Adding upstream version 110.0.1.upstream/110.0.1upstream

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

15 files changed, 3333 insertions, 0 deletions

diff --git a/netwerk/protocol/res/ExtensionProtocolHandler.cpp b/netwerk/protocol/res/ExtensionProtocolHandler.cpp
new file mode 100644
index 0000000000..c4f711807f
--- /dev/null
+++ b/netwerk/protocol/res/ExtensionProtocolHandler.cpp
@@ -0,0 +1,1041 @@
+/* -*- 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 "ExtensionProtocolHandler.h"
+
+#include "mozilla/BinarySearch.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/ExtensionPolicyService.h"
+#include "mozilla/FileUtils.h"
+#include "mozilla/ipc/FileDescriptor.h"
+#include "mozilla/ipc/IPCStreamUtils.h"
+#include "mozilla/ipc/URIUtils.h"
+#include "mozilla/net/NeckoChild.h"
+#include "mozilla/Omnijar.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ResultExtensions.h"
+
+#include "FileDescriptorFile.h"
+#include "LoadInfo.h"
+#include "nsContentUtils.h"
+#include "nsServiceManagerUtils.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsICancelable.h"
+#include "nsIFile.h"
+#include "nsIFileChannel.h"
+#include "nsIFileStreams.h"
+#include "nsIFileURL.h"
+#include "nsIJARChannel.h"
+#include "nsIMIMEService.h"
+#include "nsIURL.h"
+#include "nsIChannel.h"
+#include "nsIInputStreamPump.h"
+#include "nsIJARURI.h"
+#include "nsIStreamListener.h"
+#include "nsIInputStream.h"
+#include "nsIStreamConverterService.h"
+#include "nsNetUtil.h"
+#include "nsReadableUtils.h"
+#include "nsURLHelper.h"
+#include "prio.h"
+#include "SimpleChannel.h"
+
+#if defined(XP_WIN)
+#  include "nsILocalFileWin.h"
+#  include "WinUtils.h"
+#endif
+
+#if defined(XP_MACOSX)
+#  include "nsMacUtilsImpl.h"
+#endif
+
+#define EXTENSION_SCHEME "moz-extension"
+using mozilla::dom::Promise;
+using mozilla::ipc::FileDescriptor;
+
+// A list of file extensions containing purely static data, which can be loaded
+// from an extension before the extension is fully ready. The main purpose of
+// this is to allow image resources from theme XPIs to load quickly during
+// browser startup.
+//
+// The layout of this array is chosen in order to prevent the need for runtime
+// relocation, which an array of char* pointers would require. It also has the
+// benefit of being more compact when the difference in length between the
+// longest and average string is less than 8 bytes. The length of the
+// char[] array must match the size of the longest entry in the list.
+//
+// This list must be kept sorted.
+static const char sStaticFileExtensions[][5] = {
+    // clang-format off
+  "bmp",
+  "gif",
+  "ico",
+  "jpeg",
+  "jpg",
+  "png",
+  "svg",
+    // clang-format on
+};
+
+namespace mozilla {
+
+namespace net {
+
+using extensions::URLInfo;
+
+LazyLogModule gExtProtocolLog("ExtProtocol");
+#undef LOG
+#define LOG(...) MOZ_LOG(gExtProtocolLog, LogLevel::Debug, (__VA_ARGS__))
+
+StaticRefPtr<ExtensionProtocolHandler> ExtensionProtocolHandler::sSingleton;
+
+/**
+ * Helper class used with SimpleChannel to asynchronously obtain an input
+ * stream or file descriptor from the parent for a remote moz-extension load
+ * from the child.
+ */
+class ExtensionStreamGetter final : public nsICancelable {
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSICANCELABLE
+
+ public:
+  // To use when getting a remote input stream for a resource
+  // in an unpacked extension.
+  ExtensionStreamGetter(nsIURI* aURI, nsILoadInfo* aLoadInfo)
+      : mURI(aURI), mLoadInfo(aLoadInfo), mIsJarChannel(false) {
+    MOZ_ASSERT(aURI);
+    MOZ_ASSERT(aLoadInfo);
+
+    SetupEventTarget();
+  }
+
+  // To use when getting an FD for a packed extension JAR file
+  // in order to load a resource.
+  ExtensionStreamGetter(nsIURI* aURI, nsILoadInfo* aLoadInfo,
+                        already_AddRefed<nsIJARChannel>&& aJarChannel,
+                        nsIFile* aJarFile)
+      : mURI(aURI),
+        mLoadInfo(aLoadInfo),
+        mJarChannel(std::move(aJarChannel)),
+        mJarFile(aJarFile),
+        mIsJarChannel(true) {
+    MOZ_ASSERT(aURI);
+    MOZ_ASSERT(aLoadInfo);
+    MOZ_ASSERT(mJarChannel);
+    MOZ_ASSERT(aJarFile);
+
+    SetupEventTarget();
+  }
+
+  void SetupEventTarget() {
+    mMainThreadEventTarget = nsContentUtils::GetEventTargetByLoadInfo(
+        mLoadInfo, TaskCategory::Other);
+    if (!mMainThreadEventTarget) {
+      mMainThreadEventTarget = GetMainThreadSerialEventTarget();
+    }
+  }
+
+  // Get an input stream or file descriptor from the parent asynchronously.
+  RequestOrReason GetAsync(nsIStreamListener* aListener, nsIChannel* aChannel);
+
+  // Handle an input stream being returned from the parent
+  void OnStream(already_AddRefed<nsIInputStream> aStream);
+
+  // Handle file descriptor being returned from the parent
+  void OnFD(const FileDescriptor& aFD);
+
+  static void CancelRequest(nsIStreamListener* aListener, nsIChannel* aChannel,
+                            nsresult aResult);
+
+ private:
+  ~ExtensionStreamGetter() = default;
+
+  nsCOMPtr<nsIURI> mURI;
+  nsCOMPtr<nsILoadInfo> mLoadInfo;
+  nsCOMPtr<nsIJARChannel> mJarChannel;
+  nsCOMPtr<nsIInputStreamPump> mPump;
+  nsCOMPtr<nsIFile> mJarFile;
+  nsCOMPtr<nsIStreamListener> mListener;
+  nsCOMPtr<nsIChannel> mChannel;
+  nsCOMPtr<nsISerialEventTarget> mMainThreadEventTarget;
+  bool mIsJarChannel;
+  bool mCanceled{false};
+  nsresult mStatus{NS_OK};
+};
+
+NS_IMPL_ISUPPORTS(ExtensionStreamGetter, nsICancelable)
+
+class ExtensionJARFileOpener final : public nsISupports {
+ public:
+  ExtensionJARFileOpener(nsIFile* aFile,
+                         NeckoParent::GetExtensionFDResolver& aResolve)
+      : mFile(aFile), mResolve(aResolve) {
+    MOZ_ASSERT(aFile);
+    MOZ_ASSERT(aResolve);
+  }
+
+  NS_IMETHOD OpenFile() {
+    MOZ_ASSERT(!NS_IsMainThread());
+    AutoFDClose prFileDesc;
+
+#if defined(XP_WIN)
+    nsresult rv;
+    nsCOMPtr<nsILocalFileWin> winFile = do_QueryInterface(mFile, &rv);
+    MOZ_ASSERT(winFile);
+    if (NS_SUCCEEDED(rv)) {
+      rv = winFile->OpenNSPRFileDescShareDelete(PR_RDONLY, 0,
+                                                &prFileDesc.rwget());
+    }
+#else
+    nsresult rv = mFile->OpenNSPRFileDesc(PR_RDONLY, 0, &prFileDesc.rwget());
+#endif /* XP_WIN */
+
+    if (NS_SUCCEEDED(rv)) {
+      mFD = FileDescriptor(FileDescriptor::PlatformHandleType(
+          PR_FileDesc2NativeHandle(prFileDesc)));
+    }
+
+    nsCOMPtr<nsIRunnable> event =
+        mozilla::NewRunnableMethod("ExtensionJarFileFDResolver", this,
+                                   &ExtensionJARFileOpener::SendBackFD);
+
+    rv = NS_DispatchToMainThread(event, nsIEventTarget::DISPATCH_NORMAL);
+    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "NS_DispatchToMainThread");
+    return NS_OK;
+  }
+
+  NS_IMETHOD SendBackFD() {
+    MOZ_ASSERT(NS_IsMainThread());
+    mResolve(mFD);
+    mResolve = nullptr;
+    return NS_OK;
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+ private:
+  virtual ~ExtensionJARFileOpener() = default;
+
+  nsCOMPtr<nsIFile> mFile;
+  NeckoParent::GetExtensionFDResolver mResolve;
+  FileDescriptor mFD;
+};
+
+NS_IMPL_ISUPPORTS(ExtensionJARFileOpener, nsISupports)
+
+// The amount of time, in milliseconds, that the file opener thread will remain
+// allocated after it is used. This value chosen because to match other uses
+// of LazyIdleThread.
+#define DEFAULT_THREAD_TIMEOUT_MS 30000
+
+// Request an FD or input stream from the parent.
