error;
+  RefPtr<GDBusProxy> proxyLocation =
+      dont_AddRef(g_dbus_proxy_new_finish(aResult, getter_Transfers(error)));
+  if (!proxyLocation) {
+    if (!g_error_matches(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED)) {
+      GCL_LOG(Warning, "Failed to connect to location: %s\n", error->message);
+    }
+    return;
+  }
+
+  RefPtr self = static_cast<GCLocProviderPriv*>(aUserData);
+  /*
+   * nsGeoPositionCoords will convert NaNs to null for optional properties of
+   * the JavaScript Coordinates object.
+   */
+  double lat = UnspecifiedNaN<double>();
+  double lon = UnspecifiedNaN<double>();
+  double alt = UnspecifiedNaN<double>();
+  double hError = UnspecifiedNaN<double>();
+  const double vError = UnspecifiedNaN<double>();
+  double heading = UnspecifiedNaN<double>();
+  double speed = UnspecifiedNaN<double>();
+  struct {
+    const gchar* name;
+    double* out;
+  } props[] = {
+      {"Latitude", &lat},    {"Longitude", &lon},   {"Altitude", &alt},
+      {"Accuracy", &hError}, {"Heading", &heading}, {"Speed", &speed},
+  };
+
+  for (auto& prop : props) {
+    if (!GetLocationProperty(proxyLocation, prop.name, prop.out)) {
+      return;
+    }
+  }
+
+  if (alt < kGCMinAlt) {
+    alt = UnspecifiedNaN<double>();
+  }
+  if (speed < 0) {
+    speed = UnspecifiedNaN<double>();
+  }
+  if (heading < 0 || IsNaN(speed) || speed == 0) {
+    heading = UnspecifiedNaN<double>();
+  }
+
+  GCL_LOG(Info, "New location: %f %f +-%fm @ %gm; hdg %f spd %fm/s\n", lat, lon,
+          hError, alt, heading, speed);
+
+  self->mLastPosition =
+      new nsGeoPosition(lat, lon, alt, hError, vError, heading, speed,
+                        PR_Now() / PR_USEC_PER_MSEC);
+  self->UpdateLastPosition();
+}
+
+void GCLocProviderPriv::SetLocationTimer() {
+  MOZ_DIAGNOSTIC_ASSERT(mLastPosition, "no last position to report");
+
+  StopLocationTimer();
+
+  RefPtr<LocationTimerCallback> timerCallback = new LocationTimerCallback(this);
+  NS_NewTimerWithCallback(getter_AddRefs(mLocationTimer), timerCallback, 1000,
+                          nsITimer::TYPE_ONE_SHOT);
+}
+
+void GCLocProviderPriv::StopLocationTimer() {
+  if (!mLocationTimer) {
+    return;
+  }
+
+  mLocationTimer->Cancel();
+  mLocationTimer = nullptr;
+}
+
+// Did we made some D-Bus call and are still waiting for its response?
+bool GCLocProviderPriv::InDBusCall() {
+  return mClientState == ClientState::Initing ||
+         mClientState == ClientState::SettingAccuracy ||
+         mClientState == ClientState::SettingAccuracyForStart ||
+         mClientState == ClientState::Starting ||
+         mClientState == ClientState::Stopping ||
+         mClientState == ClientState::StoppingForRestart;
+}
+
+bool GCLocProviderPriv::InDBusStoppingCall() {
+  return mClientState == ClientState::Stopping ||
+         mClientState == ClientState::StoppingForRestart;
+}
+
+/*
+ * Did we made some D-Bus call while stopped and
+ * are still waiting for its response?
+ */
+bool GCLocProviderPriv::InDBusStoppedCall() {
+  return mClientState == ClientState::SettingAccuracy ||
+         mClientState == ClientState::SettingAccuracyForStart;
+}
+
+void GCLocProviderPriv::DeleteManager() {
+  if (!mProxyManager) {
+    return;
+  }
+
+  g_signal_handlers_disconnect_matched(mProxyManager, G_SIGNAL_MATCH_DATA, 0, 0,
+                                       nullptr, nullptr, this);
+  mProxyManager = nullptr;
+}
+
+void GCLocProviderPriv::DoShutdown(bool aDeleteClient, bool aDeleteManager) {
+  MOZ_DIAGNOSTIC_ASSERT(
+      !aDeleteManager || aDeleteClient,
+      "deleting manager proxy requires deleting client one, too");
+
+  // Invalidate the cached last position
+  StopLocationTimer();
+  mLastPosition = nullptr;
+
+  /*
+   * Do we need to delete the D-Bus proxy (or proxies)?
+   * Either because that's what our caller wanted, or because we are set to
+   * never reuse them, or because we are in a middle of some D-Bus call while
+   * having the service running (and so not being able to issue an immediate
+   * Stop call).
+   */
+  if (aDeleteClient || !kGCReuseDBusProxy ||
+      (InDBusCall() && !InDBusStoppingCall() && !InDBusStoppedCall())) {
+    if (mClientState == ClientState::Started) {
+      StopClientNoWait();
+      GCLP_SETSTATE(this, Idle);
+    }
+    if (mProxyClient) {
+      g_signal_handlers_disconnect_matched(mProxyClient, G_SIGNAL_MATCH_DATA, 0,
+                                           0, nullptr, nullptr, this);
+    }
+    if (mCancellable) {
+      g_cancellable_cancel(mCancellable);
+      mCancellable = nullptr;
+    }
+    mProxyClient = nullptr;
+
+    if (aDeleteManager || !kGCReuseDBusProxy) {
+      DeleteManager();
+    }
+
+    GCLP_SETSTATE(this, Uninit);
+  } else if (mClientState == ClientState::Started) {
+    StopClient(false);
+  } else if (mClientState == ClientState::SettingAccuracyForStart) {
+    GCLP_SETSTATE(this, SettingAccuracy);
+  } else if (mClientState == ClientState::StoppingForRestart) {
+    GCLP_SETSTATE(this, Stopping);
+  }
+}
+
+void GCLocProviderPriv::DoShutdownClearCallback(bool aDestroying) {
+  mCallback = nullptr;
+  StopMLSFallback();
+  DoShutdown(aDestroying, aDestroying);
+}
+
+NS_IMPL_ISUPPORTS(GCLocProviderPriv, nsIGeolocationProvider)
+
+// nsIGeolocationProvider
+//
+
+/*
+ * The Startup() method should only succeed if Geoclue is available on D-Bus
+ * so it can be used for determining whether to continue with this geolocation
+ * provider in Geolocation.cpp
+ */
+NS_IMETHODIMP
+GCLocProviderPriv::Startup() {
+  if (mProxyManager) {
+    return NS_OK;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(mClientState == ClientState::Uninit,
+                        "Client in a initialized state but no manager");
+
+  GUniquePtr<GError> error;
+  mProxyManager = dont_AddRef(g_dbus_proxy_new_for_bus_sync(
+      G_BUS_TYPE_SYSTEM, G_DBUS_PROXY_FLAGS_NONE, nullptr, kGeoclueBusName,
+      kGCManagerPath, kGCManagerInterface, nullptr, getter_Transfers(error)));
+  if (!mProxyManager) {
+    GCL_LOG(Info, "Cannot connect to the Manager interface: %s\n",
+            error->message);
+    return NS_ERROR_FAILURE;
+  }
+
+  g_signal_connect(mProxyManager, "notify::g-name-owner",
+                   G_CALLBACK(GCManagerOwnerNotify), this);
+
+  GUniquePtr<gchar> managerOwner(g_dbus_proxy_get_name_owner(mProxyManager));
+  if (!managerOwner) {
+    GCL_LOG(Info, "The Manager interface has no owner\n");
+    DeleteManager();
+    return NS_ERROR_FAILURE;
+  }
+
+  GCL_LOG(Info, "Manager interface connected successfully\n");
+
+  return NS_OK;
+}
+
+void GCLocProviderPriv::WatchStart() {
+  if (mClientState == ClientState::Idle) {
+    StartClient();
+  } else if (mClientState == ClientState::Started) {
+    if (mLastPosition && !mLocationTimer) {
+      GCL_LOG(Verbose,
+              "Will report the existing location if new one doesn't come up\n");
+      SetLocationTimer();
+    }
+  } else if (mClientState == ClientState::SettingAccuracy) {
+    GCLP_SETSTATE(this, SettingAccuracyForStart);
+  } else if (mClientState == ClientState::Stopping) {
+    GCLP_SETSTATE(this, StoppingForRestart);
+  }
+}
+
+NS_IMETHODIMP
+GCLocProviderPriv::Watch(nsIGeolocationUpdate* aCallback) {
+  mCallback = aCallback;
+
+  if (!mCancellable) {
+    mCancellable = dont_AddRef(g_cancellable_new());
+  }
+
+  if (mClientState != ClientState::Uninit) {
+    WatchStart();
+    return NS_OK;
+  }
+
+  if (!mProxyManager) {
+    GCL_LOG(Debug, "watch request falling back to MLS");
+    return FallbackToMLS();
+  }
+
+  StopMLSFallback();
+
+  GCLP_SETSTATE(this, Initing);
+  g_dbus_proxy_call(mProxyManager, "GetClient", nullptr, G_DBUS_CALL_FLAGS_NONE,
+                    -1, mCancellable,
+                    reinterpret_cast<GAsyncReadyCallback>(GetClientResponse),
+                    this);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+GCLocProviderPriv::Shutdown() {
+  DoShutdownClearCallback(false);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+GCLocProviderPriv::SetHighAccuracy(bool aHigh) {
+  GCL_LOG(Verbose, "Want %s accuracy\n", aHigh ? "high" : "low");
+  if (!aHigh && AlwaysHighAccuracy()) {
+    GCL_LOG(Verbose, "Forcing high accuracy due to pref\n");
+    aHigh = true;
+  }
+
+  mAccuracyWanted = aHigh ? Accuracy::High : Accuracy::Low;
+  MaybeRestartForAccuracy();
+
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(GCLocProviderPriv::LocationTimerCallback, nsITimerCallback,
+                  nsINamed)
+
+NS_IMETHODIMP
+GCLocProviderPriv::LocationTimerCallback::Notify(nsITimer* aTimer) {
+  if (mParent) {
+    RefPtr<GCLocProviderPriv> parent(mParent);
+    parent->UpdateLastPosition();
+  }
+
+  return NS_OK;
+}
+
+GeoclueLocationProvider::GeoclueLocationProvider() {
+  mPriv = new GCLocProviderPriv;
+}
+
+// nsISupports
+//
+
+NS_IMPL_ISUPPORTS(GeoclueLocationProvider, nsIGeolocationProvider)
+
+// nsIGeolocationProvider
+//
+
+NS_IMETHODIMP
+GeoclueLocationProvider::Startup() { return mPriv->Startup(); }
+
+NS_IMETHODIMP
+GeoclueLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  return mPriv->Watch(aCallback);
+}
+
+NS_IMETHODIMP
+GeoclueLocationProvider::Shutdown() { return mPriv->Shutdown(); }
+
+NS_IMETHODIMP
+GeoclueLocationProvider::SetHighAccuracy(bool aHigh) {
+  return mPriv->SetHighAccuracy(aHigh);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/system/linux/GeoclueLocationProvider.h b/dom/system/linux/GeoclueLocationProvider.h
new file mode 100644
index 0000000000..908cd25e37
--- /dev/null
+++ b/dom/system/linux/GeoclueLocationProvider.h
@@ -0,0 +1,32 @@
+/*
+ * 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 GeoclueLocationProvider_h
+#define GeoclueLocationProvider_h
+
+#include "mozilla/RefPtr.h"
+#include "nsIGeolocationProvider.h"
+
+namespace mozilla::dom {
+
+class GCLocProviderPriv;
+
+class GeoclueLocationProvider final : public nsIGeolocationProvider {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  GeoclueLocationProvider();
+
+ private:
+  ~GeoclueLocationProvider() = default;
+
+  RefPtr<GCLocProviderPriv> mPriv;
+};
+
+}  // namespace mozilla::dom
+
+#endif /* GeoclueLocationProvider_h */
diff --git a/dom/system/linux/GpsdLocationProvider.cpp b/dom/system/linux/GpsdLocationProvider.cpp
new file mode 100644
index 0000000000..0a3f5d7660
--- /dev/null
+++ b/dom/system/linux/GpsdLocationProvider.cpp
@@ -0,0 +1,446 @@
+/* -*- 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 "GpsdLocationProvider.h"
+#include <errno.h>
+#include <gps.h>
+#include "MLSFallback.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/LazyIdleThread.h"
+#include "mozilla/dom/GeolocationPositionErrorBinding.h"
+#include "GeolocationPosition.h"
+#include "nsProxyRelease.h"
+#include "nsThreadUtils.h"
+#include "prtime.h"
+
+namespace mozilla {
+namespace dom {
+
+//
+// MLSGeolocationUpdate
+//
+
+/**
+ * |MLSGeolocationUpdate| provides a fallback if gpsd is not supported.
+ */
+class GpsdLocationProvider::MLSGeolocationUpdate final
+    : public nsIGeolocationUpdate {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONUPDATE
+
+  explicit MLSGeolocationUpdate(nsIGeolocationUpdate* aCallback);
+
+ protected:
+  ~MLSGeolocationUpdate() = default;
+
+ private:
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+};
+
+GpsdLocationProvider::MLSGeolocationUpdate::MLSGeolocationUpdate(
+    nsIGeolocationUpdate* aCallback)
+    : mCallback(aCallback) {
+  MOZ_ASSERT(mCallback);
+}
+
+// nsISupports
+//
+
+NS_IMPL_ISUPPORTS(GpsdLocationProvider::MLSGeolocationUpdate,
+                  nsIGeolocationUpdate);
+
+// nsIGeolocationUpdate
+//
+
+NS_IMETHODIMP
+GpsdLocationProvider::MLSGeolocationUpdate::Update(
+    nsIDOMGeoPosition* aPosition) {
+  nsCOMPtr<nsIDOMGeoPositionCoords> coords;
+  aPosition->GetCoords(getter_AddRefs(coords));
+  if (!coords) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return mCallback->Update(aPosition);
+}
+
+NS_IMETHODIMP
+GpsdLocationProvider::MLSGeolocationUpdate::NotifyError(uint16_t aError) {
+  return mCallback->NotifyError(aError);
+}
+
+//
+// UpdateRunnable
+//
+
+class GpsdLocationProvider::UpdateRunnable final : public Runnable {
+ public:
+  UpdateRunnable(
+      const nsMainThreadPtrHandle<GpsdLocationProvider>& aLocationProvider,
+      nsIDOMGeoPosition* aPosition)
+      : Runnable("GpsdU"),
+        mLocationProvider(aLocationProvider),
+        mPosition(aPosition) {
+    MOZ_ASSERT(mLocationProvider);
+    MOZ_ASSERT(mPosition);
+  }
+
+  // nsIRunnable
+  //
+
+  NS_IMETHOD Run() override {
+    mLocationProvider->Update(mPosition);
+    return NS_OK;
+  }
+
+ private:
+  nsMainThreadPtrHandle<GpsdLocationProvider> mLocationProvider;
+  RefPtr<nsIDOMGeoPosition> mPosition;
+};
+
+//
+// NotifyErrorRunnable
+//
+
+class GpsdLocationProvider::NotifyErrorRunnable final : public Runnable {
+ public:
+  NotifyErrorRunnable(
+      const nsMainThreadPtrHandle<GpsdLocationProvider>& aLocationProvider,
+      int aError)
+      : Runnable("GpsdNE"),
+        mLocationProvider(aLocationProvider),
+        mError(aError) {
+    MOZ_ASSERT(mLocationProvider);
+  }
+
+  // nsIRunnable
+  //
+
+  NS_IMETHOD Run() override {
+    mLocationProvider->NotifyError(mError);
+    return NS_OK;
+  }
+
+ private:
+  nsMainThreadPtrHandle<GpsdLocationProvider> mLocationProvider;
+  int mError;
+};
+
+//
+// PollRunnable
+//
+
+/**
+ * |PollRunnable| does the main work of processing GPS data received
+ * from gpsd. libgps blocks while polling, so this runnable has to be
+ * executed on it's own thread. To cancel the poll runnable, invoke
+ * |StopRunning| and |PollRunnable| will stop within a reasonable time
+ * frame.
+ */
+class GpsdLocationProvider::PollRunnable final : public Runnable {
+ public:
+  PollRunnable(
+      const nsMainThreadPtrHandle<GpsdLocationProvider>& aLocationProvider)
+      : Runnable("GpsdP"),
+        mLocationProvider(aLocationProvider),
+        mRunning(true) {
+    MOZ_ASSERT(mLocationProvider);
+  }
+
+  static bool IsSupported() {
+    return GPSD_API_MAJOR_VERSION >= 5 && GPSD_API_MAJOR_VERSION <= 12;
+  }
+
+  bool IsRunning() const { return mRunning; }
+
+  void StopRunning() { mRunning = false; }
+
+  // nsIRunnable
+  //
+
+  NS_IMETHOD Run() override {
+    int err;
+
+    switch (GPSD_API_MAJOR_VERSION) {
+      case 5 ... 12:
+        err = PollLoop5();
+        break;
+      default:
+        err = GeolocationPositionError_Binding::POSITION_UNAVAILABLE;
+        break;
+    }
+
+    if (err) {
+      NS_DispatchToMainThread(
+          MakeAndAddRef<NotifyErrorRunnable>(mLocationProvider, err));
+    }
+
+    mLocationProvider = nullptr;
+
+    return NS_OK;
+  }
+
+ protected:
+  int PollLoop5() {
+#if GPSD_API_MAJOR_VERSION >= 5 && GPSD_API_MAJOR_VERSION <= 12
+    static const int GPSD_WAIT_TIMEOUT_US =
+        1000000; /* us to wait for GPS data */
+
+    struct gps_data_t gpsData;
+
+    auto res = gps_open(nullptr, nullptr, &gpsData);
+
+    if (res < 0) {
+      return ErrnoToError(errno);
+    }
+
+    gps_stream(&gpsData, WATCH_ENABLE | WATCH_JSON, NULL);
+
+    int err = 0;
+
+    // nsGeoPositionCoords will convert NaNs to null for optional properties of
+    // the JavaScript Coordinates object.
+    double lat = 0;
+    double lon = 0;
+    double alt = UnspecifiedNaN<double>();
+    double hError = 0;
+    double vError = UnspecifiedNaN<double>();
+    double heading = UnspecifiedNaN<double>();
+    double speed = UnspecifiedNaN<double>();
+
+    while (IsRunning()) {
+      errno = 0;
+      auto hasGpsData = gps_waiting(&gpsData, GPSD_WAIT_TIMEOUT_US);
+
+      if (errno) {
+        err = ErrnoToError(errno);
+        break;
+      }
+      if (!hasGpsData) {
+        continue; /* woke up from timeout */
+      }
+
+#  if GPSD_API_MAJOR_VERSION >= 7
+      res = gps_read(&gpsData, nullptr, 0);
+#  else
+
+      res = gps_read(&gpsData);
+#  endif
+
+      if (res < 0) {
+        err = ErrnoToError(errno);
+        break;
+      } else if (!res) {
+        continue; /* no data available */
+      }
+
+#  if GPSD_API_MAJOR_VERSION < 10
+      if (gpsData.status == STATUS_NO_FIX) {
+        continue;
+      }
+#  endif
+
+      switch (gpsData.fix.mode) {
+        case MODE_3D:
+          double galt;
+
+#  if GPSD_API_MAJOR_VERSION >= 9
+          galt = gpsData.fix.altMSL;
+#  else
+          galt = gpsData.fix.altitude;
+#  endif
+          if (!IsNaN(galt)) {
+            alt = galt;
+          }
+          [[fallthrough]];
+        case MODE_2D:
+          if (!IsNaN(gpsData.fix.latitude)) {
+            lat = gpsData.fix.latitude;
+          }
+          if (!IsNaN(gpsData.fix.longitude)) {
+            lon = gpsData.fix.longitude;
+          }
+          if (!IsNaN(gpsData.fix.epx) && !IsNaN(gpsData.fix.epy)) {
+            hError = std::max(gpsData.fix.epx, gpsData.fix.epy);
+          } else if (!IsNaN(gpsData.fix.epx)) {
+            hError = gpsData.fix.epx;
+          } else if (!IsNaN(gpsData.fix.epy)) {
+            hError = gpsData.fix.epy;
+          }
+          if (!IsNaN(gpsData.fix.epv)) {
+            vError = gpsData.fix.epv;
+          }
+          if (!IsNaN(gpsData.fix.track)) {
+            heading = gpsData.fix.track;
+          }
+          if (!IsNaN(gpsData.fix.speed)) {
+            speed = gpsData.fix.speed;
+          }
+          break;
+        default:
+          continue;  // There's no useful data in this fix; continue.
+      }
+
+      NS_DispatchToMainThread(MakeAndAddRef<UpdateRunnable>(
+          mLocationProvider,
+          new nsGeoPosition(lat, lon, alt, hError, vError, heading, speed,
+                            PR_Now() / PR_USEC_PER_MSEC)));
+    }
+
+    gps_stream(&gpsData, WATCH_DISABLE, NULL);
+    gps_close(&gpsData);
+
+    return err;
+#else
+    return GeolocationPositionError_Binding::POSITION_UNAVAILABLE;
+#endif  // GPSD_MAJOR_API_VERSION
+  }
+
+  static int ErrnoToError(int aErrno) {
+    switch (aErrno) {
+      case EACCES:
+        [[fallthrough]];
+      case EPERM:
+        [[fallthrough]];
+      case EROFS:
+        return GeolocationPositionError_Binding::PERMISSION_DENIED;
+      case ETIME:
+        [[fallthrough]];
+      case ETIMEDOUT:
+        return GeolocationPositionError_Binding::TIMEOUT;
+      default:
+        return GeolocationPositionError_Binding::POSITION_UNAVAILABLE;
+    }
+  }
+
+ private:
+  nsMainThreadPtrHandle<GpsdLocationProvider> mLocationProvider;
+  Atomic<bool> mRunning;
+};
+
+//
+// GpsdLocationProvider
+//
+
+const uint32_t GpsdLocationProvider::GPSD_POLL_THREAD_TIMEOUT_MS = 5000;
+
+GpsdLocationProvider::GpsdLocationProvider() {}
+
+GpsdLocationProvider::~GpsdLocationProvider() {}
+
+void GpsdLocationProvider::Update(nsIDOMGeoPosition* aPosition) {
+  if (!mCallback || !mPollRunnable) {
+    return;  // not initialized or already shut down
+  }
+
+  if (mMLSProvider) {
+    /* We got a location from gpsd, so let's cancel our MLS fallback. */
+    mMLSProvider->Shutdown();
+    mMLSProvider = nullptr;
+  }
+
+  mCallback->Update(aPosition);
+}
+
+void GpsdLocationProvider::NotifyError(int aError) {
+  if (!mCallback) {
+    return;  // not initialized or already shut down
+  }
+
+  if (!mMLSProvider) {
+    /* With gpsd failed, we restart MLS. It will be canceled once we
+     * get another location from gpsd.
+     */
+    mMLSProvider = MakeAndAddRef<MLSFallback>();
+    mMLSProvider->Startup(new MLSGeolocationUpdate(mCallback));
+  }
+
+  mCallback->NotifyError(aError);
+}
+
+// nsISupports
+//
+
+NS_IMPL_ISUPPORTS(GpsdLocationProvider, nsIGeolocationProvider)
+
+// nsIGeolocationProvider
+//
+
+NS_IMETHODIMP
+GpsdLocationProvider::Startup() {
+  if (!PollRunnable::IsSupported()) {
+    return NS_OK;  // We'll fall back to MLS.
+  }
+
+  if (mPollRunnable) {
+    return NS_OK;  // already running
+  }
+
+  RefPtr<PollRunnable> pollRunnable =
+      MakeAndAddRef<PollRunnable>(nsMainThreadPtrHandle<GpsdLocationProvider>(
+          new nsMainThreadPtrHolder<GpsdLocationProvider>("GpsdLP", this)));
+
+  // Use existing poll thread...
+  RefPtr<LazyIdleThread> pollThread = mPollThread;
+
+  // ... or create a new one.
+  if (!pollThread) {
+    pollThread = MakeAndAddRef<LazyIdleThread>(GPSD_POLL_THREAD_TIMEOUT_MS,
+                                               "Gpsd poll thread"_ns,
+                                               LazyIdleThread::ManualShutdown);
+  }
+
+  auto rv = pollThread->Dispatch(pollRunnable, NS_DISPATCH_NORMAL);
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  mPollRunnable = pollRunnable.forget();
+  mPollThread = pollThread.forget();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+GpsdLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  mCallback = aCallback;
+
+  /* The MLS fallback will kick in after a few seconds if gpsd
+   * doesn't provide location information within time. Once we
+   * see the first message from gpsd, the fallback will be
+   * disabled in |Update|.
+   */
+  mMLSProvider = MakeAndAddRef<MLSFallback>();
+  mMLSProvider->Startup(new MLSGeolocationUpdate(aCallback));
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+GpsdLocationProvider::Shutdown() {
+  if (mMLSProvider) {
+    mMLSProvider->Shutdown();
+    mMLSProvider = nullptr;
+  }
+
+  if (!mPollRunnable) {
+    return NS_OK;  // not running
+  }
+
+  mPollRunnable->StopRunning();
+  mPollRunnable = nullptr;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+GpsdLocationProvider::SetHighAccuracy(bool aHigh) { return NS_OK; }
+
+}  // namespace dom
+}  // namespace mozilla
diff --git a/dom/system/linux/GpsdLocationProvider.h b/dom/system/linux/GpsdLocationProvider.h
new file mode 100644
index 0000000000..544f0e4c69
--- /dev/null
+++ b/dom/system/linux/GpsdLocationProvider.h
@@ -0,0 +1,51 @@
+/* -*- 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 GpsdLocationProvider_h
+#define GpsdLocationProvider_h
+
+#include "nsCOMPtr.h"
+#include "Geolocation.h"
+#include "nsIGeolocationProvider.h"
+
+class MLSFallback;
+
+namespace mozilla {
+
+class LazyIdleThread;
+
+namespace dom {
+
+class GpsdLocationProvider final : public nsIGeolocationProvider {
+  class MLSGeolocationUpdate;
+  class NotifyErrorRunnable;
+  class PollRunnable;
+  class UpdateRunnable;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  GpsdLocationProvider();
+
+ private:
+  ~GpsdLocationProvider();
+
+  void Update(nsIDOMGeoPosition* aPosition);
+  void NotifyError(int aError);
+
+  static const uint32_t GPSD_POLL_THREAD_TIMEOUT_MS;
+
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+  RefPtr<LazyIdleThread> mPollThread;
+  RefPtr<PollRunnable> mPollRunnable;
+  RefPtr<MLSFallback> mMLSProvider;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif /* GpsLocationProvider_h */
diff --git a/dom/system/linux/PortalLocationProvider.cpp b/dom/system/linux/PortalLocationProvider.cpp
new file mode 100644
index 0000000000..6ebb1854dc
--- /dev/null
+++ b/dom/system/linux/PortalLocationProvider.cpp
@@ -0,0 +1,351 @@
+/* -*- 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 "PortalLocationProvider.h"
+#include "MLSFallback.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/Logging.h"
+#include "mozilla/dom/GeolocationPositionErrorBinding.h"
+#include "GeolocationPosition.h"
+#include "prtime.h"
+#include "mozilla/GUniquePtr.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/XREAppData.h"
+
+#include <gio/gio.h>
+#include <glib-object.h>
+
+extern const mozilla::XREAppData* gAppData;
+
+namespace mozilla::dom {
+
+#ifdef MOZ_LOGGING
+static LazyLogModule sPortalLog("Portal");
+#  define LOG_PORTAL(...) MOZ_LOG(sPortalLog, LogLevel::Debug, (__VA_ARGS__))
+#else
+#  define LOG_PORTAL(...)
+#endif /* MOZ_LOGGING */
+
+const char kDesktopBusName[] = "org.freedesktop.portal.Desktop";
+const char kSessionInterfaceName[] = "org.freedesktop.portal.Session";
+
+/**
+ * |MLSGeolocationUpdate| provides a fallback if Portal is not supported.
+ */
+class PortalLocationProvider::MLSGeolocationUpdate final
+    : public nsIGeolocationUpdate {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONUPDATE
+
+  explicit MLSGeolocationUpdate(nsIGeolocationUpdate* aCallback);
+
+ protected:
+  ~MLSGeolocationUpdate() = default;
+
+ private:
+  const nsCOMPtr<nsIGeolocationUpdate> mCallback;
+};
+
+PortalLocationProvider::MLSGeolocationUpdate::MLSGeolocationUpdate(
+    nsIGeolocationUpdate* aCallback)
+    : mCallback(aCallback) {
+  MOZ_ASSERT(mCallback);
+}
+
+NS_IMPL_ISUPPORTS(PortalLocationProvider::MLSGeolocationUpdate,
+                  nsIGeolocationUpdate);
+
+// nsIGeolocationUpdate
+//
+
+NS_IMETHODIMP
+PortalLocationProvider::MLSGeolocationUpdate::Update(
+    nsIDOMGeoPosition* aPosition) {
+  nsCOMPtr<nsIDOMGeoPositionCoords> coords;
+  aPosition->GetCoords(getter_AddRefs(coords));
+  if (!coords) {
+    return NS_ERROR_FAILURE;
+  }
+  LOG_PORTAL("MLS is updating position\n");
+  return mCallback->Update(aPosition);
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::MLSGeolocationUpdate::NotifyError(uint16_t aError) {
+  nsCOMPtr<nsIGeolocationUpdate> callback(mCallback);
+  return callback->NotifyError(aError);
+}
+
+//
+// PortalLocationProvider
+//
+
+PortalLocationProvider::PortalLocationProvider() = default;
+
+PortalLocationProvider::~PortalLocationProvider() {
+  if (mDBUSLocationProxy || mRefreshTimer || mMLSProvider) {
+    NS_WARNING(
+        "PortalLocationProvider: Shutdown() had not been called before "
+        "destructor.");
+    Shutdown();
+  }
+}
+
+void PortalLocationProvider::Update(nsIDOMGeoPosition* aPosition) {
+  if (!mCallback) {
+    return;  // not initialized or already shut down
+  }
+
+  if (mMLSProvider) {
+    LOG_PORTAL(
+        "Update from location portal received: Cancelling fallback MLS "
+        "provider\n");
+    mMLSProvider->Shutdown();
+    mMLSProvider = nullptr;
+  }
+
+  LOG_PORTAL("Send updated location to the callback %p", mCallback.get());
+  mCallback->Update(aPosition);
+
+  aPosition->GetCoords(getter_AddRefs(mLastGeoPositionCoords));
+  // Schedule sending repetitive updates because we don't get more until
+  // position is changed from portal. That would lead to timeout on the
+  // Firefox side.