+RequestOrReason ExtensionStreamGetter::GetAsync(nsIStreamListener* aListener,
+                                                nsIChannel* aChannel) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_ASSERT(mMainThreadEventTarget);
+
+  mListener = aListener;
+  mChannel = aChannel;
+
+  nsCOMPtr<nsICancelable> cancelableRequest(this);
+
+  RefPtr<ExtensionStreamGetter> self = this;
+  if (mIsJarChannel) {
+    // Request an FD for this moz-extension URI
+    gNeckoChild->SendGetExtensionFD(mURI)->Then(
+        mMainThreadEventTarget, __func__,
+        [self](const FileDescriptor& fd) { self->OnFD(fd); },
+        [self](const mozilla::ipc::ResponseRejectReason) {
+          self->OnFD(FileDescriptor());
+        });
+    return RequestOrCancelable(WrapNotNull(cancelableRequest));
+  }
+
+  // Request an input stream for this moz-extension URI
+  gNeckoChild->SendGetExtensionStream(mURI)->Then(
+      mMainThreadEventTarget, __func__,
+      [self](const RefPtr<nsIInputStream>& stream) {
+        self->OnStream(do_AddRef(stream));
+      },
+      [self](const mozilla::ipc::ResponseRejectReason) {
+        self->OnStream(nullptr);
+      });
+  return RequestOrCancelable(WrapNotNull(cancelableRequest));
+}
+
+// Called to cancel the ongoing async request.
+NS_IMETHODIMP
+ExtensionStreamGetter::Cancel(nsresult aStatus) {
+  if (mCanceled) {
+    return NS_OK;
+  }
+
+  mCanceled = true;
+  mStatus = aStatus;
+
+  if (mPump) {
+    mPump->CancelWithReason(aStatus, "ExtensionStreamGetter::Cancel"_ns);
+    mPump = nullptr;
+  }
+
+  if (mIsJarChannel && mJarChannel) {
+    mJarChannel->CancelWithReason(aStatus, "ExtensionStreamGetter::Cancel"_ns);
+  }
+
+  return NS_OK;
+}
+
+// static
+void ExtensionStreamGetter::CancelRequest(nsIStreamListener* aListener,
+                                          nsIChannel* aChannel,
+                                          nsresult aResult) {
+  MOZ_ASSERT(aListener);
+  MOZ_ASSERT(aChannel);
+
+  aListener->OnStartRequest(aChannel);
+  aListener->OnStopRequest(aChannel, aResult);
+  aChannel->CancelWithReason(NS_BINDING_ABORTED,
+                             "ExtensionStreamGetter::CancelRequest"_ns);
+}
+
+// Handle an input stream sent from the parent.
+void ExtensionStreamGetter::OnStream(already_AddRefed<nsIInputStream> aStream) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_ASSERT(mChannel);
+  MOZ_ASSERT(mListener);
+  MOZ_ASSERT(mMainThreadEventTarget);
+
+  nsCOMPtr<nsIInputStream> stream = std::move(aStream);
+  nsCOMPtr<nsIChannel> channel = std::move(mChannel);
+
+  // We must keep an owning reference to the listener
+  // until we pass it on to AsyncRead.
+  nsCOMPtr<nsIStreamListener> listener = std::move(mListener);
+
+  if (mCanceled) {
+    // The channel that has created this stream getter has been canceled.
+    CancelRequest(listener, channel, mStatus);
+    return;
+  }
+
+  if (!stream) {
+    // The parent didn't send us back a stream.
+    CancelRequest(listener, channel, NS_ERROR_FILE_ACCESS_DENIED);
+    return;
+  }
+
+  nsCOMPtr<nsIInputStreamPump> pump;
+  nsresult rv = NS_NewInputStreamPump(getter_AddRefs(pump), stream.forget(), 0,
+                                      0, false, mMainThreadEventTarget);
+  if (NS_FAILED(rv)) {
+    CancelRequest(listener, channel, rv);
+    return;
+  }
+
+  rv = pump->AsyncRead(listener);
+  if (NS_FAILED(rv)) {
+    CancelRequest(listener, channel, rv);
+    return;
+  }
+
+  mPump = pump;
+}
+
+// Handle an FD sent from the parent.
+void ExtensionStreamGetter::OnFD(const FileDescriptor& aFD) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_ASSERT(mChannel);
+  MOZ_ASSERT(mListener);
+
+  nsCOMPtr<nsIChannel> channel = std::move(mChannel);
+
+  // We must keep an owning reference to the listener
+  // until we pass it on to AsyncOpen.
+  nsCOMPtr<nsIStreamListener> listener = std::move(mListener);
+
+  if (mCanceled) {
+    // The channel that has created this stream getter has been canceled.
+    CancelRequest(listener, channel, mStatus);
+    return;
+  }
+
+  if (!aFD.IsValid()) {
+    // The parent didn't send us back a valid file descriptor.
+    CancelRequest(listener, channel, NS_ERROR_FILE_ACCESS_DENIED);
+    return;
+  }
+
+  RefPtr<FileDescriptorFile> fdFile = new FileDescriptorFile(aFD, mJarFile);
+  mJarChannel->SetJarFile(fdFile);
+  nsresult rv = mJarChannel->AsyncOpen(listener);
+  if (NS_FAILED(rv)) {
+    CancelRequest(listener, channel, rv);
+  }
+}
+
+NS_IMPL_QUERY_INTERFACE(ExtensionProtocolHandler,
+                        nsISubstitutingProtocolHandler, nsIProtocolHandler,
+                        nsIProtocolHandlerWithDynamicFlags,
+                        nsISupportsWeakReference)
+NS_IMPL_ADDREF_INHERITED(ExtensionProtocolHandler, SubstitutingProtocolHandler)
+NS_IMPL_RELEASE_INHERITED(ExtensionProtocolHandler, SubstitutingProtocolHandler)
+
+already_AddRefed<ExtensionProtocolHandler>
+ExtensionProtocolHandler::GetSingleton() {
+  if (!sSingleton) {
+    sSingleton = new ExtensionProtocolHandler();
+    ClearOnShutdown(&sSingleton);
+  }
+  return do_AddRef(sSingleton);
+}
+
+ExtensionProtocolHandler::ExtensionProtocolHandler()
+    : SubstitutingProtocolHandler(EXTENSION_SCHEME) {
+  // Note, extensions.webextensions.protocol.remote=false is for
+  // debugging purposes only. With process-level sandboxing, child
+  // processes (specifically content and extension processes), will
+  // not be able to load most moz-extension URI's when the pref is
+  // set to false.
+  mUseRemoteFileChannels =
+      IsNeckoChild() &&
+      Preferences::GetBool("extensions.webextensions.protocol.remote");
+}
+
+static inline ExtensionPolicyService& EPS() {
+  return ExtensionPolicyService::GetSingleton();
+}
+
+nsresult ExtensionProtocolHandler::GetFlagsForURI(nsIURI* aURI,
+                                                  uint32_t* aFlags) {
+  uint32_t flags =
+      URI_STD | URI_IS_LOCAL_RESOURCE | URI_IS_POTENTIALLY_TRUSTWORTHY;
+
+  URLInfo url(aURI);
+  if (auto* policy = EPS().GetByURL(url)) {
+    // In general a moz-extension URI is only loadable by chrome, but an
+    // allowlist subset are web-accessible (and cross-origin fetchable).
+    // The allowlist is checked using EPS.SourceMayLoadExtensionURI in
+    // BasePrincipal and nsScriptSecurityManager.
+    if (policy->IsWebAccessiblePath(url.FilePath())) {
+      if (policy->ManifestVersion() < 3) {
+        flags |= URI_LOADABLE_BY_ANYONE | URI_FETCHABLE_BY_ANYONE;
+      } else {
+        flags |= WEBEXT_URI_WEB_ACCESSIBLE;
+      }
+    } else if (policy->Type() == nsGkAtoms::theme) {
+      // Static themes cannot set web accessible resources, however using this
+      // flag here triggers SourceMayAccessPath calls necessary to allow another
+      // extension to access static theme resources in this extension.
+      flags |= WEBEXT_URI_WEB_ACCESSIBLE;
+    } else {
+      flags |= URI_DANGEROUS_TO_LOAD;
+    }
+
+    // Disallow in private windows if the extension does not have permission.
+    if (!policy->PrivateBrowsingAllowed()) {
+      flags |= URI_DISALLOW_IN_PRIVATE_CONTEXT;
+    }
+  } else {
+    // In case there is no policy, then default to treating moz-extension URIs
+    // as unsafe and generally only allow chrome: to load such.
+    flags |= URI_DANGEROUS_TO_LOAD;
+  }
+
+  *aFlags = flags;
+  return NS_OK;
+}
+
+bool ExtensionProtocolHandler::ResolveSpecialCases(const nsACString& aHost,
+                                                   const nsACString& aPath,
+                                                   const nsACString& aPathname,
+                                                   nsACString& aResult) {
+  MOZ_DIAGNOSTIC_ASSERT(NS_IsMainThread(),
+                        "The ExtensionPolicyService is not thread safe");
+  // Create special moz-extension://foo/_generated_background_page.html page
+  // for all registered extensions. We can't just do this as a substitution
+  // because substitutions can only match on host.
+  if (!SubstitutingProtocolHandler::HasSubstitution(aHost)) {
+    return false;
+  }
+
+  if (aPathname.EqualsLiteral("/_generated_background_page.html")) {
+    Unused << EPS().GetGeneratedBackgroundPageUrl(aHost, aResult);
+    return !aResult.IsEmpty();
+  }
+
+  return false;
+}
+
+// For file or JAR URI's, substitute in a remote channel.