+  SetRefreshTimer(5000);
+}
+
+void PortalLocationProvider::NotifyError(int aError) {
+  LOG_PORTAL("*****NotifyError %d\n", aError);
+  if (!mCallback) {
+    return;  // not initialized or already shut down
+  }
+
+  if (!mMLSProvider) {
+    /* With Portal failed, we restart MLS. It will be canceled once we
+     * get another location from Portal. Start it immediately.
+     */
+    mMLSProvider = MakeAndAddRef<MLSFallback>(0);
+    mMLSProvider->Startup(new MLSGeolocationUpdate(mCallback));
+  }
+
+  nsCOMPtr<nsIGeolocationUpdate> callback(mCallback);
+  callback->NotifyError(aError);
+}
+
+NS_IMPL_ISUPPORTS(PortalLocationProvider, nsIGeolocationProvider)
+
+static void location_updated_signal_cb(GDBusProxy* proxy, gchar* sender_name,
+                                       gchar* signal_name, GVariant* parameters,
+                                       gpointer user_data) {
+  LOG_PORTAL("Signal: %s received from: %s\n", sender_name, signal_name);
+
+  if (g_strcmp0(signal_name, "LocationUpdated")) {
+    LOG_PORTAL("Unexpected signal %s received", signal_name);
+    return;
+  }
+
+  auto* locationProvider = static_cast<PortalLocationProvider*>(user_data);
+  RefPtr<GVariant> response_data;
+  gchar* session_handle;
+  g_variant_get(parameters, "(o@a{sv})", &session_handle,
+                response_data.StartAssignment());
+  if (!response_data) {
+    LOG_PORTAL("Missing response data from portal\n");
+    locationProvider->NotifyError(
+        GeolocationPositionError_Binding::POSITION_UNAVAILABLE);
+    return;
+  }
+  LOG_PORTAL("Session handle: %s Response data: %s\n", session_handle,
+             GUniquePtr<gchar>(g_variant_print(response_data, TRUE)).get());
+  g_free(session_handle);
+
+  double lat = 0;
+  double lon = 0;
+  if (!g_variant_lookup(response_data, "Latitude", "d", &lat) ||
+      !g_variant_lookup(response_data, "Longitude", "d", &lon)) {
+    locationProvider->NotifyError(
+        GeolocationPositionError_Binding::POSITION_UNAVAILABLE);
+    return;
+  }
+
+  double alt = UnspecifiedNaN<double>();
+  g_variant_lookup(response_data, "Altitude", "d", &alt);
+  double vError = 0;
+  double hError = UnspecifiedNaN<double>();
+  g_variant_lookup(response_data, "Accuracy", "d", &hError);
+  double heading = UnspecifiedNaN<double>();
+  g_variant_lookup(response_data, "Heading", "d", &heading);
+  double speed = UnspecifiedNaN<double>();
+  g_variant_lookup(response_data, "Speed", "d", &speed);
+
+  locationProvider->Update(new nsGeoPosition(lat, lon, alt, hError, vError,
+                                             heading, speed,
+                                             PR_Now() / PR_USEC_PER_MSEC));
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::Startup() {
+  LOG_PORTAL("Starting location portal");
+  if (mDBUSLocationProxy) {
+    LOG_PORTAL("Proxy already started.\n");
+    return NS_OK;
+  }
+
+  // Create dbus proxy for the Location portal
+  GUniquePtr<GError> error;
+  mDBUSLocationProxy = dont_AddRef(g_dbus_proxy_new_for_bus_sync(
+      G_BUS_TYPE_SESSION, G_DBUS_PROXY_FLAGS_NONE,
+      nullptr, /* GDBusInterfaceInfo */
+      kDesktopBusName, "/org/freedesktop/portal/desktop",
+      "org.freedesktop.portal.Location", nullptr, /* GCancellable */
+      getter_Transfers(error)));
+  if (!mDBUSLocationProxy) {
+    g_printerr("Error creating location dbus proxy: %s\n", error->message);
+    return NS_OK;  // fallback to MLS
+  }
+
+  // Listen to signals which will be send to us with the location data
+  mDBUSSignalHandler =
+      g_signal_connect(mDBUSLocationProxy, "g-signal",
+                       G_CALLBACK(location_updated_signal_cb), this);
+
+  // Call CreateSession of the location portal
+  GVariantBuilder builder;
+
+  nsAutoCString appName;
+  gAppData->GetDBusAppName(appName);
+  g_variant_builder_init(&builder, G_VARIANT_TYPE_VARDICT);
+  g_variant_builder_add(&builder, "{sv}", "session_handle_token",
+                        g_variant_new_string(appName.get()));
+
+  RefPtr<GVariant> result = dont_AddRef(g_dbus_proxy_call_sync(
+      mDBUSLocationProxy, "CreateSession", g_variant_new("(a{sv})", &builder),
+      G_DBUS_CALL_FLAGS_NONE, -1, nullptr, getter_Transfers(error)));
+
+  g_variant_builder_clear(&builder);
+
+  if (!result) {
+    g_printerr("Error calling CreateSession method: %s\n", error->message);
+    return NS_OK;  // fallback to MLS
+  }
+
+  // Start to listen to the location changes
+  g_variant_builder_init(&builder, G_VARIANT_TYPE_VARDICT);
+
+  // TODO Use wayland:handle as described in
+  // https://flatpak.github.io/xdg-desktop-portal/#parent_window
+  const gchar* parent_window = "";
+  gchar* portalSession;
+  g_variant_get_child(result, 0, "o", &portalSession);
+  mPortalSession.reset(portalSession);
+
+  result = g_dbus_proxy_call_sync(
+      mDBUSLocationProxy, "Start",
+      g_variant_new("(osa{sv})", mPortalSession.get(), parent_window, &builder),
+      G_DBUS_CALL_FLAGS_NONE, -1, nullptr, getter_Transfers(error));
+
+  g_variant_builder_clear(&builder);
+
+  if (!result) {
+    g_printerr("Error calling Start method: %s\n", error->message);
+    return NS_OK;  // fallback to MLS
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  mCallback = aCallback;
+
+  if (mLastGeoPositionCoords) {
+    // We cannot immediately call the Update there becase the window is not
+    // yet ready for that.
+    LOG_PORTAL(
+        "Update location in 1ms because we have the valid coords cached.");
+    SetRefreshTimer(1);
+    return NS_OK;
+  }
+
+  /* The MLS fallback will kick in after 12 seconds if portal
+   * doesn't provide location information within time. Once we
+   * see the first message from portal, the fallback will be
+   * disabled in |Update|.
+   */
+  mMLSProvider = MakeAndAddRef<MLSFallback>(12000);
+  mMLSProvider->Startup(new MLSGeolocationUpdate(aCallback));
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP PortalLocationProvider::GetName(nsACString& aName) {
+  aName.AssignLiteral("PortalLocationProvider");
+  return NS_OK;
+}
+
+void PortalLocationProvider::SetRefreshTimer(int aDelay) {
+  LOG_PORTAL("SetRefreshTimer for %p to %d ms\n", this, aDelay);
+  if (!mRefreshTimer) {
+    NS_NewTimerWithCallback(getter_AddRefs(mRefreshTimer), this, aDelay,
+                            nsITimer::TYPE_ONE_SHOT);
+  } else {
+    mRefreshTimer->Cancel();
+    mRefreshTimer->InitWithCallback(this, aDelay, nsITimer::TYPE_ONE_SHOT);
+  }
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::Notify(nsITimer* timer) {
+  // We need to reschedule the timer because we won't get any update
+  // from portal until the location is changed. That would cause
+  // watchPosition to fail with TIMEOUT error.
+  SetRefreshTimer(5000);
+  if (mLastGeoPositionCoords) {
+    LOG_PORTAL("Update location callback with latest coords.");
+    mCallback->Update(
+        new nsGeoPosition(mLastGeoPositionCoords, PR_Now() / PR_USEC_PER_MSEC));
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::Shutdown() {
+  LOG_PORTAL("Shutdown location provider");
+  if (mRefreshTimer) {
+    mRefreshTimer->Cancel();
+    mRefreshTimer = nullptr;
+  }
+  mLastGeoPositionCoords = nullptr;
+  if (mDBUSLocationProxy) {
+    g_signal_handler_disconnect(mDBUSLocationProxy, mDBUSSignalHandler);
+    LOG_PORTAL("calling Close method to the session interface...\n");
+    RefPtr<GDBusMessage> message = dont_AddRef(g_dbus_message_new_method_call(
+        kDesktopBusName, mPortalSession.get(), kSessionInterfaceName, "Close"));
+    mPortalSession = nullptr;
+    if (message) {
+      GUniquePtr<GError> error;
+      GDBusConnection* connection =
+          g_dbus_proxy_get_connection(mDBUSLocationProxy);
+      g_dbus_connection_send_message(
+          connection, message, G_DBUS_SEND_MESSAGE_FLAGS_NONE,
+          /*out_serial=*/nullptr, getter_Transfers(error));
+      if (error) {
+        g_printerr("Failed to close the session: %s\n", error->message);
+      }
+    }
+    mDBUSLocationProxy = nullptr;
+  }
+  if (mMLSProvider) {
+    mMLSProvider->Shutdown();
+    mMLSProvider = nullptr;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+PortalLocationProvider::SetHighAccuracy(bool aHigh) { return NS_OK; }
+
+}  // namespace mozilla::dom
diff --git a/dom/system/linux/PortalLocationProvider.h b/dom/system/linux/PortalLocationProvider.h
new file mode 100644
index 0000000000..e7ead0ab5c
--- /dev/null
+++ b/dom/system/linux/PortalLocationProvider.h
@@ -0,0 +1,54 @@
+/* -*- 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 PortalLocationProvider_h
+#define PortalLocationProvider_h
+
+#include "nsCOMPtr.h"
+#include "mozilla/GRefPtr.h"
+#include "mozilla/GUniquePtr.h"
+#include "Geolocation.h"
+#include "nsIGeolocationProvider.h"
+#include <gio/gio.h>
+
+class MLSFallback;
+
+namespace mozilla::dom {
+
+class PortalLocationProvider final : public nsIGeolocationProvider,
+                                     public nsITimerCallback,
+                                     public nsINamed {
+  class MLSGeolocationUpdate;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSINAMED
+
+  PortalLocationProvider();
+
+  void Update(nsIDOMGeoPosition* aPosition);
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  void NotifyError(int aError);
+
+ private:
+  ~PortalLocationProvider();
+  void SetRefreshTimer(int aDelay);
+
+  RefPtr<GDBusProxy> mDBUSLocationProxy;
+  gulong mDBUSSignalHandler = 0;
+
+  GUniquePtr<gchar> mPortalSession;
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+  RefPtr<MLSFallback> mMLSProvider;
+  nsCOMPtr<nsIDOMGeoPositionCoords> mLastGeoPositionCoords;
+  nsCOMPtr<nsITimer> mRefreshTimer;
+};
+
+}  // namespace mozilla::dom
+
+#endif /* GpsLocationProvider_h */
diff --git a/dom/system/linux/moz.build b/dom/system/linux/moz.build
new file mode 100644
index 0000000000..f33e81eb13
--- /dev/null
+++ b/dom/system/linux/moz.build
@@ -0,0 +1,24 @@
+# -*- Mode: python; c-basic-offset: 4; 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/.
+
+if CONFIG["MOZ_GPSD"]:
+    SOURCES += ["GpsdLocationProvider.cpp"]
+
+    CXXFLAGS += CONFIG["MOZ_GPSD_CFLAGS"]
+
+    OS_LIBS += CONFIG["MOZ_GPSD_LIBS"]
+
+    LOCAL_INCLUDES += ["/dom/geolocation"]
+
+
+if CONFIG["MOZ_ENABLE_DBUS"]:
+    SOURCES += ["GeoclueLocationProvider.cpp"]
+    SOURCES += ["PortalLocationProvider.cpp"]
+    LOCAL_INCLUDES += ["/dom/geolocation"]
+    CXXFLAGS += CONFIG["MOZ_DBUS_GLIB_CFLAGS"]
+    CXXFLAGS += CONFIG["MOZ_GTK3_CFLAGS"]
+
+FINAL_LIBRARY = "xul"
diff --git a/dom/system/mac/CoreLocationLocationProvider.h b/dom/system/mac/CoreLocationLocationProvider.h
new file mode 100644
index 0000000000..27b990cf9a
--- /dev/null
+++ b/dom/system/mac/CoreLocationLocationProvider.h
@@ -0,0 +1,61 @@
+/* -*- 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 "nsIGeolocationProvider.h"
+#include "mozilla/Attributes.h"
+
+/*
+ * The CoreLocationObjects class contains the CoreLocation objects
+ * we'll need.
+ *
+ * Declaring them directly in CoreLocationLocationProvider
+ * would require Objective-C++ syntax, which would contaminate all
+ * files that include this header and require them to be Objective-C++
+ * as well.
+ *
+ * The solution then is to forward-declare CoreLocationObjects here and
+ * hold a pointer to it in CoreLocationLocationProvider, and only actually
+ * define it in CoreLocationLocationProvider.mm, thus making it safe
+ * for Geolocation.cpp, which is C++-only, to include this header.
+ */
+class CoreLocationObjects;
+class MLSFallback;
+
+class CoreLocationLocationProvider : public nsIGeolocationProvider {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  CoreLocationLocationProvider();
+  // MOZ_CAN_RUN_SCRIPT_BOUNDARY because we can't mark Objective-C methods as
+  // MOZ_CAN_RUN_SCRIPT as far as I can tell, and this method is called from
+  // Objective-C.
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  void NotifyError(uint16_t aErrorCode);
+  void Update(nsIDOMGeoPosition* aSomewhere);
+  void CreateMLSFallbackProvider();
+  void CancelMLSFallbackProvider();
+
+ private:
+  virtual ~CoreLocationLocationProvider() = default;
+
+  CoreLocationObjects* mCLObjects;
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+  RefPtr<MLSFallback> mMLSFallbackProvider;
+
+  class MLSUpdate : public nsIGeolocationUpdate {
+   public:
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSIGEOLOCATIONUPDATE
+
+    explicit MLSUpdate(CoreLocationLocationProvider& parentProvider);
+
+   private:
+    CoreLocationLocationProvider& mParentLocationProvider;
+    virtual ~MLSUpdate() = default;
+  };
+};
diff --git a/dom/system/mac/CoreLocationLocationProvider.mm b/dom/system/mac/CoreLocationLocationProvider.mm
new file mode 100644
index 0000000000..781eac9d46
--- /dev/null
+++ b/dom/system/mac/CoreLocationLocationProvider.mm
@@ -0,0 +1,246 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "GeolocationPosition.h"
+#include "nsIConsoleService.h"
+#include "nsServiceManagerUtils.h"
+#include "CoreLocationLocationProvider.h"
+#include "nsCocoaFeatures.h"
+#include "prtime.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/GeolocationPositionErrorBinding.h"
+#include "MLSFallback.h"
+
+#include <CoreLocation/CLError.h>
+#include <CoreLocation/CLLocation.h>
+#include <CoreLocation/CLLocationManager.h>
+#include <CoreLocation/CLLocationManagerDelegate.h>
+
+#include <objc/objc.h>
+#include <objc/objc-runtime.h>
+
+#include "nsObjCExceptions.h"
+
+using namespace mozilla;
+
+static const CLLocationAccuracy kHIGH_ACCURACY = kCLLocationAccuracyBest;
+static const CLLocationAccuracy kDEFAULT_ACCURACY = kCLLocationAccuracyNearestTenMeters;
+
+@interface LocationDelegate : NSObject <CLLocationManagerDelegate> {
+  CoreLocationLocationProvider* mProvider;
+}
+
+- (id)init:(CoreLocationLocationProvider*)aProvider;
+- (void)locationManager:(CLLocationManager*)aManager didFailWithError:(NSError*)aError;
+- (void)locationManager:(CLLocationManager*)aManager didUpdateLocations:(NSArray*)locations;
+
+@end
+
+@implementation LocationDelegate
+- (id)init:(CoreLocationLocationProvider*)aProvider {
+  if ((self = [super init])) {
+    mProvider = aProvider;
+  }
+
+  return self;
+}
+
+- (void)locationManager:(CLLocationManager*)aManager didFailWithError:(NSError*)aError {
+  nsCOMPtr<nsIConsoleService> console = do_GetService(NS_CONSOLESERVICE_CONTRACTID);
+
+  NS_ENSURE_TRUE_VOID(console);
+
+  NSString* message =
+      [@"Failed to acquire position: " stringByAppendingString:[aError localizedDescription]];
+
+  console->LogStringMessage(NS_ConvertUTF8toUTF16([message UTF8String]).get());
+
+  if ([aError code] == kCLErrorDenied) {
+    mProvider->NotifyError(dom::GeolocationPositionError_Binding::PERMISSION_DENIED);
+    return;
+  }
+
+  // The CL provider does not fallback to GeoIP, so use NetworkGeolocationProvider for this.
+  // The concept here is: on error, hand off geolocation to MLS, which will then report
+  // back a location or error.
+  mProvider->CreateMLSFallbackProvider();
+}
+
+- (void)locationManager:(CLLocationManager*)aManager didUpdateLocations:(NSArray*)aLocations {
+  if (aLocations.count < 1) {
+    return;
+  }
+
+  mProvider->CancelMLSFallbackProvider();
+
+  CLLocation* location = [aLocations objectAtIndex:0];
+
+  double altitude;
+  double altitudeAccuracy;
+
+  // A negative verticalAccuracy indicates that the altitude value is invalid.
+  if (location.verticalAccuracy >= 0) {
+    altitude = location.altitude;
+    altitudeAccuracy = location.verticalAccuracy;
+  } else {
+    altitude = UnspecifiedNaN<double>();
+    altitudeAccuracy = UnspecifiedNaN<double>();
+  }
+
+  double speed = location.speed >= 0 ? location.speed : UnspecifiedNaN<double>();
+
+  double heading = location.course >= 0 ? location.course : UnspecifiedNaN<double>();
+
+  // nsGeoPositionCoords will convert NaNs to null for optional properties of
+  // the JavaScript Coordinates object.
+  nsCOMPtr<nsIDOMGeoPosition> geoPosition = new nsGeoPosition(
+      location.coordinate.latitude, location.coordinate.longitude, altitude,
+      location.horizontalAccuracy, altitudeAccuracy, heading, speed, PR_Now() / PR_USEC_PER_MSEC);
+
+  mProvider->Update(geoPosition);
+  Telemetry::Accumulate(Telemetry::GEOLOCATION_OSX_SOURCE_IS_MLS, false);
+}
+@end
+
+NS_IMPL_ISUPPORTS(CoreLocationLocationProvider::MLSUpdate, nsIGeolocationUpdate);
+
+CoreLocationLocationProvider::MLSUpdate::MLSUpdate(CoreLocationLocationProvider& parentProvider)
+    : mParentLocationProvider(parentProvider) {}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::MLSUpdate::Update(nsIDOMGeoPosition* position) {
+  nsCOMPtr<nsIDOMGeoPositionCoords> coords;
+  position->GetCoords(getter_AddRefs(coords));
+  if (!coords) {
+    return NS_ERROR_FAILURE;
+  }
+  mParentLocationProvider.Update(position);
+  Telemetry::Accumulate(Telemetry::GEOLOCATION_OSX_SOURCE_IS_MLS, true);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::MLSUpdate::NotifyError(uint16_t error) {
+  mParentLocationProvider.NotifyError(error);
+  return NS_OK;
+}
+
+class CoreLocationObjects {
+ public:
+  nsresult Init(CoreLocationLocationProvider* aProvider) {
+    mLocationManager = [[CLLocationManager alloc] init];
+    NS_ENSURE_TRUE(mLocationManager, NS_ERROR_NOT_AVAILABLE);
+
+    mLocationDelegate = [[LocationDelegate alloc] init:aProvider];
+    NS_ENSURE_TRUE(mLocationDelegate, NS_ERROR_NOT_AVAILABLE);
+
+    mLocationManager.desiredAccuracy = kDEFAULT_ACCURACY;
+    mLocationManager.delegate = mLocationDelegate;
+
+    return NS_OK;
+  }
+
+  ~CoreLocationObjects() {
+    if (mLocationManager) {
+      [mLocationManager release];
+    }
+
+    if (mLocationDelegate) {
+      [mLocationDelegate release];
+    }
+  }
+
+  LocationDelegate* mLocationDelegate;
+  CLLocationManager* mLocationManager;
+};
+
+NS_IMPL_ISUPPORTS(CoreLocationLocationProvider, nsIGeolocationProvider)
+
+CoreLocationLocationProvider::CoreLocationLocationProvider()
+    : mCLObjects(nullptr), mMLSFallbackProvider(nullptr) {}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::Startup() {
+  if (!mCLObjects) {
+    auto clObjs = MakeUnique<CoreLocationObjects>();
+
+    nsresult rv = clObjs->Init(this);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    mCLObjects = clObjs.release();
+  }
+
+  // Must be stopped before starting or response (success or failure) is not guaranteed
+  [mCLObjects->mLocationManager stopUpdatingLocation];
+  [mCLObjects->mLocationManager startUpdatingLocation];
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  if (mCallback) {
+    return NS_OK;
+  }
+
+  mCallback = aCallback;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::Shutdown() {
+  NS_ENSURE_STATE(mCLObjects);
+
+  [mCLObjects->mLocationManager stopUpdatingLocation];
+
+  delete mCLObjects;
+  mCLObjects = nullptr;
+
+  if (mMLSFallbackProvider) {
+    mMLSFallbackProvider->Shutdown();
+    mMLSFallbackProvider = nullptr;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+CoreLocationLocationProvider::SetHighAccuracy(bool aEnable) {
+  NS_ENSURE_STATE(mCLObjects);
+
+  mCLObjects->mLocationManager.desiredAccuracy = (aEnable ? kHIGH_ACCURACY : kDEFAULT_ACCURACY);
+
+  return NS_OK;
+}
+
+void CoreLocationLocationProvider::Update(nsIDOMGeoPosition* aSomewhere) {
+  if (aSomewhere && mCallback) {
+    mCallback->Update(aSomewhere);
+  }
+}
+void CoreLocationLocationProvider::NotifyError(uint16_t aErrorCode) {
+  nsCOMPtr<nsIGeolocationUpdate> callback(mCallback);
+  callback->NotifyError(aErrorCode);
+}
+void CoreLocationLocationProvider::CreateMLSFallbackProvider() {
+  if (mMLSFallbackProvider) {
+    return;
+  }
+
+  mMLSFallbackProvider = new MLSFallback(0);
+  mMLSFallbackProvider->Startup(new MLSUpdate(*this));
+}
+
+void CoreLocationLocationProvider::CancelMLSFallbackProvider() {
+  if (!mMLSFallbackProvider) {
+    return;
+  }
+
+  mMLSFallbackProvider->Shutdown();
+  mMLSFallbackProvider = nullptr;
+}
diff --git a/dom/system/mac/moz.build b/dom/system/mac/moz.build
new file mode 100644
index 0000000000..6a10090793
--- /dev/null
+++ b/dom/system/mac/moz.build
@@ -0,0 +1,21 @@
+# -*- 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/.
+
+SOURCES += [
+    "CoreLocationLocationProvider.mm",
+    "nsOSPermissionRequest.mm",
+]
+
+EXPORTS += [
+    "nsOSPermissionRequest.h",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+LOCAL_INCLUDES += [
+    "/dom/geolocation",
+]
diff --git a/dom/system/mac/nsOSPermissionRequest.h b/dom/system/mac/nsOSPermissionRequest.h
new file mode 100644
index 0000000000..62e4360fee
--- /dev/null
+++ b/dom/system/mac/nsOSPermissionRequest.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 nsOSPermissionRequest_h__
+#define nsOSPermissionRequest_h__
+
+#include "nsOSPermissionRequestBase.h"
+
+class nsOSPermissionRequest : public nsOSPermissionRequestBase {
+ public:
+  nsOSPermissionRequest(){};
+
+  NS_IMETHOD GetAudioCapturePermissionState(uint16_t* aAudio) override;
+
+  NS_IMETHOD GetVideoCapturePermissionState(uint16_t* aVideo) override;
+
+  NS_IMETHOD GetScreenCapturePermissionState(uint16_t* aScreen) override;
+
+  NS_IMETHOD RequestVideoCapturePermission(
+      JSContext* aCx, mozilla::dom::Promise** aPromiseOut) override;
+
+  NS_IMETHOD RequestAudioCapturePermission(
+      JSContext* aCx, mozilla::dom::Promise** aPromiseOut) override;
+
+  NS_IMETHOD MaybeRequestScreenCapturePermission() override;
+};
+
+#endif
diff --git a/dom/system/mac/nsOSPermissionRequest.mm b/dom/system/mac/nsOSPermissionRequest.mm
new file mode 100644
index 0000000000..ccb4516fdc
--- /dev/null
+++ b/dom/system/mac/nsOSPermissionRequest.mm
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsOSPermissionRequest.h"
+
+#include "mozilla/dom/Promise.h"
+#include "nsCocoaFeatures.h"
+#include "nsCocoaUtils.h"
+
+using namespace mozilla;
+
+using mozilla::dom::Promise;
+
+NS_IMETHODIMP
+nsOSPermissionRequest::GetAudioCapturePermissionState(uint16_t* aAudio) {
+  MOZ_ASSERT(aAudio);
+
+  if (!nsCocoaFeatures::OnMojaveOrLater()) {
+    return nsOSPermissionRequestBase::GetAudioCapturePermissionState(aAudio);
+  }
+
+  return nsCocoaUtils::GetAudioCapturePermissionState(*aAudio);
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequest::GetVideoCapturePermissionState(uint16_t* aVideo) {
+  MOZ_ASSERT(aVideo);
+
+  if (!nsCocoaFeatures::OnMojaveOrLater()) {
+    return nsOSPermissionRequestBase::GetVideoCapturePermissionState(aVideo);
+  }
+
+  return nsCocoaUtils::GetVideoCapturePermissionState(*aVideo);
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequest::GetScreenCapturePermissionState(uint16_t* aScreen) {
+  MOZ_ASSERT(aScreen);
+
+  if (!nsCocoaFeatures::OnCatalinaOrLater()) {
+    return nsOSPermissionRequestBase::GetScreenCapturePermissionState(aScreen);
+  }
+
+  return nsCocoaUtils::GetScreenCapturePermissionState(*aScreen);
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequest::RequestVideoCapturePermission(JSContext* aCx, Promise** aPromiseOut) {
+  if (!nsCocoaFeatures::OnMojaveOrLater()) {
+    return nsOSPermissionRequestBase::RequestVideoCapturePermission(aCx, aPromiseOut);
+  }
+
+  RefPtr<Promise> promiseHandle;
+  nsresult rv = GetPromise(aCx, promiseHandle);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  rv = nsCocoaUtils::RequestVideoCapturePermission(promiseHandle);
+  promiseHandle.forget(aPromiseOut);
+  return rv;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequest::RequestAudioCapturePermission(JSContext* aCx, Promise** aPromiseOut) {
+  if (!nsCocoaFeatures::OnMojaveOrLater()) {
+    return nsOSPermissionRequestBase::RequestAudioCapturePermission(aCx, aPromiseOut);
+  }
+
+  RefPtr<Promise> promiseHandle;
+  nsresult rv = GetPromise(aCx, promiseHandle);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  rv = nsCocoaUtils::RequestAudioCapturePermission(promiseHandle);
+  promiseHandle.forget(aPromiseOut);
+  return rv;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequest::MaybeRequestScreenCapturePermission() {
+  if (!nsCocoaFeatures::OnCatalinaOrLater()) {
+    return nsOSPermissionRequestBase::MaybeRequestScreenCapturePermission();
+  }
+
+  return nsCocoaUtils::MaybeRequestScreenCapturePermission();
+}
diff --git a/dom/system/moz.build b/dom/system/moz.build
new file mode 100644
index 0000000000..240c56a3f2
--- /dev/null
+++ b/dom/system/moz.build
@@ -0,0 +1,122 @@
+# -*- 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/.
+
+# This picks up *hapticfeedback* which is graveyard
+with Files("**"):
+    BUG_COMPONENT = ("Core", "DOM: Core & HTML")
+
+with Files("*OSFile*"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("*ocationProvider*"):
+    BUG_COMPONENT = ("Core", "DOM: Geolocation")
+
+with Files("windows/*LocationProvider*"):
+    BUG_COMPONENT = ("Core", "DOM: Geolocation")
+
+with Files("IOUtils*"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("PathUtils*"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("mac/*LocationProvider*"):
+    BUG_COMPONENT = ("Core", "DOM: Geolocation")
+
+with Files("mac/*OSPermissionRequest*"):
+    BUG_COMPONENT = ("Firefox", "Site Permissions")
+
+with Files("linux/*LocationProvider*"):
+    BUG_COMPONENT = ("Core", "DOM: Geolocation")
+
+with Files("android/*LocationProvider*"):
+    BUG_COMPONENT = ("Core", "DOM: Geolocation")
+
+with Files("tests/chrome.ini"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("tests/*constants*"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("tests/ioutils/**"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("tests/mochitest.ini"):
+    BUG_COMPONENT = ("Core", "DOM: Device Interfaces")
+
+with Files("test/*pathutils*"):
+    BUG_COMPONENT = ("Toolkit", "OS.File")
+
+with Files("tests/*1197901*"):
+    BUG_COMPONENT = ("Core", "DOM: Device Interfaces")
+
+
+toolkit = CONFIG["MOZ_WIDGET_TOOLKIT"]
+
+if toolkit == "windows":
+    DIRS += ["windows"]
+elif toolkit == "cocoa":
+    DIRS += ["mac"]
+elif toolkit == "android":
+    DIRS += ["android"]
+elif toolkit == "gtk":
+    DIRS += ["linux"]
+
+if toolkit != "cocoa":
+    EXPORTS += [
+        "nsOSPermissionRequest.h",
+    ]
+
+XPIDL_SOURCES += [
+    "nsIOSFileConstantsService.idl",
+    "nsIOSPermissionRequest.idl",
+]
+
+XPIDL_MODULE = "dom_system"
+
+EXPORTS += [
+    "nsDeviceSensors.h",
+    "nsOSPermissionRequestBase.h",
+]
+
+EXPORTS.mozilla += [
+    "OSFileConstants.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "IOUtils.h",
+    "PathUtils.h",
+]
+
+UNIFIED_SOURCES += [
+    "IOUtils.cpp",
+    "nsDeviceSensors.cpp",
+    "nsOSPermissionRequestBase.cpp",
+    "OSFileConstants.cpp",
+    "PathUtils.cpp",
+]
+
+EXTRA_JS_MODULES += [
+    "NetworkGeolocationProvider.jsm",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+# We fire the nsDOMDeviceAcceleration
+LOCAL_INCLUDES += [
+    "/dom/base",
+    "/dom/bindings",
+    "/js/xpconnect/loader",
+    "/xpcom/base",
+]
+
+MOCHITEST_CHROME_MANIFESTS += ["tests/chrome.ini", "tests/ioutils/chrome.ini"]
+MOCHITEST_MANIFESTS += ["tests/mochitest.ini"]
diff --git a/dom/system/nsDeviceSensors.cpp b/dom/system/nsDeviceSensors.cpp
new file mode 100644
index 0000000000..5b8250ae7e
--- /dev/null
+++ b/dom/system/nsDeviceSensors.cpp
@@ -0,0 +1,556 @@
+/* -*- 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/Hal.h"
+#include "mozilla/HalSensor.h"
+
+#include "nsContentUtils.h"
+#include "nsDeviceSensors.h"
+
+#include "nsGlobalWindowInner.h"
+#include "nsPIDOMWindow.h"
+#include "nsIScriptObjectPrincipal.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_device.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/BrowsingContext.h"
+#include "mozilla/dom/DeviceLightEvent.h"
+#include "mozilla/dom/DeviceOrientationEvent.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/UserProximityEvent.h"
+#include "mozilla/ErrorResult.h"
+
+#include <cmath>
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using namespace hal;
+
+class nsIDOMWindow;
+
+#undef near
+
+#define DEFAULT_SENSOR_POLL 100
+
+static const nsTArray<nsIDOMWindow*>::index_type NoIndex =
+    nsTArray<nsIDOMWindow*>::NoIndex;
+
+class nsDeviceSensorData final : public nsIDeviceSensorData {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIDEVICESENSORDATA
+
+  nsDeviceSensorData(unsigned long type, double x, double y, double z);
+
+ private:
+  ~nsDeviceSensorData();
+
+ protected:
+  unsigned long mType;
+  double mX, mY, mZ;
+};
+
+nsDeviceSensorData::nsDeviceSensorData(unsigned long type, double x, double y,
+                                       double z)
+    : mType(type), mX(x), mY(y), mZ(z) {}
+
+NS_INTERFACE_MAP_BEGIN(nsDeviceSensorData)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIDeviceSensorData)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_ADDREF(nsDeviceSensorData)
+NS_IMPL_RELEASE(nsDeviceSensorData)
+
+nsDeviceSensorData::~nsDeviceSensorData() = default;
+
+NS_IMETHODIMP nsDeviceSensorData::GetType(uint32_t* aType) {
+  NS_ENSURE_ARG_POINTER(aType);
+  *aType = mType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsDeviceSensorData::GetX(double* aX) {
+  NS_ENSURE_ARG_POINTER(aX);
+  *aX = mX;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsDeviceSensorData::GetY(double* aY) {
+  NS_ENSURE_ARG_POINTER(aY);
+  *aY = mY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsDeviceSensorData::GetZ(double* aZ) {
+  NS_ENSURE_ARG_POINTER(aZ);
+  *aZ = mZ;
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(nsDeviceSensors, nsIDeviceSensors)
+
+nsDeviceSensors::nsDeviceSensors() {
+  mIsUserProximityNear = false;
+  mLastDOMMotionEventTime = TimeStamp::Now();
+
+  for (int i = 0; i < NUM_SENSOR_TYPE; i++) {
+    nsTArray<nsIDOMWindow*>* windows = new nsTArray<nsIDOMWindow*>();
+    mWindowListeners.AppendElement(windows);
+  }
+
+  mLastDOMMotionEventTime = TimeStamp::Now();
+}
+
+nsDeviceSensors::~nsDeviceSensors() {
+  for (int i = 0; i < NUM_SENSOR_TYPE; i++) {
+    if (IsSensorEnabled(i)) UnregisterSensorObserver((SensorType)i, this);
+  }
+
+  for (int i = 0; i < NUM_SENSOR_TYPE; i++) {
+    delete mWindowListeners[i];
+  }
+}
+
+NS_IMETHODIMP nsDeviceSensors::HasWindowListener(uint32_t aType,
+                                                 nsIDOMWindow* aWindow,
+                                                 bool* aRetVal) {
+  if (!IsSensorAllowedByPref(aType, aWindow))
+    *aRetVal = false;
+  else
+    *aRetVal = mWindowListeners[aType]->IndexOf(aWindow) != NoIndex;
+
+  return NS_OK;
+}
+
+class DeviceSensorTestEvent : public Runnable {
+ public:
+  DeviceSensorTestEvent(nsDeviceSensors* aTarget, uint32_t aType)
+      : mozilla::Runnable("DeviceSensorTestEvent"),
+        mTarget(aTarget),
+        mType(aType) {}
+
+  NS_IMETHOD Run() override {
+    SensorData sensorData;
+    sensorData.sensor() = static_cast<SensorType>(mType);
+    sensorData.timestamp() = PR_Now();
+    sensorData.values().AppendElement(0.5f);
+    sensorData.values().AppendElement(0.5f);
+    sensorData.values().AppendElement(0.5f);
+    sensorData.values().AppendElement(0.5f);
+    mTarget->Notify(sensorData);
+    return NS_OK;
+  }
+
+ private:
+  RefPtr<nsDeviceSensors> mTarget;
+  uint32_t mType;
+};
+
+NS_IMETHODIMP nsDeviceSensors::AddWindowListener(uint32_t aType,
+                                                 nsIDOMWindow* aWindow) {
+  if (!IsSensorAllowedByPref(aType, aWindow)) return NS_OK;
+
+  if (mWindowListeners[aType]->IndexOf(aWindow) != NoIndex) return NS_OK;
+
+  if (!IsSensorEnabled(aType)) {
+    RegisterSensorObserver((SensorType)aType, this);
+  }
+
+  mWindowListeners[aType]->AppendElement(aWindow);
+
+  if (StaticPrefs::device_sensors_test_events()) {
+    nsCOMPtr<nsIRunnable> event = new DeviceSensorTestEvent(this, aType);
+    NS_DispatchToCurrentThread(event);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsDeviceSensors::RemoveWindowListener(uint32_t aType,
+                                                    nsIDOMWindow* aWindow) {
+  if (mWindowListeners[aType]->IndexOf(aWindow) == NoIndex) return NS_OK;
+
+  mWindowListeners[aType]->RemoveElement(aWindow);
+
+  if (mWindowListeners[aType]->Length() == 0)
+    UnregisterSensorObserver((SensorType)aType, this);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsDeviceSensors::RemoveWindowAsListener(nsIDOMWindow* aWindow) {
+  for (int i = 0; i < NUM_SENSOR_TYPE; i++) {
+    RemoveWindowListener((SensorType)i, aWindow);
+  }
+  return NS_OK;
+}
+
+static bool WindowCannotReceiveSensorEvent(nsPIDOMWindowInner* aWindow) {
+  // Check to see if this window is in the background.
+  if (!aWindow || !aWindow->IsCurrentInnerWindow()) {
+    return true;
+  }
+
+  nsPIDOMWindowOuter* windowOuter = aWindow->GetOuterWindow();
+  BrowsingContext* topBC = aWindow->GetBrowsingContext()->Top();
+  if (windowOuter->IsBackground() || !topBC->GetIsActiveBrowserWindow()) {
+    return true;
+  }
+
+  // Check to see if this window is a cross-origin iframe:
+  if (!topBC->IsInProcess()) {
+    return true;
+  }
+
+  nsCOMPtr<nsIScriptObjectPrincipal> sop = do_QueryInterface(aWindow);
+  nsCOMPtr<nsIScriptObjectPrincipal> topSop =
+      do_QueryInterface(topBC->GetDOMWindow());
+  if (!sop || !topSop) {
+    return true;
+  }
+
+  nsIPrincipal* principal = sop->GetPrincipal();
+  nsIPrincipal* topPrincipal = topSop->GetPrincipal();
+  if (!principal || !topPrincipal) {
+    return true;
+  }
+
+  return !principal->Subsumes(topPrincipal);
+}
+
+// Holds the device orientation in Euler angle degrees (azimuth, pitch, roll).
+struct Orientation {
+  enum OrientationReference { kRelative = 0, kAbsolute };
+
+  static Orientation RadToDeg(const Orientation& aOrient) {
+    const static double kRadToDeg = 180.0 / M_PI;
+    return {aOrient.alpha * kRadToDeg, aOrient.beta * kRadToDeg,
+            aOrient.gamma * kRadToDeg};
+  }
+
+  double alpha;
+  double beta;
+  double gamma;
+};
+
+static Orientation RotationVectorToOrientation(double aX, double aY, double aZ,
+                                               double aW) {
+  double mat[9];
+
+  mat[0] = 1 - 2 * aY * aY - 2 * aZ * aZ;
+  mat[1] = 2 * aX * aY - 2 * aZ * aW;
+  mat[2] = 2 * aX * aZ + 2 * aY * aW;
+
+  mat[3] = 2 * aX * aY + 2 * aZ * aW;
+  mat[4] = 1 - 2 * aX * aX - 2 * aZ * aZ;
+  mat[5] = 2 * aY * aZ - 2 * aX * aW;
+
+  mat[6] = 2 * aX * aZ - 2 * aY * aW;
+  mat[7] = 2 * aY * aZ + 2 * aX * aW;
+  mat[8] = 1 - 2 * aX * aX - 2 * aY * aY;
+
+  Orientation orient;
+
+  if (mat[8] > 0) {
+    orient.alpha = atan2(-mat[1], mat[4]);
+    orient.beta = asin(mat[7]);
+    orient.gamma = atan2(-mat[6], mat[8]);
+  } else if (mat[8] < 0) {
+    orient.alpha = atan2(mat[1], -mat[4]);
+    orient.beta = -asin(mat[7]);
+    orient.beta += (orient.beta >= 0) ? -M_PI : M_PI;
+    orient.gamma = atan2(mat[6], -mat[8]);
+  } else {
+    if (mat[6] > 0) {
+      orient.alpha = atan2(-mat[1], mat[4]);
+      orient.beta = asin(mat[7]);
+      orient.gamma = -M_PI_2;
+    } else if (mat[6] < 0) {
+      orient.alpha = atan2(mat[1], -mat[4]);
+      orient.beta = -asin(mat[7]);
+      orient.beta += (orient.beta >= 0) ? -M_PI : M_PI;
+      orient.gamma = -M_PI_2;
+    } else {
+      orient.alpha = atan2(mat[3], mat[0]);
+      orient.beta = (mat[7] > 0) ? M_PI_2 : -M_PI_2;
+      orient.gamma = 0;
+    }
+  }
+
+  if (orient.alpha < 0) {
+    orient.alpha += 2 * M_PI;
+  }
+
+  return Orientation::RadToDeg(orient);
+}
+
+void nsDeviceSensors::Notify(const mozilla::hal::SensorData& aSensorData) {
+  uint32_t type = aSensorData.sensor();
+
+  const nsTArray<float>& values = aSensorData.values();
+  size_t len = values.Length();
+  double x = len > 0 ? values[0] : 0.0;
+  double y = len > 1 ? values[1] : 0.0;
+  double z = len > 2 ? values[2] : 0.0;
+  double w = len > 3 ? values[3] : 0.0;
+  PRTime timestamp = aSensorData.timestamp();
+
+  nsCOMArray<nsIDOMWindow> windowListeners;
+  for (uint32_t i = 0; i < mWindowListeners[type]->Length(); i++) {
+    windowListeners.AppendObject(mWindowListeners[type]->SafeElementAt(i));
+  }
+
+  for (uint32_t i = windowListeners.Count(); i > 0;) {
+    --i;
+
+    nsCOMPtr<nsPIDOMWindowInner> pwindow =
+        do_QueryInterface(windowListeners[i]);
+    if (WindowCannotReceiveSensorEvent(pwindow)) {
+      continue;
+    }
+
+    if (nsCOMPtr<Document> doc = pwindow->GetDoc()) {
+      nsCOMPtr<mozilla::dom::EventTarget> target =
+          do_QueryInterface(windowListeners[i]);
+      if (type == nsIDeviceSensorData::TYPE_ACCELERATION ||
+          type == nsIDeviceSensorData::TYPE_LINEAR_ACCELERATION ||
+          type == nsIDeviceSensorData::TYPE_GYROSCOPE) {
+        FireDOMMotionEvent(doc, target, type, timestamp, x, y, z);
+      } else if (type == nsIDeviceSensorData::TYPE_ORIENTATION) {
+        FireDOMOrientationEvent(target, x, y, z, Orientation::kAbsolute);
+      } else if (type == nsIDeviceSensorData::TYPE_ROTATION_VECTOR) {
+        const Orientation orient = RotationVectorToOrientation(x, y, z, w);
+        FireDOMOrientationEvent(target, orient.alpha, orient.beta, orient.gamma,
+                                Orientation::kAbsolute);
+      } else if (type == nsIDeviceSensorData::TYPE_GAME_ROTATION_VECTOR) {
+        const Orientation orient = RotationVectorToOrientation(x, y, z, w);
+        FireDOMOrientationEvent(target, orient.alpha, orient.beta, orient.gamma,
+                                Orientation::kRelative);
+      } else if (type == nsIDeviceSensorData::TYPE_PROXIMITY) {
+        MaybeFireDOMUserProximityEvent(target, x, z);
+      } else if (type == nsIDeviceSensorData::TYPE_LIGHT) {
+        FireDOMLightEvent(target, x);
+      }
+    }
+  }
+}
+
+void nsDeviceSensors::FireDOMLightEvent(mozilla::dom::EventTarget* aTarget,
+                                        double aValue) {
+  DeviceLightEventInit init;
+  init.mBubbles = true;
+  init.mCancelable = false;
+  init.mValue = round(aValue);
+  RefPtr<DeviceLightEvent> event =
+      DeviceLightEvent::Constructor(aTarget, u"devicelight"_ns, init);
+
+  event->SetTrusted(true);
+
+  aTarget->DispatchEvent(*event);
+}
+
+void nsDeviceSensors::MaybeFireDOMUserProximityEvent(
+    mozilla::dom::EventTarget* aTarget, double aValue, double aMax) {
+  bool near = (aValue < aMax);
+  if (mIsUserProximityNear != near) {
+    mIsUserProximityNear = near;
+    FireDOMUserProximityEvent(aTarget, mIsUserProximityNear);
+  }
+}
+
+void nsDeviceSensors::FireDOMUserProximityEvent(
+    mozilla::dom::EventTarget* aTarget, bool aNear) {
+  UserProximityEventInit init;
+  init.mBubbles = true;
+  init.mCancelable = false;
+  init.mNear = aNear;
+  RefPtr<UserProximityEvent> event =
+      UserProximityEvent::Constructor(aTarget, u"userproximity"_ns, init);
+
+  event->SetTrusted(true);
+
+  aTarget->DispatchEvent(*event);
+}
+
+void nsDeviceSensors::FireDOMOrientationEvent(EventTarget* aTarget,
+                                              double aAlpha, double aBeta,
+                                              double aGamma, bool aIsAbsolute) {
+  DeviceOrientationEventInit init;
+  init.mBubbles = true;
+  init.mCancelable = false;
+  init.mAlpha.SetValue(aAlpha);
+  init.mBeta.SetValue(aBeta);
+  init.mGamma.SetValue(aGamma);
+  init.mAbsolute = aIsAbsolute;
+
+  auto Dispatch = [&](EventTarget* aEventTarget, const nsAString& aType) {
+    RefPtr<DeviceOrientationEvent> event =
+        DeviceOrientationEvent::Constructor(aEventTarget, aType, init);
+    event->SetTrusted(true);
+    aEventTarget->DispatchEvent(*event);
+  };
+
+  Dispatch(aTarget, aIsAbsolute ? u"deviceorientationabsolute"_ns
+                                : u"deviceorientation"_ns);
+
+  // This is used to determine whether relative events have been dispatched
+  // during the current session, in which case we don't dispatch the additional
+  // compatibility events.
+  static bool sIsDispatchingRelativeEvents = false;
+  sIsDispatchingRelativeEvents = sIsDispatchingRelativeEvents || !aIsAbsolute;
+
+  // Android devices with SENSOR_GAME_ROTATION_VECTOR support dispatch
+  // relative events for "deviceorientation" by default, while other platforms
+  // and devices without such support dispatch absolute events by default.
+  if (aIsAbsolute && !sIsDispatchingRelativeEvents) {
+    // For absolute events on devices without support for relative events,
+    // we need to additionally dispatch type "deviceorientation" to keep
+    // backwards-compatibility.
+    Dispatch(aTarget, u"deviceorientation"_ns);
+  }
+}
+
+void nsDeviceSensors::FireDOMMotionEvent(Document* doc, EventTarget* target,
+                                         uint32_t type, PRTime timestamp,
+                                         double x, double y, double z) {
+  // Attempt to coalesce events
+  TimeDuration sensorPollDuration =
+      TimeDuration::FromMilliseconds(DEFAULT_SENSOR_POLL);
+  bool fireEvent =
+      (TimeStamp::Now() > mLastDOMMotionEventTime + sensorPollDuration) ||
+      StaticPrefs::device_sensors_test_events();
+
+  switch (type) {
+    case nsIDeviceSensorData::TYPE_LINEAR_ACCELERATION:
+      if (!mLastAcceleration) {
+        mLastAcceleration.emplace();
+      }
+      mLastAcceleration->mX.SetValue(x);
+      mLastAcceleration->mY.SetValue(y);
+      mLastAcceleration->mZ.SetValue(z);
+      break;
+    case nsIDeviceSensorData::TYPE_ACCELERATION:
+      if (!mLastAccelerationIncludingGravity) {
+        mLastAccelerationIncludingGravity.emplace();
+      }
+      mLastAccelerationIncludingGravity->mX.SetValue(x);
+      mLastAccelerationIncludingGravity->mY.SetValue(y);
+      mLastAccelerationIncludingGravity->mZ.SetValue(z);
+      break;
+    case nsIDeviceSensorData::TYPE_GYROSCOPE:
+      if (!mLastRotationRate) {
+        mLastRotationRate.emplace();
+      }
+      mLastRotationRate->mAlpha.SetValue(x);
+      mLastRotationRate->mBeta.SetValue(y);
+      mLastRotationRate->mGamma.SetValue(z);
+      break;
+  }
+
+  if (fireEvent) {
+    if (!mLastAcceleration) {
+      mLastAcceleration.emplace();
+    }
+    if (!mLastAccelerationIncludingGravity) {
+      mLastAccelerationIncludingGravity.emplace();
+    }
+    if (!mLastRotationRate) {
+      mLastRotationRate.emplace();
+    }
+  } else if (!mLastAcceleration || !mLastAccelerationIncludingGravity ||
+             !mLastRotationRate) {
+    return;
+  }
+
+  IgnoredErrorResult ignored;
+  RefPtr<Event> event =
+      doc->CreateEvent(u"DeviceMotionEvent"_ns, CallerType::System, ignored);
+  if (!event) {
+    return;
+  }
+
+  DeviceMotionEvent* me = static_cast<DeviceMotionEvent*>(event.get());
+
+  me->InitDeviceMotionEvent(
+      u"devicemotion"_ns, true, false, *mLastAcceleration,
+      *mLastAccelerationIncludingGravity, *mLastRotationRate,
+      Nullable<double>(DEFAULT_SENSOR_POLL), Nullable<uint64_t>(timestamp));
+
+  event->SetTrusted(true);
+
+  target->DispatchEvent(*event);
+
+  mLastRotationRate.reset();
+  mLastAccelerationIncludingGravity.reset();
+  mLastAcceleration.reset();
+  mLastDOMMotionEventTime = TimeStamp::Now();
+}
+
+bool nsDeviceSensors::IsSensorAllowedByPref(uint32_t aType,
+                                            nsIDOMWindow* aWindow) {
+  // checks "device.sensors.enabled" master pref
+  if (!StaticPrefs::device_sensors_enabled()) {
+    return false;
+  }
+
+  nsCOMPtr<nsPIDOMWindowInner> window = do_QueryInterface(aWindow);
+  nsCOMPtr<Document> doc;
+  if (window) {
+    doc = window->GetExtantDoc();
+  }
+
+  switch (aType) {
+    case nsIDeviceSensorData::TYPE_LINEAR_ACCELERATION:
+    case nsIDeviceSensorData::TYPE_ACCELERATION:
+    case nsIDeviceSensorData::TYPE_GYROSCOPE:
+      // checks "device.sensors.motion.enabled" pref
+      if (!StaticPrefs::device_sensors_motion_enabled()) {
+        return false;
+      }
+      if (doc) {
+        doc->WarnOnceAbout(DeprecatedOperations::eMotionEvent);
+      }
+      break;
+    case nsIDeviceSensorData::TYPE_GAME_ROTATION_VECTOR:
+    case nsIDeviceSensorData::TYPE_ORIENTATION:
+    case nsIDeviceSensorData::TYPE_ROTATION_VECTOR:
+      // checks "device.sensors.orientation.enabled" pref
+      if (!StaticPrefs::device_sensors_orientation_enabled()) {
+        return false;
+      }
+      if (doc) {
+        doc->WarnOnceAbout(DeprecatedOperations::eOrientationEvent);
+      }
+      break;
+    case nsIDeviceSensorData::TYPE_PROXIMITY:
+      // checks "device.sensors.proximity.enabled" pref
+      if (!StaticPrefs::device_sensors_proximity_enabled()) {
+        return false;
+      }
+      if (doc) {
+        doc->WarnOnceAbout(DeprecatedOperations::eProximityEvent, true);
+      }
+      break;
+    case nsIDeviceSensorData::TYPE_LIGHT:
+      // checks "device.sensors.ambientLight.enabled" pref
+      if (!StaticPrefs::device_sensors_ambientLight_enabled()) {
+        return false;
+      }
+      if (doc) {
+        doc->WarnOnceAbout(DeprecatedOperations::eAmbientLightEvent, true);
+      }
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Device sensor type not recognised");
+      return false;
+  }
+
+  if (!window) {
+    return true;
+  }
+  return !nsGlobalWindowInner::Cast(window)->ShouldResistFingerprinting();
+}
diff --git a/dom/system/nsDeviceSensors.h b/dom/system/nsDeviceSensors.h
new file mode 100644
index 0000000000..dd079088e6
--- /dev/null
+++ b/dom/system/nsDeviceSensors.h
@@ -0,0 +1,72 @@
+/* -*- 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 nsDeviceSensors_h
+#define nsDeviceSensors_h
+
+#include "nsIDeviceSensors.h"
+#include "nsCOMArray.h"
+#include "nsTArray.h"
+#include "nsCOMPtr.h"
+#include "mozilla/dom/DeviceMotionEvent.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/HalSensor.h"
+
+class nsIDOMWindow;
+
+namespace mozilla::dom {
+class Document;
+class EventTarget;
+}  // namespace mozilla::dom
+
+class nsDeviceSensors : public nsIDeviceSensors,
+                        public mozilla::hal::ISensorObserver {
+  using DeviceAccelerationInit = mozilla::dom::DeviceAccelerationInit;
+  using DeviceRotationRateInit = mozilla::dom::DeviceRotationRateInit;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIDEVICESENSORS
+
+  nsDeviceSensors();
+
+  void Notify(const mozilla::hal::SensorData& aSensorData) override;
+
+ private:
+  virtual ~nsDeviceSensors();
+
+  // sensor -> window listener
+  nsTArray<nsTArray<nsIDOMWindow*>*> mWindowListeners;
+
+  void FireDOMLightEvent(mozilla::dom::EventTarget* aTarget, double value);
+
+  void MaybeFireDOMUserProximityEvent(mozilla::dom::EventTarget* aTarget,
+                                      double aValue, double aMax);
+
+  void FireDOMUserProximityEvent(mozilla::dom::EventTarget* aTarget,
+                                 bool aNear);
+
+  void FireDOMOrientationEvent(mozilla::dom::EventTarget* target, double aAlpha,
+                               double aBeta, double aGamma, bool aIsAbsolute);
+
+  void FireDOMMotionEvent(mozilla::dom::Document* domDoc,
+                          mozilla::dom::EventTarget* target, uint32_t type,
+                          PRTime timestamp, double x, double y, double z);
+
+  inline bool IsSensorEnabled(uint32_t aType) {
+    return mWindowListeners[aType]->Length() > 0;
+  }
+
+  bool IsSensorAllowedByPref(uint32_t aType, nsIDOMWindow* aWindow);
+
+  mozilla::TimeStamp mLastDOMMotionEventTime;
+  bool mIsUserProximityNear;
+  mozilla::Maybe<DeviceAccelerationInit> mLastAcceleration;
+  mozilla::Maybe<DeviceAccelerationInit> mLastAccelerationIncludingGravity;
+  mozilla::Maybe<DeviceRotationRateInit> mLastRotationRate;
+};
+
+#endif
diff --git a/dom/system/nsIOSFileConstantsService.idl b/dom/system/nsIOSFileConstantsService.idl
new file mode 100644
index 0000000000..2cdde3de97
--- /dev/null
+++ b/dom/system/nsIOSFileConstantsService.idl
@@ -0,0 +1,28 @@
+/* -*- Mode: c++; c-basic-offset: 2; indent-tabs-mode: nil; tab-width: 40 -*- */
+/* vim: set ts=2 et sw=2 tw=40: */
+/* 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 "nsISupports.idl"
+
+[scriptable, uuid(d6dd239f-34d6-4b34-baa1-f69ab4a20bc4)]
+interface nsIOSFileConstantsService: nsISupports
+{
+  /**
+   * Inject module OS.Constants in the environment.
+   *
+   * This method must be called only from the main thread.
+   * Method is idempotent.
+   */
+  [implicit_jscontext]
+  void init();
+};
+
+%{ C++
+
+// {4BBE1B96-8956-457F-A03F-9C27435F2AFA}
+#define OSFILECONSTANTSSERVICE_CID {0x4BBE1B96,0x8956,0x457F,{0xA0,0x3F,0x9C,0x27,0x43,0x5F,0x2A,0xFA}}
+#define OSFILECONSTANTSSERVICE_CONTRACTID "@mozilla.org/net/osfileconstantsservice;1"
+
+%}
diff --git a/dom/system/nsIOSPermissionRequest.idl b/dom/system/nsIOSPermissionRequest.idl
new file mode 100644
index 0000000000..e0d7b531c7
--- /dev/null
+++ b/dom/system/nsIOSPermissionRequest.idl
@@ -0,0 +1,69 @@
+/* -*- Mode: c++; c-basic-offset: 2; indent-tabs-mode: nil; tab-width: 40 -*- */
+/* vim: set ts=2 et sw=2 tw=40: */
+/* 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 "nsISupports.idl"
+
+[scriptable, uuid(95790842-75a0-430d-98bf-f5ce3788ea6d)]
+interface nsIOSPermissionRequest: nsISupports
+{
+  /*
+   * The permission state is not known. As an example, on macOS
+   * this is used to indicate the user has not been prompted to
+   * authorize or deny access and there is no policy in place to
+   * deny access.
+   */
+  const uint16_t PERMISSION_STATE_NOTDETERMINED = 0;
+
+  /* A policy prevents the application from accessing the resource */
+  const uint16_t PERMISSION_STATE_RESTRICTED = 1;
+
+  /* Access to the resource is denied */
+  const uint16_t PERMISSION_STATE_DENIED = 2;
+
+  /* Access to the resource is allowed */
+  const uint16_t PERMISSION_STATE_AUTHORIZED = 3;
+
+  /* Get the permission state for both audio and video capture */
+  void getMediaCapturePermissionState(out uint16_t aVideo,
+                                      out uint16_t aAudio);
+
+  /* Get the permission state for audio capture */
+  void getAudioCapturePermissionState(out uint16_t aAudio);
+
+  /* Get the permission state for video capture */
+  void getVideoCapturePermissionState(out uint16_t aVideo);
+
+  /* Get the permission state for screen capture */
+  void getScreenCapturePermissionState(out uint16_t aScreen);
+
+  /*
+   * Request permission to access video capture devices. Returns a
+   * promise that resolves with |true| after the browser has been
+   * granted permission to capture video. If capture access is denied,
+   * the promise is resolved with |false|. The promise is rejected if
+   * an error occurs.
+   */
+  [implicit_jscontext, must_use]
+  Promise requestVideoCapturePermission();
+
+  /*
+   * Request permission to access audio capture devices. Returns a
+   * promise with the same semantics as |requestVideoCapturePermission|.
+   */
+  [implicit_jscontext, must_use]
+  Promise requestAudioCapturePermission();
+
+  /*
+   * Request permission to capture the screen using an unreliable method.
+   * Attemps to trigger a screen capture permission dialog. Whether or not
+   * the dialog is displayed and whether or not the user grants permission
+   * to record the screen is not available to the caller. This method has
+   * limited utility because it does not block to wait for a dialog
+   * prompt or the user's reponse if a dialog is displayed. And the dialog
+   * is not guaranteed to be displayed per OS restrictions.
+   */
+  void maybeRequestScreenCapturePermission();
+};
diff --git a/dom/system/nsOSPermissionRequest.h b/dom/system/nsOSPermissionRequest.h
new file mode 100644
index 0000000000..660434c863
--- /dev/null
+++ b/dom/system/nsOSPermissionRequest.h
@@ -0,0 +1,18 @@
+/* -*- 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 nsOSPermissionRequest_h__
+#define nsOSPermissionRequest_h__
+
+#include "nsOSPermissionRequestBase.h"
+
+/*
+ * The default implementation of nsOSPermissionRequestBase used on platforms
+ * that don't have a platform-specific version.