+Result<Ok, nsresult> ExtensionProtocolHandler::SubstituteRemoteChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadInfo, nsIChannel** aRetVal) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_TRY(aURI ? NS_OK : NS_ERROR_INVALID_ARG);
+  MOZ_TRY(aLoadInfo ? NS_OK : NS_ERROR_INVALID_ARG);
+
+  nsAutoCString unResolvedSpec;
+  MOZ_TRY(aURI->GetSpec(unResolvedSpec));
+
+  nsAutoCString resolvedSpec;
+  MOZ_TRY(ResolveURI(aURI, resolvedSpec));
+
+  // Use the target URI scheme to determine if this is a packed or unpacked
+  // extension URI. For unpacked extensions, we'll request an input stream
+  // from the parent. For a packed extension, we'll request a file descriptor
+  // for the JAR file.
+  nsAutoCString scheme;
+  MOZ_TRY(net_ExtractURLScheme(resolvedSpec, scheme));
+
+  if (scheme.EqualsLiteral("file")) {
+    // Unpacked extension
+    SubstituteRemoteFileChannel(aURI, aLoadInfo, resolvedSpec, aRetVal);
+    return Ok();
+  }
+
+  if (scheme.EqualsLiteral("jar")) {
+    // Packed extension
+    return SubstituteRemoteJarChannel(aURI, aLoadInfo, resolvedSpec, aRetVal);
+  }
+
+  // Only unpacked resource files and JAR files are remoted.
+  // No other moz-extension loads should be reading from the filesystem.
+  return Ok();
+}
+
+void OpenWhenReady(
+    Promise* aPromise, nsIStreamListener* aListener, nsIChannel* aChannel,
+    const std::function<nsresult(nsIStreamListener*, nsIChannel*)>& aCallback) {
+  nsCOMPtr<nsIStreamListener> listener(aListener);
+  nsCOMPtr<nsIChannel> channel(aChannel);
+
+  Unused << aPromise->ThenWithCycleCollectedArgs(
+      [channel, aCallback](
+          JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+          nsIStreamListener* aListener) -> already_AddRefed<Promise> {
+        nsresult rv = aCallback(aListener, channel);
+        if (NS_FAILED(rv)) {
+          ExtensionStreamGetter::CancelRequest(aListener, channel, rv);
+        }
+        return nullptr;
+      },
+      listener);
+}
+
+nsresult ExtensionProtocolHandler::SubstituteChannel(nsIURI* aURI,
+                                                     nsILoadInfo* aLoadInfo,
+                                                     nsIChannel** result) {
+  if (mUseRemoteFileChannels) {
+    MOZ_TRY(SubstituteRemoteChannel(aURI, aLoadInfo, result));
+  }
+
+  auto* policy = EPS().GetByURL(aURI);
+  NS_ENSURE_TRUE(policy, NS_ERROR_UNEXPECTED);
+
+  RefPtr<dom::Promise> readyPromise(policy->ReadyPromise());
+
+  nsresult rv;
+  nsCOMPtr<nsIURL> url = do_QueryInterface(aURI, &rv);
+  MOZ_TRY(rv);
+
+  nsAutoCString ext;
+  MOZ_TRY(url->GetFileExtension(ext));
+  ToLowerCase(ext);
+
+  nsCOMPtr<nsIChannel> channel;
+  if (ext.EqualsLiteral("css")) {
+    // Filter CSS files to replace locale message tokens with localized strings.
+    static const auto convert = [](nsIStreamListener* listener,
+                                   nsIChannel* channel,
+                                   nsIChannel* origChannel) -> nsresult {
+      nsresult rv;
+      nsCOMPtr<nsIStreamConverterService> convService =
+          do_GetService(NS_STREAMCONVERTERSERVICE_CONTRACTID, &rv);
+      MOZ_TRY(rv);
+
+      nsCOMPtr<nsIURI> uri;
+      MOZ_TRY(channel->GetURI(getter_AddRefs(uri)));
+
+      const char* kFromType = "application/vnd.mozilla.webext.unlocalized";
+      const char* kToType = "text/css";
+
+      nsCOMPtr<nsIStreamListener> converter;
+      MOZ_TRY(convService->AsyncConvertData(kFromType, kToType, listener, uri,
+                                            getter_AddRefs(converter)));
+
+      return origChannel->AsyncOpen(converter);
+    };
+
+    channel = NS_NewSimpleChannel(
+        aURI, aLoadInfo, *result,
+        [readyPromise](nsIStreamListener* listener, nsIChannel* channel,
+                       nsIChannel* origChannel) -> RequestOrReason {
+          if (readyPromise) {
+            nsCOMPtr<nsIChannel> chan(channel);
+            OpenWhenReady(
+                readyPromise, listener, origChannel,
+                [chan](nsIStreamListener* aListener, nsIChannel* aChannel) {
+                  return convert(aListener, chan, aChannel);
+                });
+          } else {
+            MOZ_TRY(convert(listener, channel, origChannel));
+          }
+          nsCOMPtr<nsIRequest> request(origChannel);
+          return RequestOrCancelable(WrapNotNull(request));
+        });
+  } else if (readyPromise) {
+    size_t matchIdx;
+    if (BinarySearchIf(
+            sStaticFileExtensions, 0, ArrayLength(sStaticFileExtensions),
+            [&ext](const char* aOther) {
+              return Compare(ext, nsDependentCString(aOther));
+            },
+            &matchIdx)) {
+      // This is a static resource that shouldn't depend on the extension being
+      // ready. Don't bother waiting for it.
+      return NS_OK;
+    }
+
+    channel = NS_NewSimpleChannel(
+        aURI, aLoadInfo, *result,
+        [readyPromise](nsIStreamListener* listener, nsIChannel* channel,
+                       nsIChannel* origChannel) -> RequestOrReason {
+          OpenWhenReady(readyPromise, listener, origChannel,
+                        [](nsIStreamListener* aListener, nsIChannel* aChannel) {
+                          return aChannel->AsyncOpen(aListener);
+                        });
+
+          nsCOMPtr<nsIRequest> request(origChannel);
+          return RequestOrCancelable(WrapNotNull(request));
+        });
+  } else {
+    return NS_OK;
+  }
+
+  NS_ENSURE_TRUE(channel, NS_ERROR_OUT_OF_MEMORY);
+  if (aLoadInfo) {
+    nsCOMPtr<nsILoadInfo> loadInfo =
+        static_cast<LoadInfo*>(aLoadInfo)->CloneForNewRequest();
+    (*result)->SetLoadInfo(loadInfo);
+  }
+
+  channel.swap(*result);
+  return NS_OK;
+}
+
+Result<bool, nsresult> ExtensionProtocolHandler::AllowExternalResource(
+    nsIFile* aExtensionDir, nsIFile* aRequestedFile) {
+  MOZ_ASSERT(!IsNeckoChild());
+
+#if defined(XP_WIN)
+  // On Windows, dev builds don't use symlinks so we never need to
+  // allow a resource from outside of the extension dir.
+  return false;
+#else
+  if (!mozilla::IsDevelopmentBuild()) {
+    return false;
+  }
+
+  // On Mac and Linux unpackaged dev builds, system extensions use
+  // symlinks to point to resources in the repo dir which we have to
+  // allow loading. Before we allow an unpacked extension to load a
+  // resource outside of the extension dir, we make sure the extension
+  // dir is within the app directory.
+  bool result;
+  MOZ_TRY_VAR(result, AppDirContains(aExtensionDir));
+  if (!result) {
+    return false;
+  }
+
+#  if defined(XP_MACOSX)
+  // Additionally, on Mac dev builds, we make sure that the requested
+  // resource is within the repo dir. We don't perform this check on Linux
+  // because we don't have a reliable path to the repo dir on Linux.
+  return DevRepoContains(aRequestedFile);
+#  else /* XP_MACOSX */
+  return true;
+#  endif
+#endif /* defined(XP_WIN) */
+}
+
+#if defined(XP_MACOSX)
+// The |aRequestedFile| argument must already be Normalize()'d
+Result<bool, nsresult> ExtensionProtocolHandler::DevRepoContains(
+    nsIFile* aRequestedFile) {
+  MOZ_ASSERT(mozilla::IsDevelopmentBuild());
+  MOZ_ASSERT(!IsNeckoChild());
+
+  // On the first invocation, set mDevRepo
+  if (!mAlreadyCheckedDevRepo) {
+    mAlreadyCheckedDevRepo = true;
+    MOZ_TRY(nsMacUtilsImpl::GetRepoDir(getter_AddRefs(mDevRepo)));
+    if (MOZ_LOG_TEST(gExtProtocolLog, LogLevel::Debug)) {
+      nsAutoCString repoPath;
+      Unused << mDevRepo->GetNativePath(repoPath);
+      LOG("Repo path: %s", repoPath.get());
+    }
+  }
+
+  bool result = false;
+  if (mDevRepo) {
+    MOZ_TRY(mDevRepo->Contains(aRequestedFile, &result));
+  }
+  return result;
+}
+#endif /* XP_MACOSX */
+
+#if !defined(XP_WIN)
+Result<bool, nsresult> ExtensionProtocolHandler::AppDirContains(
+    nsIFile* aExtensionDir) {
+  MOZ_ASSERT(mozilla::IsDevelopmentBuild());
+  MOZ_ASSERT(!IsNeckoChild());
+
+  // On the first invocation, set mAppDir
+  if (!mAlreadyCheckedAppDir) {
+    mAlreadyCheckedAppDir = true;
+    MOZ_TRY(NS_GetSpecialDirectory(NS_GRE_DIR, getter_AddRefs(mAppDir)));
+    if (MOZ_LOG_TEST(gExtProtocolLog, LogLevel::Debug)) {
+      nsAutoCString appDirPath;
+      Unused << mAppDir->GetNativePath(appDirPath);
+      LOG("AppDir path: %s", appDirPath.get());
+    }
+  }
+
+  bool result = false;
+  if (mAppDir) {
+    MOZ_TRY(mAppDir->Contains(aExtensionDir, &result));
+  }
+  return result;
+}
+#endif /* !defined(XP_WIN) */
+
+static void LogExternalResourceError(nsIFile* aExtensionDir,
+                                     nsIFile* aRequestedFile) {
+  MOZ_ASSERT(aExtensionDir);
+  MOZ_ASSERT(aRequestedFile);
+
+  LOG("Rejecting external unpacked extension resource [%s] from "
+      "extension directory [%s]",
+      aRequestedFile->HumanReadablePath().get(),
+      aExtensionDir->HumanReadablePath().get());
+}
+
+Result<nsCOMPtr<nsIInputStream>, nsresult> ExtensionProtocolHandler::NewStream(
+    nsIURI* aChildURI, bool* aTerminateSender) {
+  MOZ_ASSERT(!IsNeckoChild());
+  MOZ_TRY(aChildURI ? NS_OK : NS_ERROR_INVALID_ARG);
+  MOZ_TRY(aTerminateSender ? NS_OK : NS_ERROR_INVALID_ARG);
+
+  *aTerminateSender = true;
+  nsresult rv;
+
+  // We should never receive a URI that isn't for a moz-extension because
+  // these requests ordinarily come from the child's ExtensionProtocolHandler.