+ */
+class nsOSPermissionRequest : public nsOSPermissionRequestBase {};
+
+#endif /* nsOSPermissionRequest_h__ */
diff --git a/dom/system/nsOSPermissionRequestBase.cpp b/dom/system/nsOSPermissionRequestBase.cpp
new file mode 100644
index 0000000000..32b32c38cd
--- /dev/null
+++ b/dom/system/nsOSPermissionRequestBase.cpp
@@ -0,0 +1,96 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsOSPermissionRequestBase.h"
+
+#include "mozilla/dom/Promise.h"
+
+using namespace mozilla;
+
+using mozilla::dom::Promise;
+
+NS_IMPL_ISUPPORTS(nsOSPermissionRequestBase, nsIOSPermissionRequest,
+                  nsISupportsWeakReference)
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::GetMediaCapturePermissionState(
+    uint16_t* aCamera, uint16_t* aMicrophone) {
+  nsresult rv = GetVideoCapturePermissionState(aCamera);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  return GetAudioCapturePermissionState(aMicrophone);
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::GetAudioCapturePermissionState(uint16_t* aAudio) {
+  MOZ_ASSERT(aAudio);
+  *aAudio = PERMISSION_STATE_AUTHORIZED;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::GetVideoCapturePermissionState(uint16_t* aVideo) {
+  MOZ_ASSERT(aVideo);
+  *aVideo = PERMISSION_STATE_AUTHORIZED;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::GetScreenCapturePermissionState(uint16_t* aScreen) {
+  MOZ_ASSERT(aScreen);
+  *aScreen = PERMISSION_STATE_AUTHORIZED;
+  return NS_OK;
+}
+
+nsresult nsOSPermissionRequestBase::GetPromise(JSContext* aCx,
+                                               RefPtr<Promise>& aPromiseOut) {
+  nsIGlobalObject* globalObject = xpc::CurrentNativeGlobal(aCx);
+  if (NS_WARN_IF(!globalObject)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  ErrorResult result;
+  aPromiseOut = Promise::Create(globalObject, result);
+  if (NS_WARN_IF(result.Failed())) {
+    return result.StealNSResult();
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::RequestVideoCapturePermission(
+    JSContext* aCx, Promise** aPromiseOut) {
+  RefPtr<Promise> promiseHandle;
+  nsresult rv = GetPromise(aCx, promiseHandle);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  promiseHandle->MaybeResolve(true /* access authorized */);
+  promiseHandle.forget(aPromiseOut);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::RequestAudioCapturePermission(
+    JSContext* aCx, Promise** aPromiseOut) {
+  RefPtr<Promise> promiseHandle;
+  nsresult rv = GetPromise(aCx, promiseHandle);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  promiseHandle->MaybeResolve(true /* access authorized */);
+  promiseHandle.forget(aPromiseOut);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsOSPermissionRequestBase::MaybeRequestScreenCapturePermission() {
+  return NS_OK;
+}
diff --git a/dom/system/nsOSPermissionRequestBase.h b/dom/system/nsOSPermissionRequestBase.h
new file mode 100644
index 0000000000..e1037b6047
--- /dev/null
+++ b/dom/system/nsOSPermissionRequestBase.h
@@ -0,0 +1,38 @@
+/* -*- 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 nsOSPermissionRequestBase_h__
+#define nsOSPermissionRequestBase_h__
+
+#include "nsIOSPermissionRequest.h"
+#include "nsWeakReference.h"
+
+namespace mozilla::dom {
+class Promise;
+}  // namespace mozilla::dom
+
+using mozilla::dom::Promise;
+
+/*
+ * The base implementation of nsIOSPermissionRequest to be subclassed on
+ * platforms that require permission requests for access to resources such
+ * as media captures devices. This implementation always returns results
+ * indicating access is permitted.
+ */
+class nsOSPermissionRequestBase : public nsIOSPermissionRequest,
+                                  public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOSPERMISSIONREQUEST
+
+  nsOSPermissionRequestBase() = default;
+
+ protected:
+  nsresult GetPromise(JSContext* aCx, RefPtr<Promise>& aPromiseOut);
+  virtual ~nsOSPermissionRequestBase() = default;
+};
+
+#endif
diff --git a/dom/system/tests/chrome.ini b/dom/system/tests/chrome.ini
new file mode 100644
index 0000000000..ffdd4393a4
--- /dev/null
+++ b/dom/system/tests/chrome.ini
@@ -0,0 +1,9 @@
+[DEFAULT]
+support-files =
+  worker_constants.js
+
+[test_constants.xhtml]
+[test_pathutils.html]
+[test_pathutils_worker.xhtml]
+support-files =
+  pathutils_worker.js
diff --git a/dom/system/tests/file_bug1197901.html b/dom/system/tests/file_bug1197901.html
new file mode 100644
index 0000000000..e4ca9fd380
--- /dev/null
+++ b/dom/system/tests/file_bug1197901.html
@@ -0,0 +1,16 @@
+<pre>Sensor events testing</pre>
+<script>
+
+window.onmessage = function(event) {
+  if (event.data.command == "addEventListener") {
+    window.addEventListener(
+      "devicemotion", function() {
+          event.source.postMessage({ result: event.data.expected,
+                                     message: event.data.message },
+                                   "*");
+        }
+      );
+  }
+};
+
+</script>
diff --git a/dom/system/tests/ioutils/chrome.ini b/dom/system/tests/ioutils/chrome.ini
new file mode 100644
index 0000000000..a961f45631
--- /dev/null
+++ b/dom/system/tests/ioutils/chrome.ini
@@ -0,0 +1,23 @@
+[DEFAULT]
+support-files =
+  file_ioutils_test_fixtures.js
+  file_ioutils_worker.js
+
+[test_ioutils.html]
+[test_ioutils_compute_hex_digest.html]
+[test_ioutils_copy_move.html]
+[test_ioutils_create_unique.html]
+[test_ioutils_dir_iteration.html]
+[test_ioutils_getfile.html]
+[test_ioutils_mac_xattr.html]
+skip-if = (os != "mac")
+[test_ioutils_mkdir.html]
+[test_ioutils_read_write.html]
+[test_ioutils_read_write_json.html]
+[test_ioutils_read_write_utf8.html]
+[test_ioutils_remove.html]
+[test_ioutils_stat_set_modification_time.html]
+[test_ioutils_worker.xhtml]
+[test_ioutils_set_permissions.html]
+[test_ioutils_windows_file_attributes.html]
+skip-if = (os != 'win')
diff --git a/dom/system/tests/ioutils/file_ioutils_test_fixtures.js b/dom/system/tests/ioutils/file_ioutils_test_fixtures.js
new file mode 100644
index 0000000000..5d2e5011c9
--- /dev/null
+++ b/dom/system/tests/ioutils/file_ioutils_test_fixtures.js
@@ -0,0 +1,78 @@
+// Utility functions.
+
+Uint8Array.prototype.equals = function equals(other) {
+  if (this.byteLength !== other.byteLength) {
+    return false;
+  }
+  return this.every((val, i) => val === other[i]);
+};
+
+async function createFile(location, contents = "") {
+  if (typeof contents === "string") {
+    contents = new TextEncoder().encode(contents);
+  }
+  await IOUtils.write(location, contents);
+  const exists = await fileExists(location);
+  ok(exists, `Created temporary file at: ${location}`);
+}
+
+async function createDir(location) {
+  await IOUtils.makeDirectory(location, {
+    ignoreExisting: true,
+    createAncestors: true,
+  });
+  const exists = await dirExists(location);
+  ok(exists, `Created temporary directory at: ${location}`);
+}
+
+async function fileHasBinaryContents(location, expectedContents) {
+  if (!(expectedContents instanceof Uint8Array)) {
+    throw new TypeError("expectedContents must be a byte array");
+  }
+  info(`Opening ${location} for reading`);
+  const bytes = await IOUtils.read(location);
+  return bytes.equals(expectedContents);
+}
+
+async function fileHasTextContents(location, expectedContents) {
+  if (typeof expectedContents !== "string") {
+    throw new TypeError("expectedContents must be a string");
+  }
+  info(`Opening ${location} for reading`);
+  const bytes = await IOUtils.read(location);
+  const contents = new TextDecoder().decode(bytes);
+  return contents === expectedContents;
+}
+
+async function fileExists(file) {
+  try {
+    let { type } = await IOUtils.stat(file);
+    return type === "regular";
+  } catch (ex) {
+    return false;
+  }
+}
+
+async function dirExists(dir) {
+  try {
+    let { type } = await IOUtils.stat(dir);
+    return type === "directory";
+  } catch (ex) {
+    return false;
+  }
+}
+
+async function cleanup(...files) {
+  for (const file of files) {
+    await IOUtils.remove(file, {
+      ignoreAbsent: true,
+      recursive: true,
+    });
+    const exists = await IOUtils.exists(file);
+    ok(!exists, `Removed temporary file: ${file}`);
+  }
+}
+
+function sleep(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
diff --git a/dom/system/tests/ioutils/file_ioutils_worker.js b/dom/system/tests/ioutils/file_ioutils_worker.js
new file mode 100644
index 0000000000..aa28183823
--- /dev/null
+++ b/dom/system/tests/ioutils/file_ioutils_worker.js
@@ -0,0 +1,102 @@
+// Any copyright is dedicated to the Public Domain.
+// - http://creativecommons.org/publicdomain/zero/1.0/
+
+/* eslint-env mozilla/chrome-worker, node */
+
+"use strict";
+
+/* import-globals-from /testing/mochitest/tests/SimpleTest/WorkerSimpleTest.js */
+importScripts("chrome://mochikit/content/tests/SimpleTest/WorkerSimpleTest.js");
+/* import-globals-from /toolkit/modules/ObjectUtils.jsm */
+importScripts("resource://gre/modules/ObjectUtils.jsm");
+
+importScripts("file_ioutils_test_fixtures.js");
+
+self.onmessage = async function(msg) {
+  const tmpDir = await PathUtils.getTempDir();
+
+  // IOUtils functionality is the same when called from the main thread, or a
+  // web worker. These tests are a modified subset of the main thread tests, and
+  // serve as a confidence check that the implementation is thread-safe.
+  await test_api_is_available_on_worker();
+  await test_full_read_and_write();
+  await test_move_file();
+  await test_copy_file();
+  await test_make_directory();
+
+  finish();
+  info("test_ioutils_worker.xhtml: Test finished");
+
+  async function test_api_is_available_on_worker() {
+    ok(self.IOUtils, "IOUtils is present in web workers");
+  }
+
+  async function test_full_read_and_write() {
+    // Write a file.
+    const tmpFileName = PathUtils.join(tmpDir, "test_ioutils_numbers.tmp");
+    const bytes = Uint8Array.of(...new Array(50).keys());
+    const bytesWritten = await IOUtils.write(tmpFileName, bytes);
+    is(bytesWritten, 50, "IOUtils::write can write entire byte array to file");
+
+    // Read it back.
+    let fileContents = await IOUtils.read(tmpFileName);
+    ok(
+      ObjectUtils.deepEqual(bytes, fileContents) &&
+        bytes.length == fileContents.length,
+      "IOUtils::read can read back entire file"
+    );
+
+    const tooManyBytes = bytes.length + 1;
+    fileContents = await IOUtils.read(tmpFileName, { maxBytes: tooManyBytes });
+    ok(
+      ObjectUtils.deepEqual(bytes, fileContents) &&
+        fileContents.length == bytes.length,
+      "IOUtils::read can read entire file when requested maxBytes is too large"
+    );
+
+    await cleanup(tmpFileName);
+  }
+
+  async function test_move_file() {
+    const src = PathUtils.join(tmpDir, "test_move_file_src.tmp");
+    const dest = PathUtils.join(tmpDir, "test_move_file_dest.tmp");
+    const bytes = Uint8Array.of(...new Array(50).keys());
+    await IOUtils.write(src, bytes);
+
+    await IOUtils.move(src, dest);
+    ok(
+      !(await fileExists(src)) && (await fileExists(dest)),
+      "IOUtils::move can move files from a worker"
+    );
+
+    await cleanup(dest);
+  }
+
+  async function test_copy_file() {
+    const tmpFileName = PathUtils.join(tmpDir, "test_ioutils_orig.tmp");
+    const destFileName = PathUtils.join(tmpDir, "test_ioutils_copy.tmp");
+    await createFile(tmpFileName, "original");
+
+    await IOUtils.copy(tmpFileName, destFileName);
+    ok(
+      (await fileExists(tmpFileName)) &&
+        (await fileHasTextContents(destFileName, "original")),
+      "IOUtils::copy can copy source to dest in same directory"
+    );
+
+    await cleanup(tmpFileName, destFileName);
+  }
+
+  async function test_make_directory() {
+    const dir = PathUtils.join(tmpDir, "test_make_dir.tmp.d");
+    await IOUtils.makeDirectory(dir);
+    const stat = await IOUtils.stat(dir);
+    is(
+      stat.type,
+      "directory",
+      "IOUtils::makeDirectory can make a new directory from a worker"
+    );
+
+    await cleanup(dir);
+  }
+};
diff --git a/dom/system/tests/ioutils/test_ioutils.html b/dom/system/tests/ioutils/test_ioutils.html
new file mode 100644
index 0000000000..cf62c4c388
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils.html
@@ -0,0 +1,26 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script>
+    "use strict";
+
+    add_task(async function test_api_is_available_on_window() {
+      ok(window.IOUtils, "IOUtils is present on the window");
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_compute_hex_digest.html b/dom/system/tests/ioutils/test_ioutils_compute_hex_digest.html
new file mode 100644
index 0000000000..1c27a28822
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_compute_hex_digest.html
@@ -0,0 +1,54 @@
+
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+    add_task(async function test_computeHexDigest() {
+      const tempDir = PathUtils.join(PathUtils.tempDir, "ioutils-test-compute-hex-digest.tmp.d");
+      await createDir(tempDir);
+
+      const path = PathUtils.join(tempDir, "file");
+      await IOUtils.writeUTF8(path, "hello world\n");
+
+      const DIGESTS = [
+        "22596363b3de40b06f981fb85d82312e8c0ed511",
+        "a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447",
+        "6b3b69ff0a404f28d75e98a066d3fc64fffd9940870cc68bece28545b9a75086b343d7a1366838083e4b8f3ca6fd3c80",
+        "db3974a97f2407b7cae1ae637c0030687a11913274d578492558e39c16c017de84eacdc8c62fe34ee4e12b4b1428817f09b6a2760c3f8a664ceae94d2434a593",
+      ];
+      const ALGORITHMS = ["sha1", "sha256", "sha384", "sha512"];
+
+      for (let i = 0; i < ALGORITHMS.length; i++) {
+        const alg = ALGORITHMS[i];
+        const expected = DIGESTS[i];
+
+        Assert.equal(
+          await IOUtils.computeHexDigest(path, alg),
+          expected,
+          `IOUtils.hashFile() has expected value for ${alg}`);
+      }
+
+      await cleanup(tempDir);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_copy_move.html b/dom/system/tests/ioutils/test_ioutils_copy_move.html
new file mode 100644
index 0000000000..ba55eb8463
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_copy_move.html
@@ -0,0 +1,359 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    add_task(async function test_move_relative_path() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_move_relative_path.tmp");
+      const dest = "relative_to_cwd.tmp";
+      await createFile(tmpFileName, "source");
+
+      info("Test moving a file to a relative destination");
+      await Assert.rejects(
+        IOUtils.move(tmpFileName, dest),
+        /Could not parse path/,
+        "IOUtils::move only works with absolute paths"
+      );
+      ok(
+        await fileHasTextContents(tmpFileName, "source"),
+        "IOUtils::move doesn't change source file when move fails"
+      );
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_move_rename() {
+      // Set up.
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_move_src.tmp");
+      const destFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_move_dest.tmp");
+      await createFile(tmpFileName, "dest");
+      // Test.
+      info("Test move to new file in same directory");
+      await IOUtils.move(tmpFileName, destFileName);
+      info(`Moved ${tmpFileName} to ${destFileName}`);
+      ok(
+        !await fileExists(tmpFileName)
+        && await fileHasTextContents(destFileName, "dest"),
+        "IOUtils::move can move source to dest in same directory"
+      )
+
+      // Set up.
+      info("Test move to existing file with no overwrite");
+      await createFile(tmpFileName, "source");
+      // Test.
+      await Assert.rejects(
+        IOUtils.move(tmpFileName, destFileName, { noOverwrite: true }),
+        /Could not move source file\(.*\) to destination\(.*\) because the destination already exists and overwrites are not allowed/,
+        "IOUtils::move will refuse to move a file if overwrites are disabled"
+      );
+      ok(
+        await fileExists(tmpFileName)
+        && await fileHasTextContents(destFileName, "dest"),
+        "Failed IOUtils::move doesn't move the source file"
+      );
+
+      // Test.
+      info("Test move to existing file with overwrite");
+      await IOUtils.move(tmpFileName, destFileName, { noOverwrite: false });
+      ok(!await fileExists(tmpFileName), "IOUtils::move moved source");
+      ok(
+        await fileHasTextContents(destFileName, "source"),
+        "IOUtils::move overwrote the destination with the source"
+      );
+
+      // Clean up.
+      await cleanup(tmpFileName, destFileName);
+    });
+
+    add_task(async function test_move_to_dir() {
+      // Set up.
+      info("Test move and rename to non-existing directory");
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_move_to_dir.tmp");
+      const destDir = PathUtils.join(PathUtils.tempDir, "test_move_to_dir.tmp.d");
+      const dest = PathUtils.join(destDir, "dest.tmp");
+      await createFile(tmpFileName);
+      // Test.
+      ok(!await IOUtils.exists(destDir), "Expected path not to exist");
+      await IOUtils.move(tmpFileName, dest);
+      ok(
+        !await fileExists(tmpFileName) && await fileExists(dest),
+        "IOUtils::move creates non-existing parents if needed"
+      );
+
+      // Set up.
+      info("Test move and rename to existing directory.")
+      await createFile(tmpFileName);
+      // Test.
+      ok(await dirExists(destDir), `Expected ${destDir} to be a directory`);
+      await IOUtils.move(tmpFileName, dest);
+      ok(
+        !await fileExists(tmpFileName)
+        && await fileExists(dest),
+        "IOUtils::move can move/rename a file into an existing dir"
+      );
+
+      // Set up.
+      info("Test move to existing directory without specifying leaf name.")
+      await createFile(tmpFileName);
+      // Test.
+      await IOUtils.move(tmpFileName, destDir);
+      ok(await dirExists(destDir), `Expected ${destDir} to be a directory`);
+      ok(
+        !await fileExists(tmpFileName)
+        && await fileExists(PathUtils.join(destDir, PathUtils.filename(tmpFileName))),
+        "IOUtils::move can move a file into an existing dir"
+      );
+
+      // Clean up.
+      await cleanup(destDir);
+    });
+
+    add_task(async function test_move_dir() {
+      // Set up.
+      info("Test rename an empty directory");
+      const srcDir = PathUtils.join(PathUtils.tempDir, "test_move_dir.tmp.d");
+      const destDir = PathUtils.join(PathUtils.tempDir, "test_move_dir_dest.tmp.d");
+      await createDir(srcDir);
+      // Test.
+      await IOUtils.move(srcDir, destDir);
+      ok(
+        !await IOUtils.exists(srcDir) && await dirExists(destDir),
+        "IOUtils::move can rename directories"
+      );
+
+      // Set up.
+      info("Test move directory and its content into another directory");
+      await createDir(srcDir);
+      await createFile(PathUtils.join(srcDir, "file.tmp"), "foo");
+      // Test.
+      await IOUtils.move(srcDir, destDir);
+      const destFile = PathUtils.join(destDir, PathUtils.filename(srcDir), "file.tmp");
+      ok(
+        !await IOUtils.exists(srcDir)
+        && await dirExists(destDir)
+        && await dirExists(PathUtils.join(destDir, PathUtils.filename(srcDir)))
+        && await fileHasTextContents(destFile, "foo"),
+        "IOUtils::move can move a directory and its contents into another one"
+      )
+
+      // Clean up.
+      await cleanup(srcDir, destDir);
+    });
+
+    add_task(async function test_move_failures() {
+      // Set up.
+      info("Test attempt to rename a non-existent source file");
+      const notExistsSrc = PathUtils.join(PathUtils.tempDir, "not_exists_src.tmp");
+      const notExistsDest = PathUtils.join(PathUtils.tempDir, "not_exists_dest.tmp");
+      // Test.
+      await Assert.rejects(
+        IOUtils.move(notExistsSrc, notExistsDest),
+        /Could not move source file\(.*\) because it does not exist/,
+        "IOUtils::move throws if source file does not exist"
+      );
+      ok(
+        !await fileExists(notExistsSrc) && !await fileExists(notExistsDest),
+        "IOUtils::move fails if source file does not exist"
+      );
+
+      // Set up.
+      info("Test attempt to move a directory to a file");
+      const destFile = PathUtils.join(PathUtils.tempDir, "test_move_failures_file_dest.tmp");
+      const srcDir = PathUtils.join(PathUtils.tempDir, "test_move_failure_src.tmp.d");
+      await createFile(destFile);
+      await createDir(srcDir);
+      // Test.
+      await Assert.rejects(
+        IOUtils.move(srcDir, destFile),
+        /Could not move the source directory\(.*\) to the destination\(.*\) because the destination is not a directory/,
+        "IOUtils::move throws if try to move dir into an existing file"
+      );
+
+      // Clean up.
+      await cleanup(destFile, srcDir);
+    });
+
+    add_task(async function test_copy() {
+      // Set up.
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_orig.tmp");
+      const destFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_copy.tmp");
+      await createFile(tmpFileName, "original");
+      // Test.
+      info("Test copy to new file in same directory");
+      await IOUtils.copy(tmpFileName, destFileName);
+      ok(
+        await fileExists(tmpFileName)
+        && await fileHasTextContents(destFileName, "original"),
+        "IOUtils::copy can copy source to dest in same directory"
+      );
+
+      // Set up.
+      info("Test copy to existing file with no overwrite");
+      await createFile(tmpFileName, "new contents");
+      // Test.
+      await Assert.rejects(
+        IOUtils.copy(tmpFileName, destFileName, { noOverwrite: true }),
+        /Could not copy source file\(.*\) to destination\(.*\) because the destination already exists and overwrites are not allowed/,
+        "IOUtils::copy will refuse to copy to existing destination if overwrites are disabled"
+      );
+      ok(
+        await fileExists(tmpFileName)
+        && await fileHasTextContents(destFileName, "original"),
+        "Failed IOUtils::move doesn't move the source file"
+      );
+
+      // Test.
+      info("Test copy to existing file with overwrite");
+      await IOUtils.copy(tmpFileName, destFileName, { noOverwrite: false });
+      ok(await fileExists(tmpFileName), "IOUtils::copy retains source");
+      ok(
+        await fileHasTextContents(destFileName, "new contents"),
+        "IOUtils::copy overwrote the destination with the source"
+      );
+
+      // Clean up.
+      await cleanup(tmpFileName, destFileName);
+    });
+
+    add_task(async function test_copy_file_to_dir() {
+      // Set up.
+      info("Test copy file to non-existing directory");
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_copy_file_to_dir.tmp");
+      const destDir = PathUtils.join(PathUtils.tempDir, "test_copy_file_to_dir.tmp.d");
+      const dest = PathUtils.join(destDir, "dest.tmp");
+      await createFile(tmpFileName);
+      // Test.
+      ok(!await IOUtils.exists(destDir), "Expected path not to exist");
+      await IOUtils.copy(tmpFileName, dest);
+      ok(
+        await fileExists(tmpFileName) && await fileExists(dest),
+        "IOUtils::copy creates non-existing parents if needed"
+      );
+
+      // Set up.
+      info("Test copy file to existing directory")
+      await createFile(tmpFileName);
+      // Test.
+      ok(await dirExists(destDir), `Expected ${destDir} to be a directory`);
+      await IOUtils.copy(tmpFileName, dest);
+      ok(
+        await fileExists(tmpFileName)
+        && await fileExists(dest),
+        "IOUtils::copy can copy a file into an existing dir"
+      );
+
+      // Set up.
+      info("Test copy file to existing directory without specifying leaf name")
+      await createFile(tmpFileName);
+      // Test.
+      await IOUtils.copy(tmpFileName, destDir);
+      ok(await dirExists(destDir), `Expected ${destDir} to be a directory`);
+      ok(
+        await fileExists(tmpFileName)
+        && await fileExists(PathUtils.join(destDir, PathUtils.filename(tmpFileName))),
+        "IOUtils::copy can copy a file into an existing dir"
+      );
+
+      // Clean up.
+      await cleanup(tmpFileName, destDir);
+    });
+
+    add_task(async function test_copy_dir_recursive() {
+      // Set up.
+      info("Test rename an empty directory");
+      const srcDir = PathUtils.join(PathUtils.tempDir, "test_copy_dir.tmp.d");
+      const destDir = PathUtils.join(PathUtils.tempDir, "test_copy_dir_dest.tmp.d");
+      await createDir(srcDir);
+      // Test.
+      await IOUtils.copy(srcDir, destDir, { recursive: true });
+      ok(
+        await dirExists(srcDir) && await dirExists(destDir),
+        "IOUtils::copy can recursively copy entire directories"
+      );
+
+      // Set up.
+      info("Test copy directory and its content into another directory");
+      await createDir(srcDir);
+      await createFile(PathUtils.join(srcDir, "file.tmp"), "foo");
+      // Test.
+      await IOUtils.copy(srcDir, destDir, { recursive: true });
+      const destFile = PathUtils.join(destDir, PathUtils.filename(srcDir), "file.tmp");
+      ok(
+        await dirExists(srcDir)
+        && await dirExists(destDir)
+        && await dirExists(PathUtils.join(destDir, PathUtils.filename(srcDir)))
+        && await fileHasTextContents(destFile, "foo"),
+        "IOUtils::copy can move a directory and its contents into another one"
+      )
+
+      // Clean up.
+      await cleanup(srcDir, destDir);
+    });
+
+    add_task(async function test_copy_failures() {
+      // Set up.
+      info("Test attempt to copy a non-existent source file");
+      const notExistsSrc = PathUtils.join(PathUtils.tempDir, "test_copy_not_exists_src.tmp");
+      const notExistsDest = PathUtils.join(PathUtils.tempDir, "test_copy_not_exists_dest.tmp");
+      // Test.
+      await Assert.rejects(
+        IOUtils.copy(notExistsSrc, notExistsDest),
+        /Could not copy source file\(.*\) because it does not exist/,
+        "IOUtils::copy throws if source file does not exist"
+      );
+      ok(
+        !await fileExists(notExistsSrc) && !await fileExists(notExistsDest),
+        "IOUtils::copy failure due to missing source file does not affect destination"
+      );
+
+      // Set up.
+      info("Test attempt to copy a directory to a file");
+      const destFile = PathUtils.join(PathUtils.tempDir, "test_copy_failures_file_dest.tmp");
+      const srcDir = PathUtils.join(PathUtils.tempDir, "test_copy_failure_src.tmp.d");
+      await createFile(destFile);
+      await createDir(srcDir);
+      // Test.
+      await Assert.rejects(
+        IOUtils.copy(srcDir, destFile, { recursive: true }),
+        /Could not copy the source directory\(.*\) to the destination\(.*\) because the destination is not a directory/,
+        "IOUtils::copy throws if try to move dir into an existing file"
+      );
+      ok(await fileHasTextContents(destFile, ""), "IOUtils::copy failure does not affect destination");
+
+      // Set up.
+      info("Test copy directory without recursive option");
+      await createDir(srcDir);
+      // Test.
+      await Assert.rejects(
+        IOUtils.copy(srcDir, notExistsDest, { recursive: false }),
+        /Refused to copy source directory\(.*\) to the destination\(.*\)/,
+        "IOUtils::copy throws if try to copy a directory with { recursive: false }"
+      );
+      console.log(`${notExistsDest} exists?`, await IOUtils.exists(notExistsDest))
+      ok(!await IOUtils.exists(notExistsDest), "IOUtils::copy failure does not affect destination");
+
+      // Clean up.
+      await cleanup(destFile, srcDir);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_create_unique.html b/dom/system/tests/ioutils/test_ioutils_create_unique.html
new file mode 100644
index 0000000000..e447964343
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_create_unique.html
@@ -0,0 +1,88 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+    function octalFormat(n) {
+      let s = n.toString(8);
+      while (s.length < 3) {
+        s = `0${s}`;
+      }
+      return `0o${s}`;
+    }
+
+    async function check(method, path, prefix, type, perms) {
+      const filename = PathUtils.filename(path);
+
+      ok(filename.startsWith(prefix), `IOUtils.${method} uses the prefix`);
+      ok(await IOUtils.exists(path), `IOUtils.${method} creates a file`);
+
+      const stat = await IOUtils.stat(path);
+      is(stat.type, type, `IOUtils.${method} creates a "${type}" file`);
+
+      is(
+        octalFormat(stat.permissions),
+        octalFormat(perms),
+        `IOUtils.${method} creates a file with the correct permissions`
+      );
+    }
+
+    add_task(async function test_createUnique() {
+      const tempDir = PathUtils.join(
+        PathUtils.tempDir,
+        "test_createUnique.tmp.d"
+      );
+
+      const filesToChmod = [];
+
+      SimpleTest.registerCleanupFunction(async function test_createUnique_cleanup() {
+        for (const file of filesToChmod) {
+          if (await IOUtils.exists(file)) {
+            await IOUtils.setPermissions(file, 0o666);
+          }
+        }
+
+        await IOUtils.remove(tempDir, { recursive: true });
+      });
+
+      const isWindows = Services.appinfo.OS === "WINNT";
+
+      info("Creating a unique directory")
+      const dir = await IOUtils.createUniqueDirectory(tempDir, "unique-dir", 0o600);
+      await check("createUniqueDirectory", dir, "unique-dir", "directory", isWindows ? 0o666 : 0o600);
+
+      info("Creating a unique directory with the same prefix")
+      const dir2 = await IOUtils.createUniqueDirectory(tempDir, "unique-dir", 0o700);
+      await check("createUniqueDirectory", dir2, "unique-dir", "directory", isWindows ? 0o666 : 0o700);
+      ok(dir !== dir2, "IOUtils.createUniqueDirectory creates unique paths");
+
+      info("Creating a unique file");
+      const file = await IOUtils.createUniqueFile(tempDir, "unique-file", 0o641);
+      await check("createUniqueFile", file, "unique-file", "regular", isWindows ? 0o666 : 0o641);
+
+      info("Creating a unique file with the same prefix");
+      const file2 = await IOUtils.createUniqueFile(tempDir, "unique-file", 0o400);
+      filesToChmod.push(file2);
+      await check("createUniqueFile", file2, "unique-file", "regular", isWindows ? 0o444 : 0o400);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_dir_iteration.html b/dom/system/tests/ioutils/test_ioutils_dir_iteration.html
new file mode 100644
index 0000000000..93e2c60039
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_dir_iteration.html
@@ -0,0 +1,94 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+    add_task(async function iterate_dir_failure() {
+      let notExists = PathUtils.join(PathUtils.tempDir, 'does_not_exist_dir.tmp.d');
+
+      await Assert.rejects(
+        IOUtils.getChildren(notExists),
+        /Could not get children of file\(.*\) because it does not exist/,
+        "IOUtils::getChildren rejects if the file does not exist"
+      );
+      ok(!await fileExists(notExists), `Expected ${notExists} not to exist`);
+
+      info('Try to get the children of a regular file');
+
+      let tmpFileName = PathUtils.join(PathUtils.tempDir, 'iterator_file.tmp');
+      await createFile(tmpFileName)
+      await Assert.rejects(IOUtils.getChildren(tmpFileName),
+        /Could not get children of file\(.*\) because it is not a directory/,
+        "IOUtils::getChildren rejects if the file is not a dir"
+      );
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function iterate_dir() {
+      info('Try to get the children of a multi-level directory hierarchy');
+
+      let root = PathUtils.join(PathUtils.tempDir, 'iterator.tmp.d');
+      let child1 = PathUtils.join(root, 'child1.tmp');
+      let child2 = PathUtils.join(root, 'child2.tmp');
+      let grandchild = PathUtils.join(child1, 'grandchild.tmp');
+
+      await createDir(grandchild); // Ancestors will be created.