+  // Ensure this request is for a moz-extension URI. A rogue child process
+  // could send us any URI.
+  if (!aChildURI->SchemeIs(EXTENSION_SCHEME)) {
+    return Err(NS_ERROR_UNKNOWN_PROTOCOL);
+  }
+
+  // For errors after this point, we want to propagate the error to
+  // the child, but we don't force the child to be terminated because
+  // the error is likely to be due to a bug in the extension.
+  *aTerminateSender = false;
+
+  /*
+   * Make sure there is a substitution installed for the host found
+   * in the child's request URI and make sure the host resolves to
+   * a directory.
+   */
+
+  nsAutoCString host;
+  MOZ_TRY(aChildURI->GetAsciiHost(host));
+
+  // Lookup the directory this host string resolves to
+  nsCOMPtr<nsIURI> baseURI;
+  MOZ_TRY(GetSubstitution(host, getter_AddRefs(baseURI)));
+
+  // The result should be a file URL for the extension base dir
+  nsCOMPtr<nsIFileURL> fileURL = do_QueryInterface(baseURI, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIFile> extensionDir;
+  MOZ_TRY(fileURL->GetFile(getter_AddRefs(extensionDir)));
+
+  bool isDirectory = false;
+  MOZ_TRY(extensionDir->IsDirectory(&isDirectory));
+  if (!isDirectory) {
+    // The host should map to a directory for unpacked extensions
+    return Err(NS_ERROR_FILE_NOT_DIRECTORY);
+  }
+
+  // Make sure the child URI resolves to a file URI then get a file
+  // channel for the request. The resultant channel should be a
+  // file channel because we only request remote streams for unpacked
+  // extension resource loads where the URI resolves to a file.
+  nsAutoCString resolvedSpec;
+  MOZ_TRY(ResolveURI(aChildURI, resolvedSpec));
+
+  nsAutoCString resolvedScheme;
+  MOZ_TRY(net_ExtractURLScheme(resolvedSpec, resolvedScheme));
+  if (!resolvedScheme.EqualsLiteral("file")) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  nsCOMPtr<nsIIOService> ioService = do_GetIOService(&rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIURI> resolvedURI;
+  MOZ_TRY(ioService->NewURI(resolvedSpec, nullptr, nullptr,
+                            getter_AddRefs(resolvedURI)));
+
+  // We use the system principal to get a file channel for the request,
+  // but only after we've checked (above) that the child URI is of
+  // moz-extension scheme and that the URI host maps to a directory.
+  nsCOMPtr<nsIChannel> channel;
+  MOZ_TRY(NS_NewChannel(getter_AddRefs(channel), resolvedURI,
+                        nsContentUtils::GetSystemPrincipal(),
+                        nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+                        nsIContentPolicy::TYPE_OTHER));
+
+  nsCOMPtr<nsIFileChannel> fileChannel = do_QueryInterface(channel, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIFile> requestedFile;
+  MOZ_TRY(fileChannel->GetFile(getter_AddRefs(requestedFile)));
+
+  /*
+   * Make sure the file we resolved to is within the extension directory.
+   */
+
+  // Normalize paths for sane comparisons. nsIFile::Contains depends on
+  // it for reliable subpath checks.
+  MOZ_TRY(extensionDir->Normalize());
+  MOZ_TRY(requestedFile->Normalize());
+#if defined(XP_WIN)
+  if (!widget::WinUtils::ResolveJunctionPointsAndSymLinks(extensionDir) ||
+      !widget::WinUtils::ResolveJunctionPointsAndSymLinks(requestedFile)) {
+    return Err(NS_ERROR_FILE_ACCESS_DENIED);
+  }
+#endif
+
+  bool isResourceFromExtensionDir = false;
+  MOZ_TRY(extensionDir->Contains(requestedFile, &isResourceFromExtensionDir));
+  if (!isResourceFromExtensionDir) {
+    bool isAllowed;
+    MOZ_TRY_VAR(isAllowed, AllowExternalResource(extensionDir, requestedFile));
+    if (!isAllowed) {
+      LogExternalResourceError(extensionDir, requestedFile);
+      return Err(NS_ERROR_FILE_ACCESS_DENIED);
+    }
+  }
+
+  nsCOMPtr<nsIInputStream> inputStream;
+  MOZ_TRY_VAR(inputStream,
+              NS_NewLocalFileInputStream(requestedFile, PR_RDONLY, -1,
+                                         nsIFileInputStream::DEFER_OPEN));
+
+  return inputStream;
+}
+
+Result<Ok, nsresult> ExtensionProtocolHandler::NewFD(
+    nsIURI* aChildURI, bool* aTerminateSender,
+    NeckoParent::GetExtensionFDResolver& aResolve) {
+  MOZ_ASSERT(!IsNeckoChild());
+  MOZ_TRY(aChildURI ? NS_OK : NS_ERROR_INVALID_ARG);
+  MOZ_TRY(aTerminateSender ? NS_OK : NS_ERROR_INVALID_ARG);
+
+  *aTerminateSender = true;
+  nsresult rv;
+
+  // Ensure this is a moz-extension URI
+  if (!aChildURI->SchemeIs(EXTENSION_SCHEME)) {
+    return Err(NS_ERROR_UNKNOWN_PROTOCOL);
+  }
+
+  // For errors after this point, we want to propagate the error to
+  // the child, but we don't force the child to be terminated.
+  *aTerminateSender = false;
+
+  nsAutoCString host;
+  MOZ_TRY(aChildURI->GetAsciiHost(host));
+
+  // We expect the host string to map to a JAR file because the URI
+  // should refer to a web accessible resource for an enabled extension.
+  nsCOMPtr<nsIURI> subURI;
+  MOZ_TRY(GetSubstitution(host, getter_AddRefs(subURI)));
+
+  nsCOMPtr<nsIJARURI> jarURI = do_QueryInterface(subURI, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIURI> innerFileURI;
+  MOZ_TRY(jarURI->GetJARFile(getter_AddRefs(innerFileURI)));
+
+  nsCOMPtr<nsIFileURL> innerFileURL = do_QueryInterface(innerFileURI, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIFile> jarFile;
+  MOZ_TRY(innerFileURL->GetFile(getter_AddRefs(jarFile)));
+
+  if (!mFileOpenerThread) {
+    mFileOpenerThread = new LazyIdleThread(DEFAULT_THREAD_TIMEOUT_MS,
+                                           "ExtensionProtocolHandler"_ns);
+  }
+
+  RefPtr<ExtensionJARFileOpener> fileOpener =
+      new ExtensionJARFileOpener(jarFile, aResolve);
+
+  nsCOMPtr<nsIRunnable> event = mozilla::NewRunnableMethod(
+      "ExtensionJarFileOpener", fileOpener, &ExtensionJARFileOpener::OpenFile);
+
+  MOZ_TRY(mFileOpenerThread->Dispatch(event, nsIEventTarget::DISPATCH_NORMAL));
+
+  return Ok();
+}
+
+// Set the channel's content type using the provided URI's type
+
+// static
+void ExtensionProtocolHandler::SetContentType(nsIURI* aURI,
+                                              nsIChannel* aChannel) {
+  nsresult rv;
+  nsCOMPtr<nsIMIMEService> mime = do_GetService("@mozilla.org/mime;1", &rv);
+  if (NS_SUCCEEDED(rv)) {
+    nsAutoCString contentType;
+    rv = mime->GetTypeFromURI(aURI, contentType);
+    if (NS_SUCCEEDED(rv)) {
+      Unused << aChannel->SetContentType(contentType);
+    }
+  }
+}
+
+// Gets a SimpleChannel that wraps the provided ExtensionStreamGetter
+
+// static
+void ExtensionProtocolHandler::NewSimpleChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadinfo, ExtensionStreamGetter* aStreamGetter,
+    nsIChannel** aRetVal) {
+  nsCOMPtr<nsIChannel> channel = NS_NewSimpleChannel(
+      aURI, aLoadinfo, aStreamGetter,
+      [](nsIStreamListener* listener, nsIChannel* simpleChannel,
+         ExtensionStreamGetter* getter) -> RequestOrReason {
+        return getter->GetAsync(listener, simpleChannel);
+      });
+
+  SetContentType(aURI, channel);
+  channel.swap(*aRetVal);
+}
+
+// Gets a SimpleChannel that wraps the provided channel
+
+// static
+void ExtensionProtocolHandler::NewSimpleChannel(nsIURI* aURI,
+                                                nsILoadInfo* aLoadinfo,
+                                                nsIChannel* aChannel,
+                                                nsIChannel** aRetVal) {
+  nsCOMPtr<nsIChannel> channel = NS_NewSimpleChannel(
+      aURI, aLoadinfo, aChannel,
+      [](nsIStreamListener* listener, nsIChannel* simpleChannel,
+         nsIChannel* origChannel) -> RequestOrReason {
+        nsresult rv = origChannel->AsyncOpen(listener);
+        if (NS_FAILED(rv)) {
+          simpleChannel->Cancel(NS_BINDING_ABORTED);
+          return Err(rv);
+        }
+        nsCOMPtr<nsIRequest> request(origChannel);
+        return RequestOrCancelable(WrapNotNull(request));
+      });
+
+  SetContentType(aURI, channel);
+  channel.swap(*aRetVal);
+}
+
+void ExtensionProtocolHandler::SubstituteRemoteFileChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadinfo, nsACString& aResolvedFileSpec,
+    nsIChannel** aRetVal) {
+  MOZ_ASSERT(IsNeckoChild());
+
+  RefPtr<ExtensionStreamGetter> streamGetter =
+      new ExtensionStreamGetter(aURI, aLoadinfo);
+
+  NewSimpleChannel(aURI, aLoadinfo, streamGetter, aRetVal);
+}
+
+static Result<Ok, nsresult> LogCacheCheck(const nsIJARChannel* aJarChannel,
+                                          nsIJARURI* aJarURI, bool aIsCached) {
+  nsresult rv;
+
+  nsCOMPtr<nsIURI> innerFileURI;
+  MOZ_TRY(aJarURI->GetJARFile(getter_AddRefs(innerFileURI)));
+
+  nsCOMPtr<nsIFileURL> innerFileURL = do_QueryInterface(innerFileURI, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIFile> jarFile;
+  MOZ_TRY(innerFileURL->GetFile(getter_AddRefs(jarFile)));
+
+  nsAutoCString uriSpec, jarSpec;
+  Unused << aJarURI->GetSpec(uriSpec);
+  Unused << innerFileURI->GetSpec(jarSpec);
+  LOG("[JARChannel %p] Cache %s: %s (%s)", aJarChannel,
+      aIsCached ? "hit" : "miss", uriSpec.get(), jarSpec.get());
+
+  return Ok();
+}
+
+Result<Ok, nsresult> ExtensionProtocolHandler::SubstituteRemoteJarChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadinfo, nsACString& aResolvedSpec,
+    nsIChannel** aRetVal) {
+  MOZ_ASSERT(IsNeckoChild());
+  nsresult rv;
+
+  // Build a JAR URI for this jar:file:// URI and use it to extract the
+  // inner file URI.
+  nsCOMPtr<nsIURI> uri;
+  MOZ_TRY(NS_NewURI(getter_AddRefs(uri), aResolvedSpec));
+
+  nsCOMPtr<nsIJARURI> jarURI = do_QueryInterface(uri, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIJARChannel> jarChannel = do_QueryInterface(*aRetVal, &rv);
+  MOZ_TRY(rv);
+
+  bool isCached = false;
+  MOZ_TRY(jarChannel->EnsureCached(&isCached));
+  if (MOZ_LOG_TEST(gExtProtocolLog, LogLevel::Debug)) {
+    Unused << LogCacheCheck(jarChannel, jarURI, isCached);
+  }
+
+  if (isCached) {
+    // Using a SimpleChannel with an ExtensionStreamGetter here (like the
+    // non-cached JAR case) isn't needed to load the extension resource
+    // because we don't need to ask the parent for an FD for the JAR, but
+    // wrapping the JARChannel in a SimpleChannel allows HTTP forwarding to
+    // moz-extension URI's to work because HTTP forwarding requires the
+    // target channel implement nsIChildChannel.
+    NewSimpleChannel(aURI, aLoadinfo, jarChannel.get(), aRetVal);
+    return Ok();
+  }
+
+  nsCOMPtr<nsIURI> innerFileURI;
+  MOZ_TRY(jarURI->GetJARFile(getter_AddRefs(innerFileURI)));
+
+  nsCOMPtr<nsIFileURL> innerFileURL = do_QueryInterface(innerFileURI, &rv);
+  MOZ_TRY(rv);
+
+  nsCOMPtr<nsIFile> jarFile;
+  MOZ_TRY(innerFileURL->GetFile(getter_AddRefs(jarFile)));
+
+  RefPtr<ExtensionStreamGetter> streamGetter =
+      new ExtensionStreamGetter(aURI, aLoadinfo, jarChannel.forget(), jarFile);
+
+  NewSimpleChannel(aURI, aLoadinfo, streamGetter, aRetVal);
+  return Ok();
+}
+
+}  // namespace net
+}  // namespace mozilla
diff --git a/netwerk/protocol/res/ExtensionProtocolHandler.h b/netwerk/protocol/res/ExtensionProtocolHandler.h
new file mode 100644
index 0000000000..0824d7dd99
--- /dev/null
+++ b/netwerk/protocol/res/ExtensionProtocolHandler.h
@@ -0,0 +1,236 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ExtensionProtocolHandler_h___
+#define ExtensionProtocolHandler_h___
+
+#include "mozilla/net/NeckoParent.h"
+#include "mozilla/LazyIdleThread.h"
+#include "mozilla/Result.h"
+#include "SubstitutingProtocolHandler.h"
+
+namespace mozilla {
+namespace net {
+
+class ExtensionStreamGetter;
+
+class ExtensionProtocolHandler final
+    : public nsISubstitutingProtocolHandler,
+      public nsIProtocolHandlerWithDynamicFlags,
+      public SubstitutingProtocolHandler,
+      public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIPROTOCOLHANDLERWITHDYNAMICFLAGS
+  NS_FORWARD_NSIPROTOCOLHANDLER(SubstitutingProtocolHandler::)
+  NS_FORWARD_NSISUBSTITUTINGPROTOCOLHANDLER(SubstitutingProtocolHandler::)
+
+  static already_AddRefed<ExtensionProtocolHandler> GetSingleton();
+
+  /**
+   * To be called in the parent process to obtain an input stream for a
+   * a web accessible resource from an unpacked WebExtension dir.
+   *
+   * @param aChildURI a moz-extension URI sent from the child that refers
+   *        to a web accessible resource file in an enabled unpacked extension
+   * @param aTerminateSender out param set to true when the params are invalid
+   *        and indicate the child should be terminated. If |aChildURI| is
+   *        not a moz-extension URI, the child is in an invalid state and
+   *        should be terminated.
+   * @return NS_OK with |aTerminateSender| set to false on success. On
+   *         failure, returns an error and sets |aTerminateSender| to indicate
+   *         whether or not the child process should be terminated.
+   *         A moz-extension URI from the child that doesn't resolve to a
+   *         resource file within the extension could be the result of a bug
+   *         in the extension and doesn't result in |aTerminateSender| being
+   *         set to true.
+   */
+  Result<nsCOMPtr<nsIInputStream>, nsresult> NewStream(nsIURI* aChildURI,
+                                                       bool* aTerminateSender);
+
+  /**
+   * To be called in the parent process to obtain a file descriptor for an
+   * enabled WebExtension JAR file.
+   *
+   * @param aChildURI a moz-extension URI sent from the child that refers
+   *        to a web accessible resource file in an enabled unpacked extension
+   * @param aTerminateSender out param set to true when the params are invalid
+   *        and indicate the child should be terminated. If |aChildURI| is
+   *        not a moz-extension URI, the child is in an invalid state and
+   *        should be terminated.
+   * @param aPromise a promise that will be resolved asynchronously when the
+   *        file descriptor is available.
+   * @return NS_OK with |aTerminateSender| set to false on success. On
+   *         failure, returns an error and sets |aTerminateSender| to indicate
+   *         whether or not the child process should be terminated.
+   *         A moz-extension URI from the child that doesn't resolve to an
+   *         enabled WebExtension JAR could be the result of a bug in the
+   *         extension and doesn't result in |aTerminateSender| being
+   *         set to true.
+   */
+  Result<Ok, nsresult> NewFD(nsIURI* aChildURI, bool* aTerminateSender,
+                             NeckoParent::GetExtensionFDResolver& aResolve);
+
+ protected:
+  ~ExtensionProtocolHandler() = default;
+
+ private:
+  explicit ExtensionProtocolHandler();
+
+  [[nodiscard]] bool ResolveSpecialCases(const nsACString& aHost,
+                                         const nsACString& aPath,
+                                         const nsACString& aPathname,
+                                         nsACString& aResult) override;
+
+  // |result| is an inout param.  On entry to this function, *result
+  // is expected to be non-null and already addrefed.  This function
+  // may release the object stored in *result on entry and write
+  // a new pointer to an already addrefed channel to *result.