+      await createDir(child2);
+
+      let entries = await IOUtils.getChildren(root);
+
+      is(entries.length, 2, `Expected 2 entries below the path at ${root}`);
+      ok(!entries.includes(grandchild), "IOUtils::getChildren does not enter subdirectories");
+
+      await cleanup(root);
+    });
+
+    add_task(async function iterate_empty_dir() {
+      info('Try to get the children of an empty directory');
+
+      let emptyDir = PathUtils.join(PathUtils.tempDir, 'iterator_empty_dir.tmp.d');
+      await createDir(emptyDir);
+
+      is(
+        (await IOUtils.getChildren(emptyDir)).length,
+        0,
+        "IOUtils::getChildren return an empty array when called on an empty dir"
+      );
+
+      await cleanup(emptyDir);
+    });
+
+    add_task(async function iterate_ignore_missing_dir() {
+      info("Try to get the children of a missing file with ignoreAbsent");
+
+      const notExists = PathUtils.join(PathUtils.tempDir, "does_not_exist_dir.tmp.d");
+
+      is(
+        (await IOUtils.getChildren(notExists, { ignoreAbsent: true })).length,
+        0,
+        "IOUtils::getChildren returns an empty array when called with ignoreAbsent on a missing file"
+      );
+      ok(!await fileExists(notExists), `Expected ${notExists} not to exist`);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_getfile.html b/dom/system/tests/ioutils/test_ioutils_getfile.html
new file mode 100644
index 0000000000..d5295f469f
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_getfile.html
@@ -0,0 +1,86 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script>
+    "use strict";
+
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    const TEST_PATH = PathUtils.join(PathUtils.tempDir, "test-ioutils-getfile");
+
+    add_task(async function test_getFile() {
+      const expectedPath = PathUtils.join(TEST_PATH, "foo", "bar", "baz", "get-file.txt");
+      const parentPath = PathUtils.parent(expectedPath);
+
+      ok(!(await IOUtils.exists(parentPath)), "Parent directory should not exist");
+
+      const file = await IOUtils.getFile(TEST_PATH, "foo", "bar", "baz", "get-file.txt");
+      const path = file.path;
+
+      is(path, expectedPath, "Should have the correct path");
+      ok(await IOUtils.exists(parentPath), "Parent directory should be created");
+      ok(!(await IOUtils.exists(path)), "File should not be created");
+
+      await IOUtils.remove(TEST_PATH, { recursive: true });
+    });
+
+    add_task(async function test_getFile_exists() {
+      const expectedPath = PathUtils.join(TEST_PATH, "foo", "bar", "baz", "get-file-exists.txt");
+
+      await IOUtils.makeDirectory(PathUtils.parent(expectedPath));
+      await IOUtils.writeUTF8(expectedPath, "hello world");
+
+      const file = await IOUtils.getFile(TEST_PATH, "foo", "bar", "baz", "get-file-exists.txt");
+      is(file.path, expectedPath, "Should have the correct path");
+      is(await IOUtils.readUTF8(file.path), "hello world", "Contents should be unchanged");
+
+      await IOUtils.remove(TEST_PATH, { recursive: true });
+    });
+
+    add_task(async function test_getDirectory() {
+      const expectedPath = PathUtils.join(TEST_PATH, "qux", "quux", "corge");
+
+      ok(!(await IOUtils.exists(PathUtils.parent(expectedPath))), "Parent directory should not exist");
+
+      const file = await IOUtils.getDirectory(TEST_PATH, "qux", "quux", "corge");
+
+      is(file.path, expectedPath, "Should have the correct path");
+      ok(await IOUtils.exists(expectedPath), "Directory should be created");
+
+      const info = await IOUtils.stat(expectedPath);
+      is(info.type, "directory", "Should create a directory");
+
+      await IOUtils.remove(TEST_PATH, { recursive: true });
+    });
+
+    add_task(async function test_getDirectory_exists() {
+      const expectedPath = PathUtils.join(TEST_PATH, "qux", "quux", "corge");
+
+      await IOUtils.makeDirectory(expectedPath);
+
+      const file = await IOUtils.getDirectory(TEST_PATH, "qux", "quux", "corge");
+      is(file.path, expectedPath, "Should have the correct path");
+      ok(await IOUtils.exists(expectedPath), "Directory should still exist");
+
+      const info = await IOUtils.stat(expectedPath);
+      is(info.type, "directory", "Should still be a directory");
+
+      await IOUtils.remove(TEST_PATH, { recursive: true });
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_mac_xattr.html b/dom/system/tests/ioutils/test_ioutils_mac_xattr.html
new file mode 100644
index 0000000000..cd6e7aeb5d
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_mac_xattr.html
@@ -0,0 +1,90 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { FileUtils } = ChromeUtils.import("resource://gre/modules/FileUtils.jsm");
+
+    const ATTR = "bogus.attr";
+    const VALUE = new TextEncoder().encode("bogus");
+
+    add_task(async function test_macXAttr() {
+      const tmpDir = PathUtils.join(PathUtils.tempDir, "ioutils-macos-xattr.tmp.d");
+
+      await createDir(tmpDir);
+
+      const path = PathUtils.join(tmpDir, "file.tmp");
+      ok(!await IOUtils.exists(path), "File should not exist");
+      await IOUtils.writeUTF8(path, "");
+
+      ok(
+        !await IOUtils.hasMacXAttr(path, ATTR),
+        "File does not have an extended attribute at creation"
+      );
+
+      info("Testing getting an attribute that does not exist");
+      await Assert.rejects(
+        IOUtils.getMacXAttr(path, ATTR),
+        /NotFoundError: The file `.+' does not have an extended attribute/,
+        "IOUtils::getMacXAttr rejects when the attribute does not exist"
+      );
+
+      info("Testing setting an attribute");
+      await IOUtils.setMacXAttr(path, ATTR, VALUE);
+      ok(
+        await IOUtils.hasMacXAttr(path, ATTR),
+        "File has extended attribute after setting"
+      );
+
+      {
+        info("Testing getting an attribute")
+        const value = await IOUtils.getMacXAttr(path, ATTR);
+        Assert.deepEqual(
+          Array.from(value),
+          Array.from(VALUE),
+          "Attribute value should match"
+        );
+      }
+
+      info("Testing removing an attribute");
+      await IOUtils.delMacXAttr(path, ATTR);
+      await Assert.rejects(
+        IOUtils.getMacXAttr(path, ATTR),
+        /NotFoundError: The file `.+' does not have an extended attribute/,
+        "IOUtils::delMacXAttr removes the attribute"
+      );
+
+      ok(
+        !await IOUtils.hasMacXAttr(path, ATTR),
+        "File does not have extended attribute after removing"
+      );
+
+      info("Testing removing an attribute that does not exist");
+      await Assert.rejects(
+        IOUtils.delMacXAttr(path, ATTR),
+        /NotFoundError: The file `.+' does not have an extended attribute/,
+        "IOUtils::delMacXAttr rejects when the attribute does not exist"
+      );
+
+      await cleanup(tmpDir);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_mkdir.html b/dom/system/tests/ioutils/test_ioutils_mkdir.html
new file mode 100644
index 0000000000..c1a073dea6
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_mkdir.html
@@ -0,0 +1,133 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+    add_task(async function test_make_directory() {
+      info("Test creating a new directory");
+      const newDirectoryName = PathUtils.join(PathUtils.tempDir, "test_ioutils_new_dir.tmp.d");
+      await IOUtils.makeDirectory(newDirectoryName);
+      ok(
+        await IOUtils.exists(newDirectoryName),
+        "IOUtils::makeDirectory can create a new directory"
+      );
+
+      info("Test creating an existing directory");
+      await IOUtils.makeDirectory(newDirectoryName, { ignoreExisting: true });
+      ok(
+        await IOUtils.exists(newDirectoryName),
+        "IOUtils::makeDirectory can ignore existing directories"
+      );
+      await Assert.rejects(
+        IOUtils.makeDirectory(newDirectoryName, { ignoreExisting: false }),
+        /Could not create directory because it already exists at .*/,
+        "IOUtils::makeDirectory can throw if the target dir exists"
+      )
+
+      info("Test creating a nested directory");
+      const parentDirName = PathUtils.join(PathUtils.tempDir, "test_ioutils_mkdir_parent.tmp.d");
+      const nestedDirName = PathUtils.join(
+        parentDirName,
+        "test_ioutils_mkdir_child.tmp.d"
+      );
+      await Assert.rejects(
+        IOUtils.makeDirectory(nestedDirName, { createAncestors: false }),
+        /Could not create directory at .*/,
+        "IOUtils::makeDirectory can fail if the target is missing parents"
+      );
+      ok(!await IOUtils.exists(nestedDirName), `Expected ${nestedDirName} not to exist`);
+      await IOUtils.makeDirectory(nestedDirName, { createAncestors: true });
+      ok(
+        await IOUtils.exists(nestedDirName),
+        "IOUtils::makeDirectory can create ancestors of the target directory"
+      );
+
+      await cleanup(newDirectoryName, parentDirName);
+    });
+
+    add_task(async function test_make_directory_failure() {
+      info("Try to create a directory where a file already exists");
+      const notADirFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_not_a_dir.tmp");
+      await createFile(notADirFileName);
+
+      await Assert.rejects(
+        IOUtils.makeDirectory(notADirFileName, { ignoreExisting: false }),
+        /Could not create directory because the target file\(.*\) exists and is not a directory/,
+        "IOUtils::makeDirectory [ignoreExisting: false] throws when the target is an existing file"
+      );
+      ok(await fileExists(notADirFileName), `Expected ${notADirFileName} to exist`);
+
+      await Assert.rejects(
+        IOUtils.makeDirectory(notADirFileName, { ignoreExisting: true }),
+        /Could not create directory because the target file\(.*\) exists and is not a directory/,
+        "IOUtils::makeDirectory [ignoreExisting: true] throws when the target is an existing file"
+      );
+      ok(await fileExists(notADirFileName), `Expected ${notADirFileName} to exist`);
+
+      await cleanup(notADirFileName);
+    });
+
+    add_task(async function test_make_directory_permissions() {
+      if (Services.appinfo.OS === "WINNT") {
+        ok(true, "Skipping test on unsupported platform (Windows)");
+        return;
+      }
+
+      const newDir = PathUtils.join(PathUtils.tempDir, "test_ioutils_mkdir_perms.tmp.d");
+
+      ok(!await IOUtils.exists(newDir), "Directory does not exist before creation");
+      await IOUtils.makeDirectory(newDir, { permissions: 0o751 });
+      ok(await IOUtils.exists(newDir), "Directory created");
+
+      const stat = await IOUtils.stat(newDir);
+      is(stat.type, "directory", "Directory stat() as directory");
+      is(stat.permissions, 0o751, "Directory created with expected permissions");
+
+      await cleanup(newDir);
+    });
+
+    add_task(async function test_make_directory_root() {
+      if (Services.appinfo.OS === "WINNT") {
+        // We don't actually know the root drive, but we can find the root drive
+        // of the profile directory.
+        let current = PathUtils.profileDir;
+        let parent = PathUtils.parent(current);
+        while (parent !== null) {
+          current = parent;
+          parent = PathUtils.parent(current);
+        }
+        // `current` will now be a valid root directory.
+        ok(await IOUtils.exists(current), "Root directory should exist");
+
+        const DRIVE_RE = /^[A-Za-z]:$/;
+        ok(
+          current.startsWith("\\\\") || DRIVE_RE.test(current),
+          `Root directory (${current}) should be a UNC path or drive`,
+        );
+        await IOUtils.makeDirectory(current, {createAncestors: false});
+      } else {
+        ok(await IOUtils.exists("/"), "Root directory should exist");
+        await IOUtils.makeDirectory("/", {createAncestors: false});
+      }
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_read_write.html b/dom/system/tests/ioutils/test_ioutils_read_write.html
new file mode 100644
index 0000000000..2b36f7de6f
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_read_write.html
@@ -0,0 +1,550 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    // This is presently only used to test compatability between OS.File and
+    // IOUtils when it comes to writing compressed files. The import and the
+    // test `test_lz4_osfile_compat` can be removed with OS.File is removed.
+    const { OS } = ChromeUtils.import("resource://gre/modules/osfile.jsm");
+
+    add_task(async function test_read_failure() {
+      const doesNotExist = PathUtils.join(PathUtils.tempDir, "does_not_exist.tmp");
+      await Assert.rejects(
+        IOUtils.read(doesNotExist),
+        /Could not open the file at .*/,
+        "IOUtils::read rejects when file does not exist"
+      );
+    });
+
+    add_task(async function test_write_no_overwrite() {
+      // Make a new file, and try to write to it with overwrites disabled.
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_overwrite.tmp");
+      const untouchableContents = new TextEncoder().encode("Can't touch this!\n");
+
+      let exists = await IOUtils.exists(tmpFileName);
+      ok(!exists, `File ${tmpFileName} should not exist before writing`);
+
+      await IOUtils.write(tmpFileName, untouchableContents);
+
+      exists = await IOUtils.exists(tmpFileName);
+      ok(exists, `File ${tmpFileName} should exist after writing`);
+
+      const newContents = new TextEncoder().encode("Nah nah nah!\n");
+      await Assert.rejects(
+        IOUtils.write(tmpFileName, newContents, {
+          mode: "create",
+        }),
+        /Refusing to overwrite the file at */,
+        "IOUtils::write rejects writing to existing file if overwrites are disabled"
+      );
+      ok(
+        await fileHasBinaryContents(tmpFileName, untouchableContents),
+        "IOUtils::write doesn't change target file when overwrite is refused"
+      );
+
+      const bytesWritten = await IOUtils.write(
+        tmpFileName,
+        newContents,
+        { mode: "overwrite" }
+      );
+      is(
+        bytesWritten,
+        newContents.length,
+        "IOUtils::write can overwrite files if specified"
+      );
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_write_with_backup() {
+      info("Test backup file option with non-existing file");
+
+      let fileContents = new TextEncoder().encode("Original file contents");
+      let destFileName = PathUtils.join(PathUtils.tempDir, "test_write_with_backup_option.tmp");
+      let backupFileName = destFileName + ".backup";
+      let bytesWritten =
+        await IOUtils.write(destFileName, fileContents, {
+          backupFile: backupFileName,
+        });
+      ok(
+        await fileHasTextContents(destFileName, "Original file contents"),
+        "IOUtils::write creates a new file with the correct contents"
+      );
+      ok(
+        !await fileExists(backupFileName),
+        "IOUtils::write does not create a backup if the target file does not exist"
+      );
+      is(
+        bytesWritten,
+        fileContents.length,
+        "IOUtils::write correctly writes to a new file without performing a backup"
+      );
+
+      info("Test backup file option with existing destination");
+      let newFileContents = new TextEncoder().encode("New file contents");
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
+      bytesWritten =
+        await IOUtils.write(destFileName, newFileContents, {
+          backupFile: backupFileName,
+        });
+      ok(
+        await fileHasTextContents(backupFileName, "Original file contents"),
+        "IOUtils::write can backup an existing file before writing"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "New file contents"),
+        "IOUtils::write can create the target with the correct contents"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::write correctly writes to the target after taking a backup"
+      );
+
+      await cleanup(destFileName, backupFileName);
+    });
+
+    add_task(async function test_write_with_backup_and_tmp() {
+      info("Test backup with tmp and backup file options, non-existing destination");
+
+      let fileContents = new TextEncoder().encode("Original file contents");
+      let destFileName = PathUtils.join(PathUtils.tempDir, "test_write_with_backup_and_tmp_options.tmp");
+      let backupFileName = destFileName + ".backup";
+      let tmpFileName = PathUtils.join(PathUtils.tempDir, "temp_file.tmp");
+      let bytesWritten =
+        await IOUtils.write(destFileName, fileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+      ok(!await fileExists(tmpFileName), "IOUtils::write cleans up the tmpFile");
+      ok(
+        !await fileExists(backupFileName),
+        "IOUtils::write does not create a backup if the target file does not exist"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "Original file contents"),
+        "IOUtils::write can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        fileContents.length,
+        "IOUtils::write can copy tmp file to destination without performing a backup"
+      );
+
+      info("Test backup with tmp and backup file options, existing destination");
+      let newFileContents = new TextEncoder().encode("New file contents");
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
+      bytesWritten =
+        await IOUtils.write(destFileName, newFileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+
+      ok(!await fileExists(tmpFileName), "IOUtils::write cleans up the tmpFile");
+      ok(
+        await fileHasTextContents(backupFileName, "Original file contents"),
+        "IOUtils::write can create a backup if the target file exists"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "New file contents"),
+        "IOUtils::write can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::write IOUtils::write can move tmp file to destination after performing a backup"
+      );
+
+      info("Test backup with tmp and backup file options, existing destination and backup");
+      newFileContents = new TextEncoder().encode("Updated new file contents");
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
+      ok(await fileExists(backupFileName), `Expected ${backupFileName} to exist`);
+      bytesWritten =
+        await IOUtils.write(destFileName, newFileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+
+      ok(!await fileExists(tmpFileName), "IOUtils::write cleans up the tmpFile");
+      ok(
+        await fileHasTextContents(backupFileName, "New file contents"),
+        "IOUtils::write can create a backup if the target file exists"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "Updated new file contents"),
+        "IOUtils::write can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::write IOUtils::write can move tmp file to destination after performing a backup"
+      );
+
+      await cleanup(destFileName, backupFileName);
+    });
+
+    add_task(async function test_partial_read() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_partial_read.tmp");
+      const bytes = Uint8Array.of(...new Array(50).keys());
+      const bytesWritten = await IOUtils.write(tmpFileName, bytes);
+      is(
+        bytesWritten,
+        50,
+        "IOUtils::write can write entire byte array to file"
+      );
+
+      // Read just the first 10 bytes.
+      const first10 = bytes.slice(0, 10);
+      const bytes10 = await IOUtils.read(tmpFileName, { maxBytes: 10 });
+      ok(
+        ObjectUtils.deepEqual(bytes10, first10),
+        "IOUtils::read can read part of a file, up to specified max bytes"
+      );
+
+      // Trying to explicitly read nothing isn't useful, but it should still
+      // succeed.
+      const bytes0 = await IOUtils.read(tmpFileName, { maxBytes: 0 });
+      is(bytes0.length, 0, "IOUtils::read can read 0 bytes");
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_empty_read_and_write() {
+      // Trying to write an empty file isn't very useful, but it should still
+      // succeed.
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_empty.tmp");
+      const emptyByteArray = new Uint8Array(0);
+      const bytesWritten = await IOUtils.write(
+        tmpFileName,
+        emptyByteArray
+      );
+      is(bytesWritten, 0, "IOUtils::write can create an empty file");
+
+      // Trying to explicitly read nothing isn't useful, but it should still
+      // succeed.
+      const bytes0 = await IOUtils.read(tmpFileName, { maxBytes: 0 });
+      is(bytes0.length, 0, "IOUtils::read can read 0 bytes");
+
+      // Implicitly try to read nothing.
+      const nothing = await IOUtils.read(tmpFileName);
+      is(nothing.length, 0, "IOUtils:: read can read empty files");
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_full_read_and_write() {
+      // Write a file.
+
+      info("Test writing to a new binary file");
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_numbers.tmp");
+      const bytes = Uint8Array.of(...new Array(50).keys());
+      const bytesWritten = await IOUtils.write(tmpFileName, bytes);
+      is(
+        bytesWritten,
+        50,
+        "IOUtils::write can write entire byte array to file"
+      );
+
+      // Read it back.
+      info("Test reading a binary file");
+      let fileContents = await IOUtils.read(tmpFileName);
+      ok(
+        ObjectUtils.deepEqual(bytes, fileContents) &&
+        bytes.length == fileContents.length,
+        "IOUtils::read can read back entire file"
+      );
+
+      const tooManyBytes = bytes.length + 1;
+      fileContents = await IOUtils.read(tmpFileName, { maxBytes: tooManyBytes });
+      ok(
+        ObjectUtils.deepEqual(bytes, fileContents) &&
+        fileContents.length == bytes.length,
+        "IOUtils::read can read entire file when requested maxBytes is too large"
+      );
+
+      // Clean up.
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_write_relative_path() {
+      const tmpFileName = "test_ioutils_write_relative_path.tmp";
+      const bytes = Uint8Array.of(...new Array(50).keys());
+
+      info("Test writing a file at a relative destination");
+      await Assert.rejects(
+        IOUtils.write(tmpFileName, bytes),
+        /Could not parse path/,
+        "IOUtils::write only works with absolute paths"
+      );
+    });
+
+    add_task(async function test_read_relative_path() {
+      const tmpFileName = "test_ioutils_read_relative_path.tmp";
+
+      info("Test reading a file at a relative destination");
+      await Assert.rejects(
+        IOUtils.read(tmpFileName),
+        /Could not parse path/,
+        "IOUtils::write only works with absolute paths"
+      );
+    });
+
+    add_task(async function test_lz4() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_lz4.tmp");
+
+      info("Test writing lz4 encoded data");
+      const varyingBytes = Uint8Array.of(...new Array(50).keys());
+      let bytesWritten = await IOUtils.write(tmpFileName, varyingBytes, { compress: true });
+      is(bytesWritten, 64, "Expected to write 64 bytes");
+
+      info("Test reading lz4 encoded data");
+      let readData = await IOUtils.read(tmpFileName, { decompress: true });
+      ok(readData.equals(varyingBytes), "IOUtils can write and read back LZ4 encoded data");
+
+      info("Test writing lz4 compressed data");
+      const repeatedBytes = Uint8Array.of(...new Array(50).fill(1));
+      bytesWritten = await IOUtils.write(tmpFileName, repeatedBytes, { compress: true });
+      is(bytesWritten, 23, "Expected 50 bytes to compress to 23 bytes");
+
+      info("Test reading lz4 encoded data");
+      readData = await IOUtils.read(tmpFileName, { decompress: true });
+      ok(readData.equals(repeatedBytes), "IOUtils can write and read back LZ4 compressed data");
+
+      info("Test writing empty lz4 compressed data")
+      const empty = new Uint8Array();
+      bytesWritten = await IOUtils.write(tmpFileName, empty, { compress: true });
+      is(bytesWritten, 12, "Expected to write just the LZ4 header, with a content length of 0");
+
+
+      info("Test reading empty lz4 compressed data")
+      const readEmpty = await IOUtils.read(tmpFileName, { decompress: true });
+      ok(readEmpty.equals(empty), "IOUtils can write and read back empty buffers with LZ4");
+      const readEmptyRaw = await IOUtils.read(tmpFileName, { decompress: false });
+      is(readEmptyRaw.length, 12, "Expected to read back just the LZ4 header");
+      const expectedHeader = Uint8Array.of(109, 111, 122, 76, 122, 52, 48, 0, 0, 0, 0, 0); // "mozLz40\0\0\0\0"
+      ok(readEmptyRaw.equals(expectedHeader), "Expected to read header with content length of 0");
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_lz4_osfile_compat() {
+      const osfileTmpFile = PathUtils.join(PathUtils.tempDir, "test_ioutils_lz4_compat_osfile.tmp");
+      const ioutilsTmpFile = PathUtils.join(PathUtils.tempDir, "test_ioutils_lz4_compat_ioutils.tmp");
+
+      info("Test OS.File and IOUtils write the same file with LZ4 compression enabled")
+      const repeatedBytes = Uint8Array.of(...new Array(50).fill(1));
+      let expectedBytes = 23;
+      let ioutilsBytes = await IOUtils.write(ioutilsTmpFile, repeatedBytes, { compress: true });
+      let osfileBytes = await OS.File.writeAtomic(osfileTmpFile, repeatedBytes, { compression: "lz4" });
+      is(ioutilsBytes, expectedBytes, "IOUtils writes the expected number of bytes for compression");
+      is(osfileBytes, ioutilsBytes, "OS.File and IOUtils write the same number of bytes for LZ4 compression");
+
+      info("Test OS.File can read a file compressed by IOUtils");
+      const osfileReadBytes = await OS.File.read(ioutilsTmpFile, { compression: "lz4" });
+      ok(osfileReadBytes.every(byte => byte === 1), "OS.File can read a file compressed by IOUtils");
+      is(osfileReadBytes.length, 50, "OS.File reads the right number of bytes from a file compressed by IOUtils")
+
+      info("Test IOUtils can read a file compressed by OS.File");
+      const ioutilsReadBytes = await IOUtils.read(osfileTmpFile, { decompress: true });
+      ok(ioutilsReadBytes.every(byte => byte === 1), "IOUtils can read a file compressed by OS.File");
+      is(ioutilsReadBytes.length, 50, "IOUtils reads the right number of bytes from a file compressed by OS.File")
+
+      await cleanup(osfileTmpFile, ioutilsTmpFile);
+    });
+
+    add_task(async function test_lz4_bad_call() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_lz4_bad_call.tmp");
+
+      info("Test decompression with invalid options");
+      const varyingBytes = Uint8Array.of(...new Array(50).keys());
+      let bytesWritten = await IOUtils.write(tmpFileName, varyingBytes, { compress: true });
+      is(bytesWritten, 64, "Expected to write 64 bytes");
+      await Assert.rejects(
+        IOUtils.read(tmpFileName, { maxBytes: 4, decompress: true }),
+        /The `maxBytes` and `decompress` options are not compatible/,
+        "IOUtils::read rejects when maxBytes and decompress options are both used"
+      );
+
+      await cleanup(tmpFileName)
+    });
+
+    add_task(async function test_lz4_failure() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_lz4_fail.tmp");
+
+      info("Test decompression of non-lz4 data");
+      const repeatedBytes = Uint8Array.of(...new Array(50).fill(1));
+      await IOUtils.write(tmpFileName, repeatedBytes, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.read(tmpFileName, { decompress: true }),
+        (actual) => {
+          is(actual.constructor, DOMException,
+             "rejection reason constructor for decompress with bad header");
+          is(actual.name, "NotReadableError",
+             "rejection error name for decompress with bad header");
+          ok(/Could not decompress file because it has an invalid LZ4 header \(wrong magic number: .*\)/
+             .test(actual.message),
+             "rejection error message for decompress with bad header. Got "
+             + actual.message);
+          return true;
+        },
+        "IOUtils::read fails to decompress LZ4 data with a bad header"
+      );
+
+      info("Test decompression of short byte buffer");
+      const elevenBytes = Uint8Array.of(...new Array(11).fill(1));
+      await IOUtils.write(tmpFileName, elevenBytes, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.read(tmpFileName, { decompress: true }),
+        /Could not decompress file because the buffer is too short/,
+        "IOUtils::read fails to decompress LZ4 data with missing header"
+      );
+
+      info("Test decompression of valid header, but corrupt contents");
+      const headerFor10bytes = [109, 111, 122, 76, 122, 52, 48, 0, 10, 0, 0, 0] // "mozlz40\0" + 4 byte length
+      const badContents = new Array(11).fill(255); // Bad leading byte, followed by uncompressed stream.