+  [[nodiscard]] virtual nsresult SubstituteChannel(
+      nsIURI* uri, nsILoadInfo* aLoadInfo, nsIChannel** result) override;
+
+  /**
+   * For moz-extension URI's that resolve to file or JAR URI's, replaces
+   * the provided channel with a channel that will proxy the load to the
+   * parent process. For moz-extension URI's that resolve to other types
+   * of URI's (not file or JAR), the provide channel is not replaced and
+   * NS_OK is returned.
+   *
+   * @param aURI the moz-extension URI
+   * @param aLoadInfo the loadinfo for the request
+   * @param aRetVal in/out channel param referring to the channel that
+   *        might need to be substituted with a remote channel.
+   * @return NS_OK if the channel does not need to be substituted or
+   *         or the replacement channel was created successfully.
+   *         Otherwise returns an error.
+   */
+  Result<Ok, nsresult> SubstituteRemoteChannel(nsIURI* aURI,
+                                               nsILoadInfo* aLoadInfo,
+                                               nsIChannel** aRetVal);
+
+  /**
+   * Replaces a file channel with a remote file channel for loading a
+   * web accessible resource for an unpacked extension from the parent.
+   *
+   * @param aURI the moz-extension URI
+   * @param aLoadInfo the loadinfo for the request
+   * @param aResolvedFileSpec the resolved URI spec for the file.
+   * @param aRetVal in/out param referring to the new remote channel.
+   *        The reference to the input param file channel is dropped and
+   *        replaced with a reference to a new channel that remotes
+   *        the file access. The new channel encapsulates a request to
+   *        the parent for an IPCStream for the file.
+   */
+  void SubstituteRemoteFileChannel(nsIURI* aURI, nsILoadInfo* aLoadinfo,
+                                   nsACString& aResolvedFileSpec,
+                                   nsIChannel** aRetVal);
+
+  /**
+   * Replaces a JAR channel with a remote JAR channel for loading a
+   * an extension JAR file from the parent.
+   *
+   * @param aURI the moz-extension URI
+   * @param aLoadInfo the loadinfo for the request
+   * @param aResolvedFileSpec the resolved URI spec for the file.
+   * @param aRetVal in/out param referring to the new remote channel.
+   *        The input param JAR channel is replaced with a new channel
+   *        that remotes the JAR file access. The new channel encapsulates
+   *        a request to the parent for the JAR file FD.
+   */
+  Result<Ok, nsresult> SubstituteRemoteJarChannel(nsIURI* aURI,
+                                                  nsILoadInfo* aLoadinfo,
+                                                  nsACString& aResolvedSpec,
+                                                  nsIChannel** aRetVal);
+
+  /**
+   * Sets the aResult outparam to true if this unpacked extension load of
+   * a resource that is outside the extension dir should be allowed. This
+   * is only allowed for system extensions on Mac and Linux dev builds.
+   *
+   * @param aExtensionDir the extension directory. Argument must be an
+   *        nsIFile for which Normalize() has already been called.
+   * @param aRequestedFile the requested web-accessible resource file. Argument
+   *        must be an nsIFile for which Normalize() has already been called.
+   */
+  Result<bool, nsresult> AllowExternalResource(nsIFile* aExtensionDir,
+                                               nsIFile* aRequestedFile);
+
+  // Set the channel's content type using the provided URI's type
+  static void SetContentType(nsIURI* aURI, nsIChannel* aChannel);
+
+  // Gets a SimpleChannel that wraps the provided ExtensionStreamGetter
+  static void NewSimpleChannel(nsIURI* aURI, nsILoadInfo* aLoadinfo,
+                               ExtensionStreamGetter* aStreamGetter,
+                               nsIChannel** aRetVal);
+
+  // Gets a SimpleChannel that wraps the provided channel
+  static void NewSimpleChannel(nsIURI* aURI, nsILoadInfo* aLoadinfo,
+                               nsIChannel* aChannel, nsIChannel** aRetVal);
+
+#if defined(XP_MACOSX)
+  /**
+   * Sets the aResult outparam to true if we are a developer build with the
+   * repo dir environment variable set and the requested file resides in the
+   * repo dir. Developer builds may load system extensions with web-accessible
+   * resources that are symlinks to files in the repo dir. This method is for
+   * checking if an unpacked resource requested by the child is from the repo.
+   * The requested file must be already Normalized(). Only compile this for
+   * Mac because the repo dir isn't always available on Linux.
+   *
+   * @param aRequestedFile the requested web-accessible resource file. Argument
+   *        must be an nsIFile for which Normalize() has already been called.
+   */
+  Result<bool, nsresult> DevRepoContains(nsIFile* aRequestedFile);
+
+  // On development builds, this points to development repo. Lazily set.
+  nsCOMPtr<nsIFile> mDevRepo;
+
+  // Set to true once we've already tried to load the dev repo path,
+  // allowing for lazy initialization of |mDevRepo|.
+  bool mAlreadyCheckedDevRepo{false};
+#endif /* XP_MACOSX */
+
+#if !defined(XP_WIN)
+  /**
+   * Sets the aResult outparam to true if we are a developer build and the
+   * provided directory is within the NS_GRE_DIR directory. Developer builds
+   * may load system extensions with web-accessible resources that are symlinks
+   * to files outside of the extension dir to the repo dir. This method is for
+   * checking if an extension directory is within NS_GRE_DIR. In that case, we
+   * consider the extension a system extension and allow it to use symlinks to
+   * resources outside of the extension dir. This exception is only applied
+   * to loads for unpacked extensions in unpackaged developer builds.
+   * The requested dir must be already Normalized().
+   *
+   * @param aExtensionDir the extension directory. Argument must be an
+   *        nsIFile for which Normalize() has already been called.
+   */
+  Result<bool, nsresult> AppDirContains(nsIFile* aExtensionDir);
+
+  // On development builds, cache the NS_GRE_DIR repo. Lazily set.
+  nsCOMPtr<nsIFile> mAppDir;
+
+  // Set to true once we've already read the AppDir, allowing for lazy
+  // initialization of |mAppDir|.
+  bool mAlreadyCheckedAppDir{false};
+#endif /* !defined(XP_WIN) */
+
+  // Used for opening JAR files off the main thread when we just need to
+  // obtain a file descriptor to send back to the child.
+  RefPtr<mozilla::LazyIdleThread> mFileOpenerThread;
+
+  // To allow parent IPDL actors to invoke methods on this handler when
+  // handling moz-extension requests from the child.
+  static StaticRefPtr<ExtensionProtocolHandler> sSingleton;
+
+  // Set to true when this instance of the handler must proxy loads of
+  // extension web-accessible resources to the parent process.
+  bool mUseRemoteFileChannels;
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* ExtensionProtocolHandler_h___ */
diff --git a/netwerk/protocol/res/PageThumbProtocolHandler.cpp b/netwerk/protocol/res/PageThumbProtocolHandler.cpp
new file mode 100644
index 0000000000..065fd7c36e
--- /dev/null
+++ b/netwerk/protocol/res/PageThumbProtocolHandler.cpp
@@ -0,0 +1,360 @@
+/* -*- 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 "PageThumbProtocolHandler.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/ipc/URIParams.h"
+#include "mozilla/ipc/URIUtils.h"
+#include "mozilla/net/NeckoChild.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ResultExtensions.h"
+
+#include "LoadInfo.h"
+#include "nsContentUtils.h"
+#include "nsServiceManagerUtils.h"
+#include "nsIFile.h"
+#include "nsIFileChannel.h"
+#include "nsIFileStreams.h"
+#include "nsIMIMEService.h"
+#include "nsIURL.h"
+#include "nsIChannel.h"
+#include "nsIPageThumbsStorageService.h"
+#include "nsIInputStreamPump.h"
+#include "nsIStreamListener.h"
+#include "nsIInputStream.h"
+#include "nsNetUtil.h"
+#include "nsURLHelper.h"
+#include "prio.h"
+#include "SimpleChannel.h"
+#include "nsICancelable.h"
+
+#ifdef MOZ_PLACES
+#  include "nsIPlacesPreviewsHelperService.h"
+#endif
+
+#define PAGE_THUMB_HOST "thumbnails"
+#define PLACES_PREVIEWS_HOST "places-previews"
+#define PAGE_THUMB_SCHEME "moz-page-thumb"
+
+namespace mozilla {
+namespace net {
+
+LazyLogModule gPageThumbProtocolLog("PageThumbProtocol");
+
+#undef LOG
+#define LOG(level, ...) \
+  MOZ_LOG(gPageThumbProtocolLog, LogLevel::level, (__VA_ARGS__))
+
+StaticRefPtr<PageThumbProtocolHandler> PageThumbProtocolHandler::sSingleton;
+
+NS_IMPL_QUERY_INTERFACE(PageThumbProtocolHandler,
+                        nsISubstitutingProtocolHandler, nsIProtocolHandler,
+                        nsISupportsWeakReference)
+NS_IMPL_ADDREF_INHERITED(PageThumbProtocolHandler, SubstitutingProtocolHandler)
+NS_IMPL_RELEASE_INHERITED(PageThumbProtocolHandler, SubstitutingProtocolHandler)
+
+already_AddRefed<PageThumbProtocolHandler>
+PageThumbProtocolHandler::GetSingleton() {
+  if (!sSingleton) {
+    sSingleton = new PageThumbProtocolHandler();
+    ClearOnShutdown(&sSingleton);
+  }
+
+  return do_AddRef(sSingleton);
+}
+
+// A moz-page-thumb URI is only loadable by chrome pages in the parent process,
+// or privileged content running in the privileged about content process.