+      const goodHeaderBadContents = Uint8Array.of(...headerFor10bytes, ...badContents);
+      await IOUtils.write(tmpFileName, goodHeaderBadContents, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.read(tmpFileName, { decompress: true }),
+        /Could not decompress file contents, the file may be corrupt/,
+        "IOUtils::read fails to read corrupt LZ4 contents with a correct header"
+      );
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_write_directory() {
+      const fileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_directory.tmp");
+      const tmpPath = `${fileName}.tmp`;
+      const bytes = Uint8Array.of(1, 2, 3, 4);
+
+      await IOUtils.makeDirectory(fileName);
+      await Assert.rejects(
+        IOUtils.write(fileName, bytes),
+        /NotAllowedError: Could not open the file at .+ for writing/);
+
+      await Assert.rejects(
+        IOUtils.write(fileName, bytes, { tmpPath }),
+        /NotAllowedError: Could not open the file at .+ for writing/);
+
+      ok(!await IOUtils.exists(PathUtils.join(fileName, PathUtils.filename(tmpPath))));
+    });
+
+    add_task(async function test_read_offset() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_read_offset.tmp");
+
+      const bytes = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+      const byteArray = Uint8Array.of(...bytes);
+
+      await IOUtils.write(tmpFileName, byteArray);
+
+      for (const offset of [0, 5]) {
+        info(`Reading bytes from offset ${offset}`);
+
+        const readBytes = await IOUtils.read(tmpFileName, { offset });
+        Assert.deepEqual(
+          Array.from(readBytes),
+          bytes.slice(offset),
+          `should have read bytes from offset ${offset}`
+        );
+      }
+
+      for (const offset of [0, 5]) {
+        info(`Reading up to 5 bytes from offset ${offset}`);
+
+        const readBytes = await IOUtils.read(tmpFileName, {offset, maxBytes: 5});
+        Assert.deepEqual(
+          Array.from(readBytes),
+          bytes.slice(offset, offset + 5),
+          `should have read 5 bytes from offset ${offset}`
+        );
+      }
+
+      {
+        info(`Reading bytes from offset 10`);
+        const readBytes = await IOUtils.read(tmpFileName, {offset: 10});
+        is(readBytes.length, 0, "should have read 0 bytes");
+      }
+
+      {
+        info(`Reading up to 10 bytes from offset 5`);
+        const readBytes = await IOUtils.read(tmpFileName, {offset: 5, maxBytes: 10});
+        is(readBytes.length, 5, "should have read 5 bytes");
+        Assert.deepEqual(
+          Array.from(readBytes),
+          bytes.slice(5, 10),
+          "should have read last 5 bytes"
+        );
+      }
+    });
+
+    add_task(async function test_write_appendOrCreate() {
+      const fileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_appendOrCreate.tmp");
+
+      await IOUtils.write(fileName, Uint8Array.of(0, 1, 2, 3, 4), { mode: "appendOrCreate" });
+
+      {
+        const contents = await IOUtils.read(fileName);
+        Assert.deepEqual(Array.from(contents), [0, 1, 2, 3, 4], "read bytes should be equal");
+      }
+
+      await IOUtils.write(fileName, Uint8Array.of(5, 6, 7, 8, 9), { mode: "appendOrCreate" });
+
+      {
+        const contents = await IOUtils.read(fileName);
+        Assert.deepEqual(Array.from(contents), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "read bytes should be equal after appendOrCreateing");
+      }
+
+      await cleanup(fileName);
+    });
+
+    add_task(async function test_write_append() {
+      const fileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_append.tmp");
+
+      await IOUtils.write(fileName, Uint8Array.of(0, 1, 2, 3, 4));
+
+      const beforeAppend = await IOUtils.read(fileName);
+      Assert.deepEqual(Array.from(beforeAppend), [0, 1, 2, 3, 4], "read bytes should be equal");
+
+      await IOUtils.write(fileName, Uint8Array.of(5, 6, 7, 8, 9), { mode: "append" });
+
+      const afterAppend = await IOUtils.read(fileName);
+      Assert.deepEqual(Array.from(afterAppend), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "read bytes should be equal after appending");
+
+      await cleanup(fileName);
+    });
+
+    add_task(async function test_write_append_no_create() {
+      const fileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_append_no_create.tmp");
+
+      await Assert.rejects(
+        IOUtils.write(fileName, Uint8Array.of(5, 6, 7, 8, 9), { mode: "append" }),
+        /NotFoundError: Could not open the file at .*/
+      );
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_read_write_json.html b/dom/system/tests/ioutils/test_ioutils_read_write_json.html
new file mode 100644
index 0000000000..e356e50c47
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_read_write_json.html
@@ -0,0 +1,165 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import(
+      "resource://gre/modules/ObjectUtils.jsm"
+    );
+
+    const OBJECT = {
+      "foo": [
+        "bar",
+        123,
+        456.789,
+        true,
+        false,
+        null,
+      ],
+      "bar": {
+        "baz": {},
+      },
+    };
+
+    const ARRAY = [1, 2.3, true, false, null, { "foo": "bar" }];
+
+    const PRIMITIVES = [123, true, false, "hello, world", null];
+
+    add_task(async function read_json() {
+      const filename = PathUtils.join(PathUtils.tempDir, "test_ioutils_read_json.tmp");
+
+      info("Testing IOUtils.readJSON() with a serialized object...");
+      await IOUtils.writeUTF8(filename, JSON.stringify(OBJECT));
+      const readObject = await IOUtils.readJSON(filename);
+      const parsedObject = JSON.parse(await IOUtils.readUTF8(filename));
+      ok(ObjectUtils.deepEqual(OBJECT, readObject), "JSON objects should round-trip");
+      ok(
+        ObjectUtils.deepEqual(parsedObject, readObject),
+        "IOUtils.readJSON() equivalent to JSON.parse() for objects"
+      );
+
+      info("Testing IOUtils.readJSON() with a serialized array...");
+      await IOUtils.writeUTF8(filename, JSON.stringify(ARRAY));
+      const readArray = await IOUtils.readJSON(filename);
+      const parsedArray = JSON.parse(await IOUtils.readUTF8(filename));
+      ok(ObjectUtils.deepEqual(ARRAY, readArray), "JSON arrays should round-trip");
+      ok(
+        ObjectUtils.deepEqual(parsedArray, readArray),
+        "IOUtils.readJSON() equivalent to JSON.parse(IOUtils.readUTF8()) for arrays"
+      );
+
+      info("Testing IOUtils.readJSON() with serialized primitives...");
+      for (const primitive of PRIMITIVES) {
+        await IOUtils.writeUTF8(filename, JSON.stringify(primitive));
+        const readPrimitive = await IOUtils.readJSON(filename);
+        const parsedPrimitive = JSON.parse(await IOUtils.readUTF8(filename));
+        ok(primitive === readPrimitive, `JSON primitive ${primitive} should round trip`);
+        ok(
+          readPrimitive === parsedPrimitive,
+          `${readPrimitive} === ${parsedPrimitive} -- IOUtils.readJSON() equivalent to JSON.parse() for primitive`
+        );
+      }
+
+      info("Testing IOUtils.readJSON() with a file that does not exist...");
+      const notExistsFilename = PathUtils.join(PathUtils.tempDir, "test_ioutils_read_json_not_exists.tmp");
+      ok(!await IOUtils.exists(notExistsFilename), `${notExistsFilename} should not exist`);
+      await Assert.rejects(
+        IOUtils.readJSON(notExistsFilename),
+        /NotFoundError: Could not open the file at/,
+        "IOUtils::readJSON rejects when file does not exist"
+      );
+
+      info("Testing IOUtils.readJSON() with a file that does not contain JSON");
+      const invalidFilename = PathUtils.join(PathUtils.tempDir, "test_ioutils_read_json_invalid.tmp");
+      await IOUtils.writeUTF8(invalidFilename, ":)");
+
+      await Assert.rejects(
+        IOUtils.readJSON(invalidFilename),
+        /SyntaxError: JSON\.parse/,
+        "IOUTils::readJSON rejects when the file contains invalid JSON"
+      );
+
+      await cleanup(filename, invalidFilename);
+    });
+
+    add_task(async function write_json() {
+      const filename = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_json.tmp");
+
+      info("Testing IOUtils.writeJSON() with an object...");
+      await IOUtils.writeJSON(filename, OBJECT);
+      const readObject = await IOUtils.readJSON(filename);
+      const readObjectStr = await IOUtils.readUTF8(filename);
+      ok(ObjectUtils.deepEqual(OBJECT, readObject), "JSON objects should round-trip");
+      ok(
+        readObjectStr === JSON.stringify(OBJECT),
+        "IOUtils.writeJSON() eqvuialent to JSON.stringify() for an object"
+      );
+
+      info("Testing IOUtils.writeJSON() with an array...");
+      await IOUtils.writeJSON(filename, ARRAY);
+      const readArray = await IOUtils.readJSON(filename);
+      const readArrayStr = await IOUtils.readUTF8(filename);
+      ok(ObjectUtils.deepEqual(ARRAY, readArray), "JSON arrays should round-trip");
+      ok(
+        readArrayStr === JSON.stringify(ARRAY),
+        "IOUtils.writeJSON() equivalent to JSON.stringify() for an array"
+      );
+
+      info("Testing IOUtils.writeJSON() with primitives...");
+      for (const primitive of PRIMITIVES) {
+        await IOUtils.writeJSON(filename, primitive);
+        const readPrimitive = await IOUtils.readJSON(filename);
+        const readPrimitiveStr = await IOUtils.readUTF8(filename);
+        ok(
+          primitive === readPrimitive,
+          `${primitive} === ${readPrimitive} -- IOUtils.writeJSON() should round trip primitive`
+        );
+        ok(
+          readPrimitiveStr === JSON.stringify(primitive),
+          `${readPrimitiveStr} === ${JSON.stringify(primitive)} -- IOUtils.writeJSON() equivalent to JSON.stringify for primitive`
+        );
+      }
+
+      info("Testing IOUtils.writeJSON() with unserializable objects...");
+      await Assert.rejects(
+        IOUtils.writeJSON(filename, window),
+        /TypeError: cyclic object value/,
+        "IOUtils.writeJSON() cannot write cyclic objects"
+      );
+
+      await cleanup(filename);
+    });
+
+    add_task(async function test_append_json() {
+      const filename = PathUtils.join(PathUtils.tempDir, "test_ioutils_append_json.tmp");
+
+      await IOUtils.writeJSON(filename, OBJECT);
+
+      await Assert.rejects(
+        IOUtils.writeJSON(filename, OBJECT, {mode: "append"}),
+        /NotSupportedError: IOUtils.writeJSON does not support appending to files/,
+        "IOUtils.writeJSON() cannot append"
+      );
+
+      await cleanup(filename);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_read_write_utf8.html b/dom/system/tests/ioutils/test_ioutils_read_write_utf8.html
new file mode 100644
index 0000000000..9ce4ff615a
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_read_write_utf8.html
@@ -0,0 +1,387 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    // This is presently only used to test compatability between OS.File and
+    // IOUtils when it comes to writing compressed files. The import and the
+    // test `test_lz4_osfile_compat` can be removed with OS.File is removed.
+    const { OS } = ChromeUtils.import("resource://gre/modules/osfile.jsm");
+
+    // This is an impossible sequence of bytes in an UTF-8 encoded file.
+    // See section 3.5.3 of this text:
+    // https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt
+    const invalidUTF8 = Uint8Array.of(0xfe, 0xfe, 0xff, 0xff);
+
+    add_task(async function test_read_utf8_failure() {
+      info("Test attempt to read non-existent file (UTF8)");
+      const doesNotExist = PathUtils.join(PathUtils.tempDir, "does_not_exist.tmp");
+      await Assert.rejects(
+        IOUtils.readUTF8(doesNotExist),
+        /Could not open the file at .*/,
+        "IOUtils::readUTF8 rejects when file does not exist"
+      );
+
+      info("Test attempt to read invalid UTF-8");
+      const invalidUTF8File = PathUtils.join(PathUtils.tempDir, "invalid_utf8.tmp");
+
+      // Deliberately write the invalid byte sequence to file.
+      await IOUtils.write(invalidUTF8File, invalidUTF8);
+
+      await Assert.rejects(
+        IOUtils.readUTF8(invalidUTF8File),
+        /Could not read file\(.*\) because it is not UTF-8 encoded/,
+        "IOUtils::readUTF8 will reject when reading a file that is not valid UTF-8"
+      );
+
+      await cleanup(invalidUTF8File);
+    });
+
+    add_task(async function test_write_utf8_no_overwrite() {
+      // Make a new file, and try to write to it with overwrites disabled.
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_write_utf8_overwrite.tmp");
+      const untouchableContents = "Can't touch this!\n";
+      await IOUtils.writeUTF8(tmpFileName, untouchableContents);
+
+      const newContents = "Nah nah nah!\n";
+      await Assert.rejects(
+        IOUtils.writeUTF8(tmpFileName, newContents, {
+          mode: "create",
+        }),
+        /Refusing to overwrite the file at */,
+        "IOUtils::writeUTF8 rejects writing to existing file if overwrites are disabled"
+      );
+      ok(
+        await fileHasTextContents(tmpFileName, untouchableContents),
+        "IOUtils::writeUTF8 doesn't change target file when overwrite is refused"
+      );
+
+      const bytesWritten = await IOUtils.writeUTF8(
+        tmpFileName,
+        newContents,
+        { mode: "overwrite" }
+      );
+      is(
+        bytesWritten,
+        newContents.length,
+        "IOUtils::writeUTF8 can overwrite files if specified"
+      );
+      ok(
+        await fileHasTextContents(tmpFileName, newContents),
+        "IOUtils::writeUTF8 overwrites with the expected contents"
+      );
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_write_with_backup() {
+      info("Test backup file option with non-existing file");
+      let fileContents = "Original file contents";
+      let destFileName = PathUtils.join(PathUtils.tempDir, "test_write_utf8_with_backup_option.tmp");
+      let backupFileName = destFileName + ".backup";
+      let bytesWritten =
+        await IOUtils.writeUTF8(destFileName, fileContents, {
+          backupFile: backupFileName,
+        });
+      ok(
+        await fileHasTextContents(destFileName, "Original file contents"),
+        "IOUtils::writeUTF8 creates a new file with the correct contents"
+      );
+      ok(
+        !await fileExists(backupFileName),
+        "IOUtils::writeUTF8 does not create a backup if the target file does not exist"
+      );
+      is(
+        bytesWritten,
+        fileContents.length,
+        "IOUtils::write correctly writes to a new file without performing a backup"
+      );
+
+      info("Test backup file option with existing destination");
+      let newFileContents = "New file contents";
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
+      bytesWritten =
+        await IOUtils.writeUTF8(destFileName, newFileContents, {
+          backupFile: backupFileName,
+        });
+      ok(
+        await fileHasTextContents(backupFileName, "Original file contents"),
+        "IOUtils::writeUTF8 can backup an existing file before writing"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "New file contents"),
+        "IOUtils::writeUTF8 can create the target with the correct contents"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::writeUTF8 correctly writes to the target after taking a backup"
+      );
+
+      await cleanup(destFileName, backupFileName);
+    });
+
+    add_task(async function test_write_with_backup_and_tmp() {
+      info("Test backup with tmp and backup file options, non-existing destination");
+      let fileContents = "Original file contents";
+      let destFileName = PathUtils.join(PathUtils.tempDir, "test_write_utf8_with_backup_and_tmp_options.tmp");
+      let backupFileName = destFileName + ".backup";
+      let tmpFileName = PathUtils.join(PathUtils.tempDir, "temp_file.tmp");
+      let bytesWritten =
+        await IOUtils.writeUTF8(destFileName, fileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+      ok(!await fileExists(tmpFileName), "IOUtils::writeUTF8 cleans up the tmpFile");
+      ok(
+        !await fileExists(backupFileName),
+        "IOUtils::writeUTF8 does not create a backup if the target file does not exist"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "Original file contents"),
+        "IOUtils::writeUTF8 can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        fileContents.length,
+        "IOUtils::writeUTF8 can copy tmp file to destination without performing a backup"
+      );
+
+      info("Test backup with tmp and backup file options, existing destination");
+      let newFileContents = "New file contents";
+      bytesWritten =
+        await IOUtils.writeUTF8(destFileName, newFileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+
+      ok(!await fileExists(tmpFileName), "IOUtils::writeUTF8 cleans up the tmpFile");
+      ok(
+        await fileHasTextContents(backupFileName, "Original file contents"),
+        "IOUtils::writeUTF8 can create a backup if the target file exists"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "New file contents"),
+        "IOUtils::writeUTF8 can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::writeUTF8 can move tmp file to destination after performing a backup"
+      );
+
+      await cleanup(destFileName, backupFileName);
+    });
+
+    add_task(async function test_empty_read_and_write_utf8() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_empty_utf8.tmp");
+      const emptyString = ""
+      const bytesWritten = await IOUtils.writeUTF8(
+        tmpFileName,
+        emptyString
+      );
+      is(bytesWritten, 0, "IOUtils::writeUTF8 can create an empty file");
+
+      const nothing = await IOUtils.readUTF8(tmpFileName);
+      is(nothing.length, 0, "IOUtils::readUTF8 can read empty files");
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_full_read_and_write_utf8() {
+      // Write a file.
+      info("Test writing emoji file");
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_emoji.tmp");
+
+      // Make sure non-ASCII text is supported for writing and reading back.
+      // For fun, a sampling of space-separated emoji characters from different
+      // Unicode versions, including multi-byte glyphs that are rendered using
+      // ZWJ sequences.
+      const emoji = "☕️ ⚧️ 😀 🖖🏿 🤠 🏳️‍🌈 🥠 🏴‍☠️ 🪐";
+      const expectedBytes = 71;
+      const bytesWritten = await IOUtils.writeUTF8(tmpFileName, emoji);
+      is(
+        bytesWritten,
+        expectedBytes,
+        "IOUtils::writeUTF8 can write emoji to file"
+      );
+
+      // Read it back.
+      info("Test reading emoji from file");
+      let fileContents = await IOUtils.readUTF8(tmpFileName);
+      ok(
+        emoji == fileContents &&
+        emoji.length == fileContents.length,
+        "IOUtils::readUTF8 can read back entire file"
+      );
+
+      // Clean up.
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_write_utf8_relative_path() {
+      const tmpFileName = "test_ioutils_write_utf8_relative_path.tmp";
+
+      info("Test writing a file at a relative destination");
+      await Assert.rejects(
+        IOUtils.writeUTF8(tmpFileName, "foo"),
+        /Could not parse path/,
+        "IOUtils::writeUTF8 only works with absolute paths"
+      );
+    });
+
+    add_task(async function test_read_utf8_relative_path() {
+      const tmpFileName = "test_ioutils_read_utf8_relative_path.tmp";
+
+      info("Test reading a file at a relative destination");
+      await Assert.rejects(
+        IOUtils.readUTF8(tmpFileName),
+        /Could not parse path/,
+        "IOUtils::readUTF8 only works with absolute paths"
+      );
+    });
+
+
+    add_task(async function test_utf8_lz4() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_utf8_lz4.tmp");
+
+      info("Test writing lz4 encoded UTF-8 string");
+      const emoji = "☕️ ⚧️ 😀 🖖🏿 🤠 🏳️‍🌈 🥠 🏴‍☠️ 🪐";
+      let bytesWritten = await IOUtils.writeUTF8(tmpFileName, emoji, { compress: true });
+      is(bytesWritten, 83, "Expected to write 64 bytes");
+
+      info("Test reading lz4 encoded UTF-8 string");
+      let readData = await IOUtils.readUTF8(tmpFileName, { decompress: true });
+      is(readData, emoji, "IOUtils can write and read back UTF-8 LZ4 encoded data");
+
+      info("Test writing lz4 compressed UTF-8 string");
+      const lotsOfCoffee = new Array(24).fill("☕️").join(""); // ☕️ is 3 bytes in UTF-8: \0xe2 \0x98 \0x95
+      bytesWritten = await IOUtils.writeUTF8(tmpFileName, lotsOfCoffee, { compress: true });
+      console.log(bytesWritten);
+      is(bytesWritten, 28, "Expected 72 bytes to compress to 28 bytes");
+
+      info("Test reading lz4 encoded UTF-8 string");
+      readData = await IOUtils.readUTF8(tmpFileName, { decompress: true });
+      is(readData, lotsOfCoffee, "IOUtils can write and read back UTF-8 LZ4 compressed data");
+
+      info("Test writing empty lz4 compressed UTF-8 string")
+      const empty = "";
+      bytesWritten = await IOUtils.writeUTF8(tmpFileName, empty, { compress: true });
+      is(bytesWritten, 12, "Expected to write just the LZ4 header");
+
+      info("Test reading empty lz4 compressed UTF-8 string")
+      const readEmpty = await IOUtils.readUTF8(tmpFileName, { decompress: true });
+      is(readEmpty, empty, "IOUtils can write and read back empty buffers with LZ4");
+      const readEmptyRaw = await IOUtils.readUTF8(tmpFileName, { decompress: false });
+      is(readEmptyRaw.length, 12, "Expected to read back just the LZ4 header");
+
+      await cleanup(tmpFileName);
+    });
+
+    add_task(async function test_utf8_lz4_osfile_compat() {
+      const osfileTmpFile = PathUtils.join(PathUtils.tempDir, "test_ioutils_utf8_lz4_compat_osfile.tmp");
+      const ioutilsTmpFile = PathUtils.join(PathUtils.tempDir, "test_ioutils_utf8_lz4_compat_ioutils.tmp");
+
+      info("Test OS.File and IOUtils write the same UTF-8 file with LZ4 compression enabled")
+      const emoji = "☕️ ⚧️ 😀 🖖🏿 🤠 🏳️‍🌈 🥠 🏴‍☠️ 🪐";
+      let expectedBytes = 83;
+      let ioutilsBytes = await IOUtils.writeUTF8(ioutilsTmpFile, emoji, { compress: true });
+      let osfileBytes = await OS.File.writeAtomic(osfileTmpFile, emoji, { compression: "lz4" });
+      is(ioutilsBytes, expectedBytes, "IOUtils writes the expected number of bytes for compression");
+      is(osfileBytes, ioutilsBytes, "OS.File and IOUtils write the same number of bytes for LZ4 compression");
+
+      info("Test OS.File can read an UTF-8 file compressed by IOUtils");
+      const osfileReadStr = await OS.File.read(ioutilsTmpFile, { compression: "lz4", encoding: "utf-8" });
+      is(osfileReadStr, emoji, "OS.File can read an UTF-8 file compressed by IOUtils")
+
+      info("Test IOUtils can read an UTF-8 file compressed by OS.File");
+      const ioutilsReadString = await IOUtils.readUTF8(ioutilsTmpFile, { decompress: true });
+      is(ioutilsReadString, emoji, "IOUtils can read an UTF-8 file compressed by OS.File");
+
+      await cleanup(osfileTmpFile, ioutilsTmpFile);
+    });
+
+    add_task(async function test_utf8_lz4_bad_call() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_utf8_lz4_bad_call.tmp");
+
+      info("readUTF8 ignores the maxBytes option if provided");
+      const emoji = "☕️ ⚧️ 😀 🖖🏿 🤠 🏳️‍🌈 🥠 🏴‍☠️ 🪐";
+      let bytesWritten = await IOUtils.writeUTF8(tmpFileName, emoji, { compress: true });
+      is(bytesWritten, 83, "Expected to write 83 bytes");
+
+      let readData = await IOUtils.readUTF8(tmpFileName, { maxBytes: 4, decompress: true });
+      is(readData, emoji, "IOUtils can write and read back UTF-8 LZ4 encoded data");
+
+      await cleanup(tmpFileName)
+    });
+
+    add_task(async function test_utf8_lz4_failure() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_utf8_lz4_fail.tmp");
+
+      info("Test decompression of non-lz4 UTF-8 string");
+      const repeatedBytes = Uint8Array.of(...new Array(50).fill(1));
+      await IOUtils.write(tmpFileName, repeatedBytes, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.readUTF8(tmpFileName, { decompress: true }),
+        /Could not decompress file because it has an invalid LZ4 header \(wrong magic number: .*\)/,
+        "IOUtils::readUTF8 fails to decompress LZ4 data with a bad header"
+      );
+
+      info("Test UTF-8 decompression of short byte buffer");
+      const elevenBytes = Uint8Array.of(...new Array(11).fill(1));
+      await IOUtils.write(tmpFileName, elevenBytes, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.readUTF8(tmpFileName, { decompress: true }),
+        /Could not decompress file because the buffer is too short/,
+        "IOUtils::readUTF8 fails to decompress LZ4 data with missing header"
+      );
+
+      info("Test UTF-8 decompression of valid header, but corrupt contents");
+      const headerFor10bytes = [109, 111, 122, 76, 122, 52, 48, 0, 10, 0, 0, 0] // "mozlz40\0" + 4 byte length
+      const badContents = new Array(11).fill(255); // Bad leading byte, followed by uncompressed stream.
+      const goodHeaderBadContents = Uint8Array.of(...headerFor10bytes, ...badContents);
+      await IOUtils.write(tmpFileName, goodHeaderBadContents, { compress: false });
+
+      await Assert.rejects(
+        IOUtils.readUTF8(tmpFileName, { decompress: true }),
+        /Could not decompress file contents, the file may be corrupt/,
+        "IOUtils::readUTF8 fails to read corrupt LZ4 contents with a correct header"
+      );
+
+      info("Testing decompression of an empty file (no header)");
+      {
+        const n = await IOUtils.writeUTF8(tmpFileName, "");
+        ok(n === 0, "Overwrote with empty file");
+      }
+      await Assert.rejects(
+        IOUtils.readUTF8(tmpFileName, { decompress: true }),
+        /Could not decompress file because the buffer is too short/,
+        "IOUtils::readUTF8 fails to decompress empty files"
+      );
+
+      await cleanup(tmpFileName);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_remove.html b/dom/system/tests/ioutils/test_ioutils_remove.html
new file mode 100644
index 0000000000..e6aaf924f2
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_remove.html
@@ -0,0 +1,90 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    add_task(async function test_create_and_remove_file() {
+      info("Test creating and removing a single file");
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutils_create_and_remove.tmp");
+      await IOUtils.write(tmpFileName, new Uint8Array(0));
+      ok(await fileExists(tmpFileName), `Expected file ${tmpFileName} to exist`);
+
+      await IOUtils.remove(tmpFileName);
+      ok(!await fileExists(tmpFileName), "IOUtils::remove can remove files");
+
+      info("Test creating and removing an empty directory");
+      const tempDirName = PathUtils.join(PathUtils.tempDir, "test_ioutils_create_and_remove.tmp.d");
+      await IOUtils.makeDirectory(tempDirName);
+      ok(await dirExists(tempDirName), `Expected directory ${tempDirName} to exist`);
+
+      await IOUtils.remove(tempDirName);
+      ok(!await dirExists(tempDirName), "IOUtils::remove can remove empty directories");
+    });
+
+    add_task(async function test_remove_non_existing() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_ioutil_remove_non_existing.tmp");
+      ok(!await fileExists(tmpFileName), `Expected file ${tmpFileName} not to exist`);
+
+      await IOUtils.remove(tmpFileName, { ignoreAbsent: true });
+      ok(!await fileExists(tmpFileName), "IOUtils::remove can ignore missing files without error");
+
+      await Assert.rejects(
+        IOUtils.remove(tmpFileName, { ignoreAbsent: false }),
+        /Could not remove the file at .* because it does not exist/,
+        "IOUtils::remove can throw an error when target file is missing"
+      );
+      ok(!await fileExists(tmpFileName), `Expected file ${tmpFileName} not to exist`);
+    });
+
+    add_task(async function test_remove_recursive() {
+      const tmpParentDir = PathUtils.join(PathUtils.tempDir, "test_ioutils_remove.tmp.d");
+      const tmpChildDir = PathUtils.join(tmpParentDir, "child.tmp.d");
+      const tmpTopLevelFileName = PathUtils.join(tmpParentDir, "top.tmp");
+      const tmpNestedFileName = PathUtils.join(tmpChildDir, "nested.tmp");
+      await createDir(tmpChildDir);
+      await createFile(tmpTopLevelFileName, "");
+      await createFile(tmpNestedFileName, "");
+
+      ok(
+        await fileExists(tmpTopLevelFileName),
+        `Expected file ${tmpTopLevelFileName} to exist`
+      );
+      ok(
+        await fileExists(tmpNestedFileName),
+        `Expected file  ${tmpNestedFileName} to exist`
+      );
+
+      await Assert.rejects(
+        IOUtils.remove(tmpParentDir, { recursive: false }),
+        /Could not remove the non-empty directory at .*/,
+        "IOUtils::remove fails if non-recursively removing directory with contents"
+      );
+
+      await IOUtils.remove(tmpParentDir, { recursive: true });
+      ok(
+        !await dirExists(tmpParentDir),
+        "IOUtils::remove can recursively remove a directory"
+      );
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_set_permissions.html b/dom/system/tests/ioutils/test_ioutils_set_permissions.html
new file mode 100644
index 0000000000..36f7dab72a
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_set_permissions.html
@@ -0,0 +1,84 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    add_task(async function test_setPermissions() {
+      const tempFile = PathUtils.join(PathUtils.tempDir, "setPermissions.tmp");
+
+      await IOUtils.writeUTF8(tempFile, "");
+      await IOUtils.setPermissions(tempFile, 0o421);
+
+      let stat = await IOUtils.stat(tempFile);
+
+      if (Services.appinfo.OS === "WINNT") {
+        // setPermissions ignores the x bit on Windows.
+        is(stat.permissions, 0o666, "Permissions munged on Windows");
+      } else {
+        let umask = Services.sysinfo.getProperty("umask");
+        is(stat.permissions, 0o421 & ~umask, "Permissions match");
+      }
+
+      await IOUtils.setPermissions(tempFile, 0o400);
+      stat = await IOUtils.stat(tempFile);
+
+      if (Services.appinfo.OS === "WINNT") {
+        is(stat.permissions, 0o444, "Permissions munged on Windows");
+
+        // We need to make the file writable to delete it on Windows.
+        await IOUtils.setPermissions(tempFile, 0o600);
+      } else {
+        is(stat.permissions, 0o400, "Permissions match");
+      }
+
+      await cleanup(tempFile);
+    });
+
+    add_task(async function test_setPermissionsWithoutHonoringUmask() {
+      const tempFile = PathUtils.join(PathUtils.tempDir, "setPermissions.tmp");
+
+      await IOUtils.writeUTF8(tempFile, "");
+      await IOUtils.setPermissions(tempFile, 0o421, false);
+
+      let stat = await IOUtils.stat(tempFile);
+
+      if (Services.appinfo.OS === "WINNT") {
+        // setPermissions ignores the x bit on Windows.
+        is(stat.permissions, 0o666, "Permissions munged on Windows");
+      } else {
+        is(stat.permissions, 0o421, "Permissions match");
+      }
+
+      await IOUtils.setPermissions(tempFile, 0o400);
+      stat = await IOUtils.stat(tempFile);
+
+      if (Services.appinfo.OS === "WINNT") {
+        is(stat.permissions, 0o444, "Permissions munged on Windows");
+
+        // We need to make the file writable to delete it on Windows.
+        await IOUtils.setPermissions(tempFile, 0o600);
+      } else {
+        is(stat.permissions, 0o400, "Permissions match");
+      }
+
+      await cleanup(tempFile);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_stat_set_modification_time.html b/dom/system/tests/ioutils/test_ioutils_stat_set_modification_time.html
new file mode 100644
index 0000000000..8f8328bd81
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_stat_set_modification_time.html
@@ -0,0 +1,241 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+    const { ObjectUtils } = ChromeUtils.import("resource://gre/modules/ObjectUtils.jsm");
+
+    add_task(async function test_stat() {
+      info("Test attempt to stat a regular empty file");
+
+      const emptyFileName = PathUtils.join(PathUtils.tempDir, "test_stat_empty.tmp");
+      await createFile(emptyFileName);
+
+      const emptyFileInfo = await IOUtils.stat(emptyFileName);
+      is(emptyFileInfo.size, 0, "IOUtils::stat can get correct (empty) file size");
+      is(emptyFileInfo.path, emptyFileName, "IOUtils::stat result contains the path");
+      is(emptyFileInfo.type, "regular", "IOUtils::stat can stat regular (empty) files");
+      Assert.less(
+        (emptyFileInfo.lastModified - new Date().valueOf()),
+        1000, // Allow for 1 second deviation in case of slow tests.
+        "IOUtils::stat can get the last modification date for a regular file"
+      );
+
+      info("Test attempt to stat a regular binary file");
+      const tempFileName = PathUtils.join(PathUtils.tempDir, "test_stat_binary.tmp");
+      const bytes = Uint8Array.of(...new Array(50).keys());
+      await createFile(tempFileName, bytes);
+
+      const fileInfo = await IOUtils.stat(tempFileName);
+      is(fileInfo.size, 50, "IOUtils::stat can get correct file size");
+      is(fileInfo.path, tempFileName, "IOUtils::stat result contains the path");
+      is(fileInfo.type, "regular", "IOUtils::stat can stat regular files");
+      Assert.less(
+        (fileInfo.lastModified - new Date().valueOf()),
+        1000, // Allow for 1 second deviation in case of slow tests.