+PageThumbProtocolHandler::PageThumbProtocolHandler()
+    : SubstitutingProtocolHandler(PAGE_THUMB_SCHEME) {}
+
+RefPtr<RemoteStreamPromise> PageThumbProtocolHandler::NewStream(
+    nsIURI* aChildURI, bool* aTerminateSender) {
+  MOZ_ASSERT(!IsNeckoChild());
+  MOZ_ASSERT(NS_IsMainThread());
+
+  if (!aChildURI || !aTerminateSender) {
+    return RemoteStreamPromise::CreateAndReject(NS_ERROR_INVALID_ARG, __func__);
+  }
+
+  *aTerminateSender = true;
+  nsresult rv;
+
+  // We should never receive a URI that isn't for a moz-page-thumb because
+  // these requests ordinarily come from the child's PageThumbProtocolHandler.
+  // Ensure this request is for a moz-page-thumb URI. A compromised child
+  // process could send us any URI.
+  bool isPageThumbScheme = false;
+  if (NS_FAILED(aChildURI->SchemeIs(PAGE_THUMB_SCHEME, &isPageThumbScheme)) ||
+      !isPageThumbScheme) {
+    return RemoteStreamPromise::CreateAndReject(NS_ERROR_UNKNOWN_PROTOCOL,
+                                                __func__);
+  }
+
+  // We should never receive a URI that does not have "thumbnails" as the host.
+  nsAutoCString host;
+  if (NS_FAILED(aChildURI->GetAsciiHost(host)) ||
+      !(host.EqualsLiteral(PAGE_THUMB_HOST) ||
+        host.EqualsLiteral(PLACES_PREVIEWS_HOST))) {
+    return RemoteStreamPromise::CreateAndReject(NS_ERROR_UNEXPECTED, __func__);
+  }
+
+  // For errors after this point, we want to propagate the error to
+  // the child, but we don't force the child process to be terminated.
+  *aTerminateSender = false;
+
+  // Make sure the child URI resolves to a file URI. We will then get a file
+  // channel for the request. The resultant channel should be a file channel
+  // because we only request remote streams for resource loads where the URI
+  // resolves to a file.
+  nsAutoCString resolvedSpec;
+  rv = ResolveURI(aChildURI, resolvedSpec);
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  nsAutoCString resolvedScheme;
+  rv = net_ExtractURLScheme(resolvedSpec, resolvedScheme);
+  if (NS_FAILED(rv) || !resolvedScheme.EqualsLiteral("file")) {
+    return RemoteStreamPromise::CreateAndReject(NS_ERROR_UNEXPECTED, __func__);
+  }
+
+  nsCOMPtr<nsIIOService> ioService = do_GetIOService(&rv);
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  nsCOMPtr<nsIURI> resolvedURI;
+  rv = ioService->NewURI(resolvedSpec, nullptr, nullptr,
+                         getter_AddRefs(resolvedURI));
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  // We use the system principal to get a file channel for the request,
+  // but only after we've checked (above) that the child URI is of
+  // moz-page-thumb scheme and that the URI host matches PAGE_THUMB_HOST.
+  nsCOMPtr<nsIChannel> channel;
+  rv = NS_NewChannel(getter_AddRefs(channel), resolvedURI,
+                     nsContentUtils::GetSystemPrincipal(),
+                     nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+                     nsIContentPolicy::TYPE_OTHER);
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  auto promiseHolder = MakeUnique<MozPromiseHolder<RemoteStreamPromise>>();
+  RefPtr<RemoteStreamPromise> promise = promiseHolder->Ensure(__func__);
+
+  nsCOMPtr<nsIMIMEService> mime = do_GetService("@mozilla.org/mime;1", &rv);
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  nsAutoCString contentType;
+  rv = mime->GetTypeFromURI(aChildURI, contentType);
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  rv = NS_DispatchBackgroundTask(
+      NS_NewRunnableFunction(
+          "PageThumbProtocolHandler::NewStream",
+          [contentType, channel, holder = std::move(promiseHolder)]() {
+            nsresult rv;
+
+            nsCOMPtr<nsIFileChannel> fileChannel =
+                do_QueryInterface(channel, &rv);
+            if (NS_FAILED(rv)) {
+              holder->Reject(rv, __func__);
+              return;
+            }
+
+            nsCOMPtr<nsIFile> requestedFile;
+            rv = fileChannel->GetFile(getter_AddRefs(requestedFile));
+            if (NS_FAILED(rv)) {
+              holder->Reject(rv, __func__);
+              return;
+            }
+
+            nsCOMPtr<nsIInputStream> inputStream;
+            rv = NS_NewLocalFileInputStream(getter_AddRefs(inputStream),
+                                            requestedFile, PR_RDONLY, -1);
+            if (NS_FAILED(rv)) {
+              holder->Reject(rv, __func__);
+              return;
+            }
+
+            RemoteStreamInfo info(inputStream, contentType, -1);
+
+            holder->Resolve(std::move(info), __func__);
+          }),
+      NS_DISPATCH_EVENT_MAY_BLOCK);
+
+  if (NS_FAILED(rv)) {
+    return RemoteStreamPromise::CreateAndReject(rv, __func__);
+  }
+
+  return promise;
+}
+
+bool PageThumbProtocolHandler::ResolveSpecialCases(const nsACString& aHost,
+                                                   const nsACString& aPath,
+                                                   const nsACString& aPathname,
+                                                   nsACString& aResult) {
+  // This should match the scheme in PageThumbs.jsm. We will only resolve
+  // URIs for thumbnails generated by PageThumbs here.
+  if (!aHost.EqualsLiteral(PAGE_THUMB_HOST) &&
+      !aHost.EqualsLiteral(PLACES_PREVIEWS_HOST)) {
+    // moz-page-thumb should always have a "thumbnails" host. We do not intend
+    // to allow substitution rules to be created for moz-page-thumb.
+    return false;
+  }
+
+  // Regardless of the outcome, the scheme will be resolved to file://.
+  aResult.Assign("file://");
+
+  if (IsNeckoChild()) {
+    // We will resolve the URI in the parent if load is performed in the child
+    // because the child does not have access to the profile directory path.
+    // Technically we could retrieve the path from dom::ContentChild, but I
+    // would prefer to obtain the path from PageThumbsStorageService (which
+    // depends on OS.Path). Here, we resolve to the same URI, with the file://
+    // scheme. This won't ever be accessed directly by the content process,
+    // and is mainly used to keep the substitution protocol handler mechanism
+    // happy.
+    aResult.Append(aHost);
+    aResult.Append(aPath);
+  } else {
+    // Resolve the URI in the parent to the thumbnail file URI since we will
+    // attempt to open the channel to load the file after this.
+    nsAutoString thumbnailUrl;
+    nsresult rv = GetThumbnailPath(aPath, aHost, thumbnailUrl);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return false;
+    }
+
+    aResult.Append(NS_ConvertUTF16toUTF8(thumbnailUrl));
+  }
+
+  return true;
+}
+
+nsresult PageThumbProtocolHandler::SubstituteChannel(nsIURI* aURI,
+                                                     nsILoadInfo* aLoadInfo,
+                                                     nsIChannel** aRetVal) {
+  // Check if URI resolves to a file URI.
+  nsAutoCString resolvedSpec;
+  MOZ_TRY(ResolveURI(aURI, resolvedSpec));
+
+  nsAutoCString scheme;
+  MOZ_TRY(net_ExtractURLScheme(resolvedSpec, scheme));
+
+  if (!scheme.EqualsLiteral("file")) {
+    NS_WARNING("moz-page-thumb URIs should only resolve to file URIs.");
+    return NS_ERROR_NO_INTERFACE;
+  }
+
+  // Load the URI remotely if accessed from a child.
+  if (IsNeckoChild()) {
+    MOZ_TRY(SubstituteRemoteChannel(aURI, aLoadInfo, aRetVal));
+  }
+
+  return NS_OK;
+}
+
+Result<Ok, nsresult> PageThumbProtocolHandler::SubstituteRemoteChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadInfo, nsIChannel** aRetVal) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_TRY(aURI ? NS_OK : NS_ERROR_INVALID_ARG);
+  MOZ_TRY(aLoadInfo ? NS_OK : NS_ERROR_INVALID_ARG);
+
+#ifdef DEBUG
+  nsAutoCString resolvedSpec;
+  MOZ_TRY(ResolveURI(aURI, resolvedSpec));
+
+  nsAutoCString scheme;
+  MOZ_TRY(net_ExtractURLScheme(resolvedSpec, scheme));
+
+  MOZ_ASSERT(scheme.EqualsLiteral("file"));
+#endif /* DEBUG */
+
+  RefPtr<RemoteStreamGetter> streamGetter =
+      new RemoteStreamGetter(aURI, aLoadInfo);
+
+  NewSimpleChannel(aURI, aLoadInfo, streamGetter, aRetVal);
+  return Ok();
+}
+
+nsresult PageThumbProtocolHandler::GetThumbnailPath(const nsACString& aPath,
+                                                    const nsACString& aHost,
+                                                    nsString& aThumbnailPath) {
+  MOZ_ASSERT(!IsNeckoChild());
+
+  // Ensures that the provided path has a query string. We will start parsing
+  // from there.