+        "IOUtils::stat can get the last modification date for a regular file"
+      );
+
+      info("Test attempt to stat a directory");
+      const tempDirName = PathUtils.join(PathUtils.tempDir, "test_stat_dir.tmp.d");
+      await IOUtils.makeDirectory(tempDirName);
+
+      const dirInfo = await IOUtils.stat(tempDirName);
+      is(dirInfo.size, -1, "IOUtils::stat reports -1 size for directories")
+      is(fileInfo.path, tempFileName, "IOUtils::stat result contains the path");
+      is(fileInfo.type, "regular", "IOUtils::stat can stat directories");
+      Assert.less(
+        (fileInfo.lastModified - new Date().valueOf()),
+        1000, // Allow for 1 second deviation in case of slow tests.
+        "IOUtils::stat can get the last modification date for a regular file"
+      );
+      Assert.less(
+        (fileInfo.lastAccessed - new Date().valueOf()),
+        1000,
+        "IOUtils::stat can get the last access date for a regular file"
+      );
+
+      await cleanup(emptyFileName, tempFileName, tempDirName)
+    });
+
+    add_task(async function test_stat_failures() {
+      info("Test attempt to stat a non-existing file");
+
+      const notExistsFile = PathUtils.join(PathUtils.tempDir, "test_stat_not_exists.tmp");
+
+      await Assert.rejects(
+        IOUtils.stat(notExistsFile),
+        /Could not stat file\(.*\) because it does not exist/,
+        "IOUtils::stat throws if the target file does not exist"
+      );
+    });
+
+    add_task(async function test_setModificationTime_and_stat() {
+      const tmpFileName = PathUtils.join(PathUtils.tempDir, "test_setModificationTime_and_stat.tmp");
+      {
+        info("Test attempt to setModificationTime a file");
+        await createFile(tmpFileName);
+
+        const oldFileInfo = await IOUtils.stat(tmpFileName);
+        await sleep(500);
+
+        // Now update the time stamp.
+        const stamp = await IOUtils.setModificationTime(tmpFileName);
+        const newFileInfo = await IOUtils.stat(tmpFileName);
+
+        ok(
+          newFileInfo.lastModified > oldFileInfo.lastModified,
+          "IOUtils::setModificationTime can update the lastModified time stamp on the file system"
+        );
+        is(
+          stamp,
+          newFileInfo.lastModified,
+          "IOUtils::setModificationTime returns the updated time stamp."
+        );
+        is(
+          newFileInfo.lastAccessed,
+          oldFileInfo.lastAccessed,
+          "IOUtils::setModificationTime does not change lastAccessed"
+        );
+
+        await sleep(500);
+
+        const newerStamp = await IOUtils.setAccessTime(tmpFileName);
+        const newerFileInfo = await IOUtils.stat(tmpFileName);
+
+        ok(
+          newerFileInfo.lastAccessed > newFileInfo.lastAccessed,
+          "IOUtils::setAccessTime can update the lastAccessed time stamp on the file system"
+        );
+        is(
+          newerStamp,
+          newerFileInfo.lastAccessed,
+          "IOUtils::setAccessTime returns the updated time stamp."
+        );
+        is(
+          newerFileInfo.lastModified,
+          newFileInfo.lastModified,
+          "IOUtils::setAccessTime does not change lastModified"
+        );
+      }
+
+      const tmpDirName = PathUtils.join(PathUtils.tempDir, "test_setModificationTime_and_stat.tmp.d");
+      {
+        info("Test attempt to setModificationTime a directory");
+        await createDir(tmpDirName);
+
+        const oldFileInfo = await IOUtils.stat(tmpDirName);
+        await sleep(500);
+
+        const stamp = await IOUtils.setModificationTime(tmpDirName);
+        const newFileInfo = await IOUtils.stat(tmpDirName);
+
+        ok(
+          newFileInfo.lastModified > oldFileInfo.lastModified,
+          "IOUtils::setModificationTime can update the lastModified time stamp on a directory"
+        );
+        is(
+          stamp,
+          newFileInfo.lastModified,
+          "IOUtils::setModificationTime returns the updated time stamp on a directory"
+        );
+      }
+
+      await cleanup(tmpFileName, tmpDirName);
+    });
+
+    add_task(async function test_setModificationTime_custom_mod_time() {
+      const tempFileName = PathUtils.join(PathUtils.tempDir, "test_setModificationTime_custom_mod_time.tmp");
+      await createFile(tempFileName);
+      const originalInfo = await IOUtils.stat(tempFileName);
+      const now = originalInfo.lastModified;
+
+      const oneMinute = 60 * 1000; // milliseconds
+
+      info("Test attempt to set modification time to the future");
+      const future = now + oneMinute;
+      let newModTime = await IOUtils.setModificationTime(tempFileName, future);
+      const futureInfo = await IOUtils.stat(tempFileName);
+      Assert.less(originalInfo.lastModified, futureInfo.lastModified, "IOUtils::setModificationTime can set a future modification time for the file");
+
+      is(newModTime, futureInfo.lastModified, "IOUtils::setModificationTime returns the updated time stamp");
+      is(newModTime, future, "IOUtils::setModificationTime return value matches the argument value exactly");
+
+      info("Test attempt to set modification time to the past");
+      const past = now - 2 * oneMinute;
+      newModTime = await IOUtils.setModificationTime(tempFileName, past);
+      const pastInfo = await IOUtils.stat(tempFileName);
+      Assert.greater(originalInfo.lastModified, pastInfo.lastModified, "IOUtils::setModificationTime can set a past modification time for the file");
+
+      is(newModTime, pastInfo.lastModified, "IOUtils::setModificationTime returns the updated time stamp");
+      is(newModTime, past, "IOUtils::setModificationTime return value matches the argument value exactly");
+
+      await cleanup(tempFileName);
+    });
+
+    add_task(async function test_stat_btime() {
+      if (["Darwin", "WINNT"].includes(Services.appinfo.OS)) {
+        const tempFileName = PathUtils.join(PathUtils.tempDir, "test_stat_btime.tmp");
+        await createFile(tempFileName);
+        const originalInfo = await IOUtils.stat(tempFileName);
+
+        const future = originalInfo.lastModified + 6000;
+        await IOUtils.setModificationTime(tempFileName, future);
+        const futureInfo = await IOUtils.stat(tempFileName);
+
+        ok(originalInfo.hasOwnProperty("creationTime"), "originalInfo has creationTime field");
+        ok(originalInfo.creationTime !== undefined && originalInfo.creationTime !== null, "originalInfo has non-null creationTime");
+
+        ok(futureInfo.hasOwnProperty("creationTime"), "futureInfo has creationTime field");
+        ok(futureInfo.creationTime !== undefined && futureInfo.creationTime !== null, "futureInfo has non-null creationTime");
+
+        is(originalInfo.creationTime, futureInfo.creationTime, "creationTime matches");
+
+        await cleanup(tempFileName);
+      } else {
+        ok(true, `skipping test_stat_btime() on unsupported platform ${Services.appinfo.OS}`);
+      }
+    });
+
+    add_task(async function test_setModificationTime_failures() {
+      info("Test attempt to setModificationTime a non-existing file");
+      const notExistsFile = PathUtils.join(PathUtils.tempDir, "test_setModificationTime_not_exists.tmp");
+
+      await Assert.rejects(
+        IOUtils.setModificationTime(notExistsFile),
+        /Could not set modification time of file\(.*\) because it does not exist/,
+        "IOUtils::setModificationTime throws if the target file does not exist"
+      );
+
+      info("Test attempt to set modification time to Epoch");
+      const tempFileName = PathUtils.join(PathUtils.tempDir, "test_setModificationTime_epoch.tmp");
+      await createFile(tempFileName);
+
+      await Assert.rejects(
+        IOUtils.setModificationTime(tempFileName, 0),
+        /Refusing to set the modification time of file\(.*\) to 0/,
+        "IOUtils::setModificationTime cannot set the file modification time to Epoch"
+      );
+
+      await cleanup(tempFileName);
+    });
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_windows_file_attributes.html b/dom/system/tests/ioutils/test_ioutils_windows_file_attributes.html
new file mode 100644
index 0000000000..b452b4f6db
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_windows_file_attributes.html
@@ -0,0 +1,135 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE HTML>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Test the IOUtils file I/O API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+  <script src="file_ioutils_test_fixtures.js"></script>
+  <script>
+    "use strict";
+
+    const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+    add_task(async function getSetWindowsAttributes() {
+      const tmpDir = PathUtils.join(PathUtils.tempDir, "ioutils-windows-attributes.tmp.d");
+      await createDir(tmpDir);
+      ok(await dirExists(tmpDir), `Expected ${tmpDir} to be a directory`);
+
+      const filePath = PathUtils.join(tmpDir, "file.tmp");
+      await createFile(filePath);
+      ok(await fileExists(filePath), `Expected ${filePath} to exist`);
+
+      {
+        info("Getting attributes for newly created file.");
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === false, `Newly created file ${filePath} is not a read-only file`);
+        ok(attrs.hidden === false, `Newly created file ${filePath} is not a hidden file`);
+        ok(attrs.system === false, `Newly created file ${filePath} is not a system file`);
+      }
+
+      {
+        info("Setting read-only on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { readOnly: true })
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === true, `Updated file ${filePath} is a read-only file`);
+        ok(attrs.hidden === false, `Updated file ${filePath} is not a hidden file`);
+        ok(attrs.system === false, `Updated file ${filePath} is not a system file`);
+      }
+
+      info("Attempting to write to a read-only file.");
+
+      await Assert.rejects(
+        IOUtils.writeUTF8(filePath, "hello, world"),
+        /NotAllowedError: Could not open the file at .+ for writing/,
+        "IOUtils::writeUTF8 on a read-only file fails."
+      );
+
+      {
+        info("Setting hidden on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { hidden: true })
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === true, `Updated file ${filePath} is still a read-only file`);
+        ok(attrs.hidden === true, `Updated file ${filePath} is a hidden file`);
+        ok(attrs.system === false, `Updated file ${filePath} is not a system file`);
+      }
+
+      {
+        info("Setting system on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { system: true })
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === true, `Updated file ${filePath} is still a read-only file`);
+        ok(attrs.hidden === true, `Updated file ${filePath} is still a hidden file`);
+        ok(attrs.system === true, `Updated file ${filePath} is a system file`);
+      }
+
+      {
+        info("Clearing all Windows attributes on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { readOnly: false, hidden: false, system: false });
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === false, `Updated file ${filePath} is not a read-only file`);
+        ok(attrs.hidden === false, `Updated file ${filePath} is not a hidden file`);
+        ok(attrs.system === false, `Updated file ${filePath} is not a system file`);
+      }
+
+      {
+        info("Setting all Windows attributes on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { readOnly: true, hidden: true, system: true });
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === true, `Updated file ${filePath} is a read-only file`);
+        ok(attrs.hidden === true, `Updated file ${filePath} is a hidden file`);
+        ok(attrs.system === true, `Updated file ${filePath} is a system file`);
+      }
+
+      {
+        info("Clearing read-only on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { readOnly: false });
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === false, `Updated file ${filePath} is no longer a read-only file`);
+        ok(attrs.hidden === true, `Updated file ${filePath} is still a hidden file`);
+        ok(attrs.system === true, `Updated file ${filePath} is still a system file`);
+      }
+
+      {
+        info("Clearing hidden on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { hidden: false });
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === false, `Updated file ${filePath} is still not a read-only file`);
+        ok(attrs.hidden === false, `Updated file ${filePath} is no longer a hidden file`);
+        ok(attrs.system === true, `Updated file ${filePath} is still a system file`);
+      }
+
+      {
+        info("Clearing system on an existing file.");
+        await IOUtils.setWindowsAttributes(filePath, { system: false });
+        const attrs = await IOUtils.getWindowsAttributes(filePath);
+
+        ok(attrs.readOnly === false, `Updated file ${filePath} is still not a read-only file`);
+        ok(attrs.hidden === false, `Updated file ${filePath} is sitll not a hidden file`);
+        ok(attrs.system === false, `Updated file ${filePath} is no longer a system file`);
+      }
+
+      await cleanup(tmpDir);
+    });
+
+  </script>
+</head>
+
+<body>
+  <p id="display"></p>
+  <div id="content" style="display: none"></div>
+  <pre id="test"></pre>
+</body>
+
+</html>
diff --git a/dom/system/tests/ioutils/test_ioutils_worker.xhtml b/dom/system/tests/ioutils/test_ioutils_worker.xhtml
new file mode 100644
index 0000000000..df67d48676
--- /dev/null
+++ b/dom/system/tests/ioutils/test_ioutils_worker.xhtml
@@ -0,0 +1,40 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing IOUtils on a chrome worker thread"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onload="test();">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+  <script src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script src="chrome://mochikit/content/tests/SimpleTest/WorkerHandler.js"/>
+
+  <script type="application/javascript">
+  <![CDATA[
+
+      // Test IOUtils in a chrome worker.
+      function test() {
+        // finish() will be called in the worker.
+        SimpleTest.waitForExplicitFinish();
+        info("test_ioutils_worker.xhtml: Starting test");
+
+        const worker = new ChromeWorker("file_ioutils_worker.js");
+        info("test_ioutils_worker.xhtml: Chrome worker created");
+
+        // Set up the worker with testing facilities, and start it.
+        listenForTests(worker, { verbose: false });
+        worker.postMessage(0);
+        info("test_ioutils_worker.xhtml: Test in progress");
+      };
+
+  ]]>
+  </script>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result" />
+</window>
diff --git a/dom/system/tests/mochitest.ini b/dom/system/tests/mochitest.ini
new file mode 100644
index 0000000000..ec18d0016d
--- /dev/null
+++ b/dom/system/tests/mochitest.ini
@@ -0,0 +1,10 @@
+[DEFAULT]
+tags = condprof
+scheme = https
+
+support-files =
+  file_bug1197901.html
+
+[test_bug1197901.html]
+skip-if = os == "android"
+  condprof  #: "Only focused window should get the sensor events"
diff --git a/dom/system/tests/pathutils_worker.js b/dom/system/tests/pathutils_worker.js
new file mode 100644
index 0000000000..da51334d09
--- /dev/null
+++ b/dom/system/tests/pathutils_worker.js
@@ -0,0 +1,44 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* eslint-env mozilla/chrome-worker */
+
+"use strict";
+
+/* import-globals-from /testing/mochitest/tests/SimpleTest/WorkerSimpleTest.js */
+importScripts("chrome://mochikit/content/tests/SimpleTest/WorkerSimpleTest.js");
+
+self.onmessage = async function(message) {
+  let expected = message.data;
+  info("ON message");
+  info(JSON.stringify(expected));
+  const profileDir = await PathUtils.getProfileDir();
+  is(
+    profileDir,
+    expected.profileDir,
+    "PathUtils.profileDir() in a worker should match PathUtils.profileDir on main thread"
+  );
+
+  const localProfileDir = await PathUtils.getLocalProfileDir();
+  is(
+    localProfileDir,
+    expected.localProfileDir,
+    "PathUtils.getLocalProfileDir() in a worker should match PathUtils.localProfileDir on main thread"
+  );
+
+  const tempDir = await PathUtils.getTempDir();
+  is(
+    tempDir,
+    expected.tempDir,
+    "PathUtils.getTempDir() in a worker should match PathUtils.tempDir on main thread"
+  );
+
+  const osTempDir = await PathUtils.getOSTempDir();
+  is(
+    osTempDir,
+    expected.osTempDir,
+    "PathUtils.getOSTempDir() in a worker should match PathUtils.osTempDir on main thread"
+  );
+
+  finish();
+};
diff --git a/dom/system/tests/test_bug1197901.html b/dom/system/tests/test_bug1197901.html
new file mode 100644
index 0000000000..7e1866ffa3
--- /dev/null
+++ b/dom/system/tests/test_bug1197901.html
@@ -0,0 +1,96 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1197901
+-->
+<head>
+  <meta charset="utf-8">
+  <title>Test for Bug 1197901</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+  <script type="application/javascript">
+
+  /** Test for Bug 1197901 **/
+  SimpleTest.requestFlakyTimeout("requestFlakyTimeout is silly");
+  SimpleTest.waitForExplicitFinish();
+  window.onload = function() {
+    SimpleTest.waitForFocus(function() {
+      SpecialPowers.pushPrefEnv({"set": [["device.sensors.test.events", true]]},
+                                doTest);
+      }, window);
+  };
+
+  function doTest() {
+    window.onmessage = function(event) {
+      ok(event.data.result, event.data.message);
+    };
+
+    // Only same-origin iframe should get the events.
+    var xo = document.getElementById("cross-origin");
+    xo.contentWindow.postMessage(
+      { command: "addEventListener",
+        expected: false,
+        message: "Cross-origin iframe shouldn't get the sensor events."},
+      "*");
+
+    var so = document.getElementById("same-origin");
+    so.contentWindow.postMessage(
+      { command: "addEventListener",
+        expected: true,
+        message: "Same-origin iframe should get the sensor events." },
+      "*");
+
+    // We need a timeout here to check that something does not happen.
+    setTimeout(function() {
+      so.remove();
+      xo.remove();
+      doWindowTest();
+    }, 500);
+  }
+
+  function doWindowTest() {
+    var win = window.open("file_bug1197901.html", "w1", "height=100,width=100");
+    win.onload = function() {
+      win.focus();
+      SimpleTest.waitForFocus(function() {
+        var win2 = window.open("file_bug1197901.html", "w2", "height=100,width=100,left=100");
+        win2.onload = function() {
+          win2.focus();
+          SimpleTest.waitForFocus(function() {
+            // Only focused window should get the events.
+            win.postMessage(
+              { command: "addEventListener",
+                expected: false,
+                message: "Only focused window should get the sensor events." },
+              "*");
+            win2.postMessage(
+              { command: "addEventListener",
+                expected: true,
+                message: "Focused window should get the sensor events." },
+              "*");
+            setTimeout(function() {
+              window.onmessage = null;
+              win.close();
+              win2.close();
+              SimpleTest.finish();
+            }, 500);
+          }, win2);
+        };
+      }, win);
+    };
+  }
+
+  </script>
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=">Mozilla Bug </a>
+<p id="display"></p>
+<div id="content" style="display: none">
+
+</div>
+<pre id="test">
+</pre>
+<iframe src="file_bug1197901.html" id="same-origin"></iframe>
+<iframe src="http://example.com/tests/dom/system/tests/file_bug1197901.html" id="cross-origin"></iframe>
+</body>
+</html>
diff --git a/dom/system/tests/test_constants.xhtml b/dom/system/tests/test_constants.xhtml
new file mode 100644
index 0000000000..82c25ce1b0
--- /dev/null
+++ b/dom/system/tests/test_constants.xhtml
@@ -0,0 +1,140 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing constants on a chrome worker thread"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+  <script src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script type="application/javascript">
+  <![CDATA[
+
+/* global OS */
+
+let worker;
+
+function test_xul() {
+  let lib;
+  isnot(null, OS.Constants.Path.libxul, "libxulpath is defined");
+  try {
+    lib = ctypes.open(OS.Constants.Path.libxul);
+    lib.declare("DumpJSStack", ctypes.default_abi, ctypes.void_t);
+  } catch (x) {
+    ok(false, "Could not open libxul " + x);
+  }
+  if (lib) {
+    lib.close();
+  }
+  ok(true, "test_xul: opened libxul successfully");
+}
+
+// Test that OS.Constants.libc is defined
+function test_libc() {
+  isnot(null, OS.Constants.libc, "OS.Constants.libc is defined");
+  is(0o001, OS.Constants.libc.S_IXOTH, "OS.Constants.libc.S_IXOTH is defined");
+  is(0o002, OS.Constants.libc.S_IWOTH, "OS.Constants.libc.S_IWOTH is defined");
+  is(0o007, OS.Constants.libc.S_IRWXO, "OS.Constants.libc.S_IRWXO is defined");
+  is(0o010, OS.Constants.libc.S_IXGRP, "OS.Constants.libc.S_IXGRP is defined");
+  is(0o020, OS.Constants.libc.S_IWGRP, "OS.Constants.libc.S_IWGRP is defined");
+  is(0o040, OS.Constants.libc.S_IRGRP, "OS.Constants.libc.S_IRGRP is defined");
+  is(0o070, OS.Constants.libc.S_IRWXG, "OS.Constants.libc.S_IRWXG is defined");
+  is(0o100, OS.Constants.libc.S_IXUSR, "OS.Constants.libc.S_IXUSR is defined");
+  is(0o200, OS.Constants.libc.S_IWUSR, "OS.Constants.libc.S_IWUSR is defined");
+  is(0o400, OS.Constants.libc.S_IRUSR, "OS.Constants.libc.S_IRUSR is defined");
+  is(0o700, OS.Constants.libc.S_IRWXU, "OS.Constants.libc.S_IRWXU is defined");
+}
+
+// Test that OS.Constants.Win is defined
+function test_Win() {
+  var xulRuntime = Cc["@mozilla.org/xre/app-info;1"]
+                           .getService(Ci.nsIXULRuntime);
+  if(xulRuntime.OS == "Windows") {
+    ok("Win" in OS.Constants, "OS.Constants.Win is defined");
+    is(OS.Constants.Win.INVALID_HANDLE_VALUE, -1,
+      "OS.Constants.Win.INVALID_HANDLE_VALUE is defined and correct");
+  }
+}
+
+// Test that OS.Constants.Sys.DEBUG is set properly on main thread
+function test_debugBuildMainThread(isDebugBuild) {
+  is(isDebugBuild, !!OS.Constants.Sys.DEBUG, "OS.Constants.Sys.DEBUG is set properly on main thread");
+}
+
+// Test that OS.Constants.Sys.umask is set properly on main thread
+function test_umaskMainThread(umask) {
+  is(umask, OS.Constants.Sys.umask,
+     "OS.Constants.Sys.umask is set properly on main thread: " +
+     ("0000"+umask.toString(8)).slice(-4));
+}
+
+var ctypes;
+function test() {
+  ok(true, "test_constants.xhtml: Starting test");
+
+  // Test 1: Load libxul from main thread
+  Cc["@mozilla.org/net/osfileconstantsservice;1"].
+    getService(Ci.nsIOSFileConstantsService).
+    init();
+  ({ctypes} = ChromeUtils.import("resource://gre/modules/ctypes.jsm"));
+  test_xul();
+  test_libc();
+  test_Win();
+
+  let isDebugBuild = Cc["@mozilla.org/xpcom/debug;1"]
+                            .getService(Ci.nsIDebug2).isDebugBuild;
+  test_debugBuildMainThread(isDebugBuild);
+
+  let umask = Cc["@mozilla.org/system-info;1"].
+    getService(Ci.nsIPropertyBag2).
+    getProperty("umask");
+  test_umaskMainThread(umask);
+
+  // Test 2: Load libxul from chrome thread
+  worker = new ChromeWorker("worker_constants.js");
+  SimpleTest.waitForExplicitFinish();
+  ok(true, "test_constants.xhtml: Chrome worker created");
+  worker.onerror = function onerror(error) {
+    error.preventDefault();
+    ok(false, "error " + error);
+  }
+  worker.onmessage = function onmessage(msg) {
+    switch (msg.data.kind) {
+    case "is":
+      SimpleTest.is(msg.data.a, msg.data.b, msg.data.description);
+      return;
+    case "isnot":
+      SimpleTest.isnot(msg.data.a, msg.data.b, msg.data.description);
+      return;
+    case "ok":
+      SimpleTest.ok(msg.data.condition, msg.data.description);
+      return;
+    case "finish":
+      SimpleTest.finish();
+      return;
+    default:
+      SimpleTest.ok(false, "test_constants.xhtml: wrong message " + JSON.stringify(msg.data));
+      return;
+    }
+  };
+
+  // pass expected values that are unavailable off-main-thread
+  // to the worker
+  worker.postMessage({
+    isDebugBuild: isDebugBuild,
+    umask: umask
+  });
+  ok(true, "test_constants.xhtml: Test in progress");
+};
+]]>
+  </script>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result"/>
+</window>
diff --git a/dom/system/tests/test_pathutils.html b/dom/system/tests/test_pathutils.html
new file mode 100644
index 0000000000..aefd764c17
--- /dev/null
+++ b/dom/system/tests/test_pathutils.html
@@ -0,0 +1,604 @@
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!doctype html>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>PathUtils tests</title>
+</head>
+<script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+<script src="chrome://mochikit/content/tests/SimpleTest/WorkerHandler.js"></script>
+<link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css">
+<script>
+  "use strict";
+
+  const { AppConstants } = ChromeUtils.import(
+    "resource://gre/modules/AppConstants.jsm"
+  );
+  const { Assert } = ChromeUtils.import("resource://testing-common/Assert.jsm");
+
+  const UNRECOGNIZED_PATH = /Could not initialize path: NS_ERROR_FILE_UNRECOGNIZED_PATH/;
+  const SPLIT_RELATIVE_ABSOLUTE = /PathUtils.splitRelative requires a relative path/;
+  const SPLIT_RELATIVE_EMPTY = /PathUtils.splitRelative: Empty directory components \(""\) not allowed by options/;
+  const SPLIT_RELATIVE_PARENT = /PathUtils.splitRelative: Parent directory components \("\.\."\) not allowed by options/;
+  const SPLIT_RELATIVE_CURRENT = /PathUtils.splitRelative: Current directory components \("\."\) not allowed by options/;
+  const EMPTY_PATH = /PathUtils does not support empty paths/;
+  const JOIN = /Could not append to path/;
+
+  add_task(function test_filename() {
+    Assert.throws(
+      () => PathUtils.filename(""),
+      EMPTY_PATH,
+      "PathUtils.filename() does not support empty paths"
+    );
+    Assert.throws(
+      () => PathUtils.filename("foo.txt"),
+      UNRECOGNIZED_PATH,
+      "PathUtils.filename() does not support relative paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.filename("C:"),
+        "C:",
+        "PathUtils.filename() with a drive path"
+      );
+      is(
+        PathUtils.filename("C:\\"),
+        "C:",
+        "PathUtils.filename() with a drive path"
+      );
+      is(
+        PathUtils.filename("C:\\Windows"),
+        "Windows",
+        "PathUtils.filename() with a path with 2 components"
+      );
+      is(
+        PathUtils.filename("C:\\Windows\\"),
+        "Windows",
+        "PathUtils.filename() with a path with 2 components and a trailing slash"
+      );
+      is(
+        PathUtils.filename("C:\\Windows\\System32"),
+        "System32",
+        "PathUtils.filename() with a path with 3 components"
+      );
+      is(
+        PathUtils.filename("\\\\server"),
+        "\\\\server",
+        "PathUtils.filename() with a UNC server path"
+      );
+      is(
+        PathUtils.filename("C:\\file.dat"),
+        "file.dat",
+        "PathUtils.filename() with a file path"
+      );
+    } else {
+      is(
+        PathUtils.filename("/"),
+        "/",
+        "PathUtils.filename() with a root path"
+      );
+      is(
+        PathUtils.filename("/usr/"),
+        "usr",
+        "PathUtils.filename() with a non-root path"
+      );
+      is(
+        PathUtils.filename("/usr/lib/libfoo.so"),
+        "libfoo.so",
+        "PathUtils.filename() with a path with 3 components"
+      );
+    }
+  });
+
+  add_task(function test_parent() {
+    Assert.throws(
+      () => PathUtils.parent("."),
+      UNRECOGNIZED_PATH,
+      "PathUtils.parent() does not support relative paths"
+    );
+    Assert.throws(
+      () => PathUtils.parent(""),
+      EMPTY_PATH,
+      "PathUtils.parent() does not support empty paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.parent("C:"),
+        null,
+        "PathUtils.parent() with a drive path"
+      );
+      is(
+        PathUtils.parent("\\\\server"),
+        null,
+        "PathUtils.parent() with a UNC server path"
+      );
+      is(
+        PathUtils.parent("\\\\server\\foo"),
+        "\\\\server",
+        "PathUtils.parent() with a UNC server path and child component"
+      );
+
+      Assert.throws(
+        () => PathUtils.parent("C:", -1),
+        /^NotSupportedError: PathUtils.parent: A depth of at least 1 is required/,
+        "PathUtils.parent() with a negative depth throws"
+      );
+      Assert.throws(
+        () => PathUtils.parent("C:", 0),
+        /^NotSupportedError: PathUtils.parent: A depth of at least 1 is required/,
+        "PathUtils.parent() with a zero depth throws"
+      );
+
+      {
+        const path = "C:\\Users\\User\\AppData\\Local\\Mozilla\\Firefox\\Profiles\\foo.default";
+
+        const expected = [
+          "C:\\Users\\User\\AppData\\Local\\Mozilla\\Firefox\\Profiles",
+          "C:\\Users\\User\\AppData\\Local\\Mozilla\\Firefox",
+          "C:\\Users\\User\\AppData\\Local\\Mozilla",
+          "C:\\Users\\User\\AppData\\Local",
+          "C:\\Users\\User\\AppData",
+          "C:\\Users\\User",
+          "C:\\Users",
+          "C:",
+          null,
+        ];
+
+        for (const [i, parent] of expected.entries()) {
+          is(PathUtils.parent(path, i + 1), parent, `PathUtils.parent() with depth=${i + 1}`)
+        }
+      }
+    } else {
+      is(
+        PathUtils.parent("/"),
+        null,
+        "PathUtils.parent() with a root path"
+      );
+      is(
+        PathUtils.parent("/var"),
+        "/",
+        "PathUtils.parent() with a 2 component path"
+      );
+      is(
+        PathUtils.parent("/var/run"),
+        "/var",
+        "PathUtils.parent() with a 3 component path"
+      );
+
+      Assert.throws(
+        () => PathUtils.parent("/", -1),
+        /^NotSupportedError: PathUtils.parent: A depth of at least 1 is required/,
+        "PathUtils.parent() with a negative depth throws"
+      );
+      Assert.throws(
+        () => PathUtils.parent("/", 0),
+        /^NotSupportedError: PathUtils.parent: A depth of at least 1 is required/,
+        "PathUtils.parent() with a zero depth throws"
+      );
+
+      {
+        const path = "/home/user/.mozilla/firefox/foo.default";
+        const expected = [
+          "/home/user/.mozilla/firefox",
+          "/home/user/.mozilla",
+          "/home/user",
+          "/home",
+          "/",
+          null,
+        ];
+
+        for (const [i, parent] of expected.entries()) {
+          is(
+            PathUtils.parent(path, i + 1),
+            parent,
+            `PathUtils.parent() with depth=${i + 1}`
+          );
+        }
+      }
+    }
+  });
+
+  add_task(function test_join() {
+    Assert.throws(
+      () => PathUtils.join(),
+      EMPTY_PATH,
+      "PathUtils.join() does not support empty paths"
+    );
+    Assert.throws(
+      () => PathUtils.join(""),
+      EMPTY_PATH,
+      "PathUtils.join() does not support empty paths"
+    );
+    Assert.throws(
+      () => PathUtils.join("foo", "bar"),
+      UNRECOGNIZED_PATH,
+      "PathUtils.join() does not support relative paths"
+    );
+    Assert.throws(
+      () => PathUtils.join("."),
+      UNRECOGNIZED_PATH,
+      "PathUtils.join() does not support relative paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.join("C:"),
+        "C:",
+        "PathUtils.join() with a single path"
+      );
+      is(
+        PathUtils.join("C:\\Windows", "System32"),
+        "C:\\Windows\\System32",
+        "PathUtils.join() with a 2 component path and an additional component"
+      );
+      is(
+        PathUtils.join("C:", "Users", "Example"),
+        "C:\\Users\\Example",
+        "PathUtils.join() with a root path and two additional components"
+      );
+      is(
+        PathUtils.join("\\\\server", "Files", "Example.dat"),
+        "\\\\server\\Files\\Example.dat",
+        "PathUtils.join() with a server path"
+      );
+    } else {
+      is(
+        PathUtils.join("/"),
+        "/",
+        "PathUtils.join() with a root path"
+      );
+      is(
+        PathUtils.join("/usr", "lib"),
+        "/usr/lib",
+        "PathUtils.join() with a 2 component path and an additional component"
+      );
+      is(
+        PathUtils.join("/", "home", "example"),
+        "/home/example",
+        "PathUtils.join() with a root path and two additional components"
+      );
+    }
+  });
+
+  add_task(function test_join_relative() {
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.joinRelative("C:", ""),
+        "C:",
+        "PathUtils.joinRelative() with an empty relative path"
+      );
+
+      is(
+        PathUtils.joinRelative("C:", "foo\\bar\\baz"),
+        "C:\\foo\\bar\\baz",
+        "PathUtils.joinRelative() with a relative path containing path separators"
+      );
+    } else {
+      is(
+        PathUtils.joinRelative("/", ""),
+        "/",
+        "PathUtils.joinRelative() with an empty relative path"
+      );
+
+      is(
+        PathUtils.joinRelative("/", "foo/bar/baz"),
+        "/foo/bar/baz",
+        "PathUtils.joinRelative() with a relative path containing path separators"
+      );
+    }
+  });
+
+  add_task(async function test_normalize() {
+    Assert.throws(
+      () => PathUtils.normalize(""),
+      EMPTY_PATH,
+      "PathUtils.normalize() does not support empty paths"
+    );
+    Assert.throws(
+      () => PathUtils.normalize("."),
+      UNRECOGNIZED_PATH,
+      "PathUtils.normalize() does not support relative paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.normalize("C:\\\\Windows\\\\..\\\\\\.\\Users\\..\\Windows"),
+        "C:\\Windows",
+        "PathUtils.normalize() with a non-normalized path"
+      );
+    } else {
+      // nsLocalFileUnix::Normalize() calls realpath, which resolves symlinks
+      // and requires the file to exist.
+      //
+      // On Darwin, the temp directory is located in `/private/var`, which is a
+      // symlink to `/var`, so we need to pre-normalize our temporary directory
+      // or expected paths won't match.
+      const tmpDir = PathUtils.join(
+        PathUtils.normalize(PathUtils.tempDir),
+        "pathutils_test"
+      );
+
+      await IOUtils.makeDirectory(tmpDir, { ignoreExisting: true });
+      info(`created tmpDir ${tmpDir}`);
+      SimpleTest.registerCleanupFunction(async () => {
+        await IOUtils.remove(tmpDir, {
+          recursive: true,
+        });
+      });
+
+      await IOUtils.makeDirectory(PathUtils.join(tmpDir, "foo", "bar"), {
+        createAncestors: true,
+      });
+
+      is(
+        PathUtils.normalize("/"),
+        "/",
+        "PathUtils.normalize() with a normalized path"
+      );
+
+      is(
+        PathUtils.normalize(
+          PathUtils.join(
+            tmpDir,
+            "foo",
+            ".",
+            "bar",
+            ".",
+          )
+        ),
+        PathUtils.join(tmpDir, "foo", "bar"),
+        "PathUtils.normalize() with a non-normalized path"
+      );
+    }
+  });
+
+  add_task(function test_split() {
+    Assert.throws(
+      () => PathUtils.split("foo"),
+      UNRECOGNIZED_PATH,
+      "PathUtils.split() does not support relative paths"
+    );
+    Assert.throws(
+      () => PathUtils.split(""),
+      EMPTY_PATH,
+      "PathUtils.split() does not support empty paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      Assert.deepEqual(
+        PathUtils.split("C:\\Users\\Example"),
+        ["C:", "Users", "Example"],
+        "PathUtils.split() on an absolute path"
+      );
+
+      Assert.deepEqual(
+        PathUtils.split("C:\\Users\\Example\\"),
+        ["C:", "Users", "Example"],
+        "PathUtils.split() on an absolute path with a trailing slash"
+      );
+
+      Assert.deepEqual(
+        PathUtils.split("\\\\server\\Files\\Example.dat"),
+        ["\\\\server", "Files", "Example.dat"],
+        "PathUtils.split() with a server as the root"
+      );
+    } else {
+      Assert.deepEqual(
+        PathUtils.split("/home/foo"),
+        ["/", "home", "foo"],
+        "PathUtils.split() on absolute path"
+      );
+
+      Assert.deepEqual(
+        PathUtils.split("/home/foo/"),
+        ["/", "home", "foo"],
+        "PathUtils.split() on absolute path with trailing slash"
+      );
+    }
+  });
+
+  add_task(function test_splitRelative() {
+    Assert.throws(
+      () => PathUtils.splitRelative(""),
+      EMPTY_PATH,
+      "PathUtils.splitRelative() should throw with empty path"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      Assert.throws(
+        () => PathUtils.splitRelative("C:\\"),
+        SPLIT_RELATIVE_ABSOLUTE,
+        "PathUtils.splitRelative() should throw with a drive path"
+      );
+
+      Assert.throws(
+        () => PathUtils.splitRelative("\\\\server\\share\\"),
+        SPLIT_RELATIVE_ABSOLUTE,
+        "PathUtils.splitRelative() should throw with a UNC path"
+      );
+
+      Assert.throws(
+        () => PathUtils.splitRelative("foo\\\\bar"),
+        SPLIT_RELATIVE_EMPTY,
+        "PathUtils.splitRelative() should throw with empty component by default"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo\\\\bar", { allowEmpty: true }),
+        ["foo", "", "bar"],
+        "PathUtils.splitRelative() with an empty component is allowed with option"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo"),
+        ["foo"],
+        "PathUtils.splitRelative() on a relative path with one component"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo\\"),
+        ["foo"],
+        "PathUtils.splitRelative() on a relative path with one component and a trailing slash"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo\\bar"),
+        ["foo", "bar"],
+        "PathUtils.splitRelative() on a relative path with two components"
+      );
+    } else {
+      Assert.throws(
+        () => PathUtils.splitRelative("/foo/bar"),
+        SPLIT_RELATIVE_ABSOLUTE,
+        "PathUtils.splitRelative() should throw with an absolute path"
+      );
+
+      Assert.throws(
+        () => PathUtils.splitRelative("foo//bar"),
+        SPLIT_RELATIVE_EMPTY,
+        "PathUtils.splitRelative() should throw with empty component by default"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo//bar", { allowEmpty: true }),
+        ["foo", "", "bar"],
+        "PathUtils.splitRelative() with an empty component is allowed with option"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo"),
+        ["foo"],
+        "PathUtils.splitRelative() on a relative path with one component"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo/"),
+        ["foo"],
+        "PathUtils.splitRelative() on a relative path with one component and a trailing slash"
+      );
+
+      Assert.deepEqual(
+        PathUtils.splitRelative("foo/bar"),
+        ["foo", "bar"],
+        "PathUtils.splitRelative() on a relative path with two components",
+      );
+    }
+
+    Assert.throws(
+      () => PathUtils.splitRelative("."),
+      SPLIT_RELATIVE_CURRENT,
+      "PathUtils.splitRelative() with a current dir component is disallowed by default"
+    );
+
+    Assert.throws(
+      () => PathUtils.splitRelative(".."),
+      SPLIT_RELATIVE_PARENT,
+      "PathUtils.splitRelative() with a parent dir component is disallowed by default"
+    );
+
+    Assert.deepEqual(
+      PathUtils.splitRelative(".", { allowCurrentDir: true }),
+      ["."],
+      "PathUtils.splitRelative() with a current dir component is allowed with option"
+    );
+
+    Assert.deepEqual(
+      PathUtils.splitRelative("..", { allowParentDir: true }),
+      [".."],
+      "PathUtils.splitRelative() with a parent dir component is allowed with option"
+    );
+  });
+
+  add_task(function test_toFileURI() {
+    Assert.throws(
+      () => PathUtils.toFileURI("."),
+      UNRECOGNIZED_PATH,
+      "PathUtils.toFileURI() does not support relative paths"
+    );
+    Assert.throws(
+      () => PathUtils.toFileURI(""),
+      EMPTY_PATH,
+      "PathUtils.toFileURI() does not support empty paths"
+    );
+
+    if (Services.appinfo.OS === "WINNT") {
+      is(
+        PathUtils.toFileURI("C:\\"),
+        "file:///C:/",
+        "PathUtils.toFileURI() with a root path"
+      );
+
+      is(
+        PathUtils.toFileURI("C:\\Windows\\"),
+        "file:///C:/Windows/",
+        "PathUtils.toFileURI() with a non-root directory path"
+      );
+
+      is(
+        PathUtils.toFileURI("C:\\Windows\\system32\\notepad.exe"),
+        "file:///C:/Windows/system32/notepad.exe",
+        "PathUtils.toFileURI() with a file path"
+      );
+    } else {
+      is(
+        PathUtils.toFileURI("/"),
+        "file:///",
+        "PathUtils.toFileURI() with a root path"
+      );
+
+      is(
+        PathUtils.toFileURI("/bin"),
+        "file:///bin/",
+        "PathUtils.toFileURI() with a non-root directory path"
+      );
+
+      is(
+        PathUtils.toFileURI("/bin/ls"),
+        "file:///bin/ls",
+        "PathUtils.toFileURI() with a file path"
+      );
+    }
+  });
+
+  add_task(async function test_isAbsolute() {
+    if (Services.appinfo.OS === "WINNT") {
+      ok(PathUtils.isAbsolute("C:"), "Drive paths are absolute paths on Windows");
+      ok(PathUtils.isAbsolute("C:\\Windows"), "Paths from the root are absolute paths on Windows");
+      ok(!PathUtils.isAbsolute("foo"), "Paths containing a single item are not absolute paths on Windows");
+      ok(!PathUtils.isAbsolute(".\\foo"), "Paths relative to the current working directory are not absolute paths on Windows");
+      ok(!PathUtils.isAbsolute("..\\foo"), "Paths relative to the parent directory are not absolute paths on Windows");
+    } else {
+      ok(PathUtils.isAbsolute("/"), "Root paths are absolute paths");
+      ok(PathUtils.isAbsolute("/home"), "Paths with a root stem are absolute paths");
+      ok(!PathUtils.isAbsolute("foo"), "Paths containing a single non-root item are not absolute paths");
+      ok(!PathUtils.isAbsolute("./foo"), "Paths relative to the current working directory are not absolute paths");
+      ok(!PathUtils.isAbsolute("../foo"), "Paths relative to the parent directory are not absolute paths");
+    }
+  });
+
+  add_task(async function test_getDirectories() {
+    // See: nsAppDirectoryServiceDefs.h
+    const tests = [
+      ["profileDir", "ProfD"],
+      ["localProfileDir", "ProfLD"],
+      ["tempDir", AppConstants.MOZ_SANDBOX ? "ContentTmpD" : "TmpD"],
+      ["osTempDir", "TmpD"],
+    ];
+
+    for (const [attrName, dirConstant] of tests) {
+      const expected = Services.dirsvc.get(dirConstant, Ci.nsIFile).path;
+
+      const attrValue = PathUtils[attrName];
+      is(attrValue, expected, `PathUtils.${attrName} == Services.dirsvc.get("${dirConstant}", Ci.nsIFile).path`);
+    }
+  });
+</script>
+
+<body>
+</body>
+
+</html>
diff --git a/dom/system/tests/test_pathutils_worker.xhtml b/dom/system/tests/test_pathutils_worker.xhtml
new file mode 100644
index 0000000000..f8b04bf130
--- /dev/null
+++ b/dom/system/tests/test_pathutils_worker.xhtml
@@ -0,0 +1,39 @@
+<?xml version="1.0"?>
+<!-- Any copyright is dedicated to the Public Domain.
+- http://creativecommons.org/publicdomain/zero/1.0/ -->
+<window title="Testing PathUtils on a chrome worker thread"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="chrome://mochikit/content/tests/SimpleTest/WorkerHandler.js"></script>
+
+  <script type="application/javascript">
+  <![CDATA[
+    function test() {
+      SimpleTest.waitForExplicitFinish();
+
+      info("test_pathtuils_worker.xhtml: Starting test");
+
+      const worker = new ChromeWorker("pathutils_worker.js");
+      info("test_pathtuils_worker.xhtml: ChromeWorker created");
+
+      listenForTests(worker, { verbose: false });
+      worker.postMessage({
+        profileDir: PathUtils.profileDir,
+        localProfileDir: PathUtils.localProfileDir,
+        tempDir: PathUtils.tempDir,
+        osTempDir: PathUtils.osTempDir,
+      });
+
+      info("test_pathtuils_worker.xhtml: Test running...");
+    }
+  ]]>
+  </script>
+  <body xmlns="https://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display: none"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result" />
+</window>
diff --git a/dom/system/tests/worker_constants.js b/dom/system/tests/worker_constants.js
new file mode 100644
index 0000000000..befc5e4239
--- /dev/null
+++ b/dom/system/tests/worker_constants.js
@@ -0,0 +1,95 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+/* eslint-env mozilla/chrome-worker */
+/* global OS */
+
+function log(text) {
+  dump("WORKER " + text + "\n");
+}
+
+function send(message) {
+  self.postMessage(message);
+}
+
+self.onmessage = function(msg) {
+  self.onmessage = function(msgInner) {
+    log("ignored message " + JSON.stringify(msgInner.data));
+  };
+  let { isDebugBuild, umask } = msg.data;
+  try {
+    test_name();
+    test_xul();
+    test_debugBuildWorkerThread(isDebugBuild);
+    test_umaskWorkerThread(umask);
+    test_bits();
+  } catch (x) {
+    log("Catching error: " + x);
+    log("Stack: " + x.stack);
+    log("Source: " + x.toSource());
+    ok(false, x.toString() + "\n" + x.stack);
+  }
+  finish();
+};
+
+function finish() {
+  send({ kind: "finish" });
+}
+
+function ok(condition, description) {
+  send({ kind: "ok", condition, description });
+}
+function is(a, b, description) {
+  send({ kind: "is", a, b, description });
+}
+function isnot(a, b, description) {
+  send({ kind: "isnot", a, b, description });
+}
+
+// Test that OS.Constants.Sys.Name is defined
+function test_name() {
+  isnot(null, OS.Constants.Sys.Name, "OS.Constants.Sys.Name is defined");
+}
+
+// Test that OS.Constants.Sys.DEBUG is set properly in ChromeWorker thread
+function test_debugBuildWorkerThread(isDebugBuild) {
+  is(
+    isDebugBuild,
+    !!OS.Constants.Sys.DEBUG,
+    "OS.Constants.Sys.DEBUG is set properly on worker thread"
+  );
+}
+
+// Test that OS.Constants.Sys.umask is set properly in ChromeWorker thread
+function test_umaskWorkerThread(umask) {
+  is(
+    umask,
+    OS.Constants.Sys.umask,
+    "OS.Constants.Sys.umask is set properly on worker thread: " +
+      ("0000" + umask.toString(8)).slice(-4)
+  );
+}
+
+// Test that OS.Constants.Path.libxul lets us open libxul
+function test_xul() {
+  let lib;
+  isnot(null, OS.Constants.Path.libxul, "libxul is defined");
+  try {
+    lib = ctypes.open(OS.Constants.Path.libxul);
+    lib.declare("DumpJSStack", ctypes.default_abi, ctypes.void_t);
+  } catch (x) {
+    ok(false, "test_xul: Could not open libxul: " + x);
+  }
+  if (lib) {
+    lib.close();
+  }
+  ok(true, "test_xul: opened libxul successfully");
+}
+
+// Check if the value of OS.Constants.Sys.bits is 32 or 64
+function test_bits() {
+  is(
+    OS.Constants.Sys.bits,
+    ctypes.int.ptr.size * 8,
+    "OS.Constants.Sys.bits is either 32 or 64"
+  );
+}
diff --git a/dom/system/windows/WindowsLocationProvider.cpp b/dom/system/windows/WindowsLocationProvider.cpp
new file mode 100644
index 0000000000..3c548cfb21
--- /dev/null
+++ b/dom/system/windows/WindowsLocationProvider.cpp
@@ -0,0 +1,297 @@
+/* -*- 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 "WindowsLocationProvider.h"
+#include "GeolocationPosition.h"
+#include "nsComponentManagerUtils.h"
+#include "prtime.h"
+#include "MLSFallback.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Logging.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/dom/GeolocationPositionErrorBinding.h"
+
+namespace mozilla::dom {
+
+LazyLogModule gWindowsLocationProviderLog("WindowsLocationProvider");
+#define LOG(...) \
+  MOZ_LOG(gWindowsLocationProviderLog, LogLevel::Debug, (__VA_ARGS__))
+
+NS_IMPL_ISUPPORTS(WindowsLocationProvider::MLSUpdate, nsIGeolocationUpdate);
+
+WindowsLocationProvider::MLSUpdate::MLSUpdate(nsIGeolocationUpdate* aCallback)
+    : mCallback(aCallback) {}
+
+NS_IMETHODIMP
+WindowsLocationProvider::MLSUpdate::Update(nsIDOMGeoPosition* aPosition) {
+  if (!mCallback) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIDOMGeoPositionCoords> coords;
+  aPosition->GetCoords(getter_AddRefs(coords));
+  if (!coords) {
+    return NS_ERROR_FAILURE;
+  }
+  Telemetry::Accumulate(Telemetry::GEOLOCATION_WIN8_SOURCE_IS_MLS, true);
+  return mCallback->Update(aPosition);
+}
+NS_IMETHODIMP
+WindowsLocationProvider::MLSUpdate::NotifyError(uint16_t aError) {
+  if (!mCallback) {
+    return NS_ERROR_FAILURE;
+  }
+  nsCOMPtr<nsIGeolocationUpdate> callback(mCallback);
+  return callback->NotifyError(aError);
+}
+
+class LocationEvent final : public ILocationEvents {
+ public:
+  LocationEvent(nsIGeolocationUpdate* aCallback,
+                WindowsLocationProvider* aProvider)
+      : mCallback(aCallback), mProvider(aProvider), mCount(0) {}
+
+  // IUnknown interface
+  STDMETHODIMP_(ULONG) AddRef() override;
+  STDMETHODIMP_(ULONG) Release() override;
+  STDMETHODIMP QueryInterface(REFIID iid, void** ppv) override;
+
+  // ILocationEvents interface
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  STDMETHODIMP OnStatusChanged(REFIID aReportType,
+                               LOCATION_REPORT_STATUS aStatus) override;
+  STDMETHODIMP OnLocationChanged(REFIID aReportType,
+                                 ILocationReport* aReport) override;
+
+ private:
+  nsCOMPtr<nsIGeolocationUpdate> mCallback;
+  RefPtr<WindowsLocationProvider> mProvider;
+  ULONG mCount;
+};
+
+STDMETHODIMP_(ULONG)
+LocationEvent::AddRef() { return InterlockedIncrement(&mCount); }
+
+STDMETHODIMP_(ULONG)
+LocationEvent::Release() {
+  ULONG count = InterlockedDecrement(&mCount);
+  if (!count) {
+    delete this;
+    return 0;
+  }
+  return count;
+}
+
+STDMETHODIMP
+LocationEvent::QueryInterface(REFIID iid, void** ppv) {
+  if (iid == IID_IUnknown) {
+    *ppv = static_cast<IUnknown*>(this);
+  } else if (iid == IID_ILocationEvents) {
+    *ppv = static_cast<ILocationEvents*>(this);
+  } else {
+    return E_NOINTERFACE;
+  }
+  AddRef();
+  return S_OK;
+}
+
+STDMETHODIMP
+LocationEvent::OnStatusChanged(REFIID aReportType,
+                               LOCATION_REPORT_STATUS aStatus) {
+  if (aReportType != IID_ILatLongReport) {
+    return S_OK;
+  }
+
+  // When registering event, REPORT_INITIALIZING is fired at first.
+  // Then, when the location is found, REPORT_RUNNING is fired.
+  if (aStatus == REPORT_RUNNING) {
+    // location is found by Windows Location provider, we use it.
+    mProvider->CancelMLSProvider();
+    return S_OK;
+  }
+
+  // Cannot get current location at this time.  We use MLS instead until
+  // Location API returns RUNNING status.
+  if (NS_SUCCEEDED(mProvider->CreateAndWatchMLSProvider(mCallback))) {
+    return S_OK;
+  }
+
+  // Cannot watch location by MLS provider.  We must return error by
+  // Location API.
+  uint16_t err;
+  switch (aStatus) {
+    case REPORT_ACCESS_DENIED:
+      err = GeolocationPositionError_Binding::PERMISSION_DENIED;
+      break;
+    case REPORT_NOT_SUPPORTED:
+    case REPORT_ERROR:
+      err = GeolocationPositionError_Binding::POSITION_UNAVAILABLE;
+      break;
+    default:
+      return S_OK;
+  }
+  nsCOMPtr<nsIGeolocationUpdate> callback(mCallback);
+  callback->NotifyError(err);
+  return S_OK;
+}
+
+STDMETHODIMP
+LocationEvent::OnLocationChanged(REFIID aReportType, ILocationReport* aReport) {
+  if (aReportType != IID_ILatLongReport) {
+    return S_OK;
+  }
+
+  RefPtr<ILatLongReport> latLongReport;
+  if (FAILED(aReport->QueryInterface(IID_ILatLongReport,
+                                     getter_AddRefs(latLongReport)))) {
+    return E_FAIL;
+  }
+
+  DOUBLE latitude = 0.0;
+  latLongReport->GetLatitude(&latitude);
+
+  DOUBLE longitude = 0.0;
+  latLongReport->GetLongitude(&longitude);
+
+  DOUBLE alt = UnspecifiedNaN<double>();
+  latLongReport->GetAltitude(&alt);
+
+  DOUBLE herror = 0.0;
+  latLongReport->GetErrorRadius(&herror);
+
+  DOUBLE verror = UnspecifiedNaN<double>();
+  latLongReport->GetAltitudeError(&verror);
+
+  double heading = UnspecifiedNaN<double>();
+  double speed = UnspecifiedNaN<double>();
+
+  // nsGeoPositionCoords will convert NaNs to null for optional properties of
+  // the JavaScript Coordinates object.
+  RefPtr<nsGeoPosition> position =
+      new nsGeoPosition(latitude, longitude, alt, herror, verror, heading,
+                        speed, PR_Now() / PR_USEC_PER_MSEC);
+  mCallback->Update(position);
+
+  Telemetry::Accumulate(Telemetry::GEOLOCATION_WIN8_SOURCE_IS_MLS, false);
+
+  return S_OK;
+}
+
+NS_IMPL_ISUPPORTS(WindowsLocationProvider, nsIGeolocationProvider)
+
+WindowsLocationProvider::WindowsLocationProvider() {
+  LOG("WindowsLocationProvider::WindowsLocationProvider(%p)\n", this);
+}
+
+WindowsLocationProvider::~WindowsLocationProvider() {
+  LOG("WindowsLocationProvider::~WindowsLocationProvider(%p, %p)\n", this,
+      mLocation.get());
+}
+
+NS_IMETHODIMP
+WindowsLocationProvider::Startup() {
+  LOG("WindowsLocationProvider::Startup(%p, %p)\n", this, mLocation.get());
+  if (mLocation) {
+    return NS_OK;
+  }
+
+  RefPtr<ILocation> location;
+  if (FAILED(::CoCreateInstance(CLSID_Location, nullptr, CLSCTX_INPROC_SERVER,
+                                IID_ILocation, getter_AddRefs(location)))) {
+    // We will use MLS provider
+    return NS_OK;
+  }
+
+  IID reportTypes[] = {IID_ILatLongReport};
+  if (FAILED(location->RequestPermissions(nullptr, reportTypes, 1, FALSE))) {
+    // We will use MLS provider
+    return NS_OK;
+  }
+
+  mLocation = location;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+WindowsLocationProvider::Watch(nsIGeolocationUpdate* aCallback) {
+  LOG("WindowsLocationProvider::Watch(%p, %p, %d)\n", this, mLocation.get(),
+      mWatching);
+  if (mLocation) {
+    if (mWatching) {
+      return NS_OK;
+    }
+    RefPtr<LocationEvent> event = new LocationEvent(aCallback, this);
+    if (SUCCEEDED(mLocation->RegisterForReport(event, IID_ILatLongReport, 0))) {
+      mWatching = true;
+      return NS_OK;
+    }
+  }
+
+  // Cannot use Location API.  We will use MLS instead.
+  LOG(" > MLS fallback\n");
+  mLocation = nullptr;
+
+  return CreateAndWatchMLSProvider(aCallback);
+}
+
+NS_IMETHODIMP
+WindowsLocationProvider::Shutdown() {
+  LOG("WindowsLocationProvider::Shutdown(%p, %p)\n", this, mLocation.get());
+  if (mLocation) {
+    if (mWatching) {
+      mLocation->UnregisterForReport(IID_ILatLongReport);
+    }
+    mLocation = nullptr;
+    mWatching = false;
+  }
+
+  CancelMLSProvider();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+WindowsLocationProvider::SetHighAccuracy(bool enable) {
+  if (!mLocation) {
+    // MLS provider doesn't support HighAccuracy
+    return NS_OK;
+  }
+
+  LOCATION_DESIRED_ACCURACY desiredAccuracy;
+  if (enable) {
+    desiredAccuracy = LOCATION_DESIRED_ACCURACY_HIGH;
+  } else {
+    desiredAccuracy = LOCATION_DESIRED_ACCURACY_DEFAULT;
+  }
+  if (FAILED(
+          mLocation->SetDesiredAccuracy(IID_ILatLongReport, desiredAccuracy))) {
+    return NS_ERROR_FAILURE;
+  }
+  return NS_OK;
+}
+
+nsresult WindowsLocationProvider::CreateAndWatchMLSProvider(
+    nsIGeolocationUpdate* aCallback) {
+  if (mMLSProvider) {
+    return NS_OK;
+  }
+
+  mMLSProvider = new MLSFallback(0);
+  return mMLSProvider->Startup(new MLSUpdate(aCallback));
+}
+
+void WindowsLocationProvider::CancelMLSProvider() {
+  if (!mMLSProvider) {
+    return;
+  }
+
+  mMLSProvider->Shutdown();
+  mMLSProvider = nullptr;
+}
+
+#undef LOG
+
+}  // namespace mozilla::dom
diff --git a/dom/system/windows/WindowsLocationProvider.h b/dom/system/windows/WindowsLocationProvider.h
new file mode 100644
index 0000000000..779236d003
--- /dev/null
+++ b/dom/system/windows/WindowsLocationProvider.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/. */
+
+#ifndef mozilla_dom_WindowsLocationProvider_h__
+#define mozilla_dom_WindowsLocationProvider_h__
+
+#include "nsCOMPtr.h"
+#include "nsIGeolocationProvider.h"
+
+#include <locationapi.h>
+
+class MLSFallback;
+
+namespace mozilla::dom {
+
+class WindowsLocationProvider final : public nsIGeolocationProvider {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIGEOLOCATIONPROVIDER
+
+  WindowsLocationProvider();
+
+  nsresult CreateAndWatchMLSProvider(nsIGeolocationUpdate* aCallback);
+  void CancelMLSProvider();
+
+  class MLSUpdate : public nsIGeolocationUpdate {
+   public:
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSIGEOLOCATIONUPDATE
+    explicit MLSUpdate(nsIGeolocationUpdate* aCallback);
+
+   private:
+    nsCOMPtr<nsIGeolocationUpdate> mCallback;
+    virtual ~MLSUpdate() {}
+  };
+
+ private:
+  ~WindowsLocationProvider();
+
+  RefPtr<ILocation> mLocation;
+  RefPtr<MLSFallback> mMLSProvider;
+  bool mWatching = false;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_WindowsLocationProvider_h__
diff --git a/dom/system/windows/moz.build b/dom/system/windows/moz.build
new file mode 100644
index 0000000000..45131af39e
--- /dev/null
+++ b/dom/system/windows/moz.build
@@ -0,0 +1,11 @@
+# -*- 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/.
+
+SOURCES += ["nsHapticFeedback.cpp", "WindowsLocationProvider.cpp"]
+
+LOCAL_INCLUDES += ["/dom/geolocation"]
+
+FINAL_LIBRARY = "xul"
diff --git a/dom/system/windows/nsHapticFeedback.cpp b/dom/system/windows/nsHapticFeedback.cpp
new file mode 100644
index 0000000000..f85c5889d9
--- /dev/null
+++ b/dom/system/windows/nsHapticFeedback.cpp
@@ -0,0 +1,15 @@
+/* -*- 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 "nsHapticFeedback.h"
+
+NS_IMPL_ISUPPORTS(nsHapticFeedback, nsIHapticFeedback)
+
+NS_IMETHODIMP
+nsHapticFeedback::PerformSimpleAction(int32_t aType) {
+  // Todo
+  return NS_OK;
+}
diff --git a/dom/system/windows/nsHapticFeedback.h b/dom/system/windows/nsHapticFeedback.h
new file mode 100644
index 0000000000..b15cb00f31
--- /dev/null
+++ b/dom/system/windows/nsHapticFeedback.h
@@ -0,0 +1,15 @@
+/* -*- 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 "nsIHapticFeedback.h"
+
+class nsHapticFeedback final : public nsIHapticFeedback {
+  ~nsHapticFeedback() {}
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIHAPTICFEEDBACK
+};