+  int32_t queryIndex = aPath.FindChar('?');
+  if (queryIndex <= 0) {
+    return NS_ERROR_MALFORMED_URI;
+  }
+
+  // Extract URL from query string.
+  nsAutoString url;
+  bool found =
+      URLParams::Extract(Substring(aPath, queryIndex + 1), u"url"_ns, url);
+  if (!found || url.IsVoid()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  nsresult rv;
+  if (aHost.EqualsLiteral(PAGE_THUMB_HOST)) {
+    nsCOMPtr<nsIPageThumbsStorageService> pageThumbsStorage =
+        do_GetService("@mozilla.org/thumbnails/pagethumbs-service;1", &rv);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+    // Use PageThumbsStorageService to get the local file path of the screenshot
+    // for the given URL.
+    rv = pageThumbsStorage->GetFilePathForURL(url, aThumbnailPath);
+#ifdef MOZ_PLACES
+  } else if (aHost.EqualsLiteral(PLACES_PREVIEWS_HOST)) {
+    nsCOMPtr<nsIPlacesPreviewsHelperService> helper =
+        do_GetService("@mozilla.org/places/previews-helper;1", &rv);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+    rv = helper->GetFilePathForURL(url, aThumbnailPath);
+#endif
+  } else {
+    MOZ_ASSERT_UNREACHABLE("Unknown thumbnail host");
+    return NS_ERROR_UNEXPECTED;
+  }
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+  return NS_OK;
+}
+
+// static
+void PageThumbProtocolHandler::NewSimpleChannel(
+    nsIURI* aURI, nsILoadInfo* aLoadinfo, RemoteStreamGetter* aStreamGetter,
+    nsIChannel** aRetVal) {
+  nsCOMPtr<nsIChannel> channel = NS_NewSimpleChannel(
+      aURI, aLoadinfo, aStreamGetter,
+      [](nsIStreamListener* listener, nsIChannel* simpleChannel,
+         RemoteStreamGetter* getter) -> RequestOrReason {
+        return getter->GetAsync(listener, simpleChannel,
+                                &NeckoChild::SendGetPageThumbStream);
+      });
+
+  channel.swap(*aRetVal);
+}
+
+#undef LOG
+
+}  // namespace net
+}  // namespace mozilla
diff --git a/netwerk/protocol/res/PageThumbProtocolHandler.h b/netwerk/protocol/res/PageThumbProtocolHandler.h
new file mode 100644
index 0000000000..da670d52d4
--- /dev/null
+++ b/netwerk/protocol/res/PageThumbProtocolHandler.h
@@ -0,0 +1,120 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 PageThumbProtocolHandler_h___
+#define PageThumbProtocolHandler_h___
+
+#include "mozilla/Result.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/net/RemoteStreamGetter.h"
+#include "SubstitutingProtocolHandler.h"
+#include "nsIInputStream.h"
+#include "nsWeakReference.h"
+
+namespace mozilla {
+namespace net {
+
+class RemoteStreamGetter;
+
+class PageThumbProtocolHandler final : public nsISubstitutingProtocolHandler,
+                                       public SubstitutingProtocolHandler,
+                                       public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_FORWARD_NSIPROTOCOLHANDLER(SubstitutingProtocolHandler::)
+  NS_FORWARD_NSISUBSTITUTINGPROTOCOLHANDLER(SubstitutingProtocolHandler::)
+
+  static already_AddRefed<PageThumbProtocolHandler> GetSingleton();
+
+  /**
+   * To be called in the parent process to obtain an input stream for the
+   * given thumbnail.
+   *
+   * @param aChildURI a moz-page-thumb URI sent from the child.
+   * @param aTerminateSender out param set to true when the params are invalid
+   *        and indicate the child should be terminated. If |aChildURI| is
+   *        not a moz-page-thumb URI, the child is in an invalid state and
+   *        should be terminated. This outparam will be set synchronously.
+   *
+   * @return RemoteStreamPromise
+   *        The RemoteStreamPromise will resolve with an RemoteStreamInfo on
+   *        success, and reject with an nsresult on failure.
+   */
+  RefPtr<RemoteStreamPromise> NewStream(nsIURI* aChildURI,
+                                        bool* aTerminateSender);
+
+ protected:
+  ~PageThumbProtocolHandler() = default;
+
+ private:
+  explicit PageThumbProtocolHandler();
+
+  [[nodiscard]] bool ResolveSpecialCases(const nsACString& aHost,
+                                         const nsACString& aPath,
+                                         const nsACString& aPathname,
+                                         nsACString& aResult) override;
+
+  /**
+   * On entry to this function, *aRetVal is expected to be non-null and already
+   * addrefed. This function may release the object stored in *aRetVal on entry
+   * and write a new pointer to an already addrefed channel to *aRetVal.
+   *
+   * @param aURI the moz-page-thumb URI.
+   * @param aLoadInfo the loadinfo for the request.
+   * @param aRetVal in/out channel param referring to the channel that
+   *        might need to be substituted with a remote channel.
+   * @return NS_OK if channel has been substituted successfully or no
+   *         substitution at all. Otherwise, returns an error. This function
+   *         will return NS_ERROR_NO_INTERFACE if the URI resolves to a
+   *         non file:// URI.
+   */
+  [[nodiscard]] virtual nsresult SubstituteChannel(
+      nsIURI* aURI, nsILoadInfo* aLoadInfo, nsIChannel** aRetVal) override;
+
+  /**
+   * This replaces the provided channel with a channel that will proxy the load
+   * to the parent process.
+   *
+   * @param aURI the moz-page-thumb URI.
+   * @param aLoadInfo the loadinfo for the request.
+   * @param aRetVal in/out channel param referring to the channel that
+   *        might need to be substituted with a remote channel.
+   * @return NS_OK if the replacement channel was created successfully.
+   *         Otherwise, returns an error.
+   */
+  Result<Ok, nsresult> SubstituteRemoteChannel(nsIURI* aURI,
+                                               nsILoadInfo* aLoadInfo,
+                                               nsIChannel** aRetVal);
+
+  /*
+   * Extracts the URL from the query string in the given moz-page-thumb URI
+   * and queries PageThumbsStorageService using the extracted URL to obtain
+   * the local file path of the screenshot. This should only be called from
+   * the parent because PageThumbsStorageService relies on the path of the
+   * profile directory, which is unavailable in the child.
+   *
+   * @param aPath the path of the moz-page-thumb URI.
+   * @param aHost the host of the moz-page-thumb URI.
+   * @param aThumbnailPath in/out string param referring to the thumbnail path.
+   * @return NS_OK if the thumbnail path was obtained successfully. Otherwise
+   *         returns an error.
+   */
+  nsresult GetThumbnailPath(const nsACString& aPath, const nsACString& aHost,
+                            nsString& aThumbnailPath);
+
+  // To allow parent IPDL actors to invoke methods on this handler when
+  // handling moz-page-thumb requests from the child.
+  static StaticRefPtr<PageThumbProtocolHandler> sSingleton;
+
+  // Gets a SimpleChannel that wraps the provided channel.
+  static void NewSimpleChannel(nsIURI* aURI, nsILoadInfo* aLoadinfo,
+                               RemoteStreamGetter* aStreamGetter,
+                               nsIChannel** aRetVal);
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* PageThumbProtocolHandler_h___ */
diff --git a/netwerk/protocol/res/RemoteStreamGetter.cpp b/netwerk/protocol/res/RemoteStreamGetter.cpp
new file mode 100644
index 0000000000..e5c218b3c0
--- /dev/null
+++ b/netwerk/protocol/res/RemoteStreamGetter.cpp
@@ -0,0 +1,138 @@
+/* -*- 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 "RemoteStreamGetter.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/net/NeckoChild.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ResultExtensions.h"
+#include "nsContentUtils.h"
+#include "nsIInputStreamPump.h"
+
+namespace mozilla {
+namespace net {
+
+NS_IMPL_ISUPPORTS(RemoteStreamGetter, nsICancelable)
+
+RemoteStreamGetter::RemoteStreamGetter(nsIURI* aURI, nsILoadInfo* aLoadInfo)
+    : mURI(aURI), mLoadInfo(aLoadInfo) {
+  MOZ_ASSERT(aURI);
+  MOZ_ASSERT(aLoadInfo);
+}
+
+// Request an input stream from the parent.
+RequestOrReason RemoteStreamGetter::GetAsync(nsIStreamListener* aListener,
+                                             nsIChannel* aChannel,
+                                             Method aMethod) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_ASSERT(aMethod);
+
+  mListener = aListener;
+  mChannel = aChannel;
+
+  nsCOMPtr<nsICancelable> cancelableRequest(this);
+
+  RefPtr<RemoteStreamGetter> self = this;
+  Maybe<LoadInfoArgs> loadInfoArgs;
+  nsresult rv = ipc::LoadInfoToLoadInfoArgs(mLoadInfo, &loadInfoArgs);
+  if (NS_FAILED(rv)) {
+    return Err(rv);
+  }
+
+  (gNeckoChild->*aMethod)(mURI, loadInfoArgs)
+      ->Then(
+          GetMainThreadSerialEventTarget(), __func__,
+          [self](const Maybe<RemoteStreamInfo>& info) { self->OnStream(info); },
+          [self](const mozilla::ipc::ResponseRejectReason) {
+            self->OnStream(Nothing());
+          });
+  return RequestOrCancelable(WrapNotNull(cancelableRequest));
+}
+
+// Called to cancel the ongoing async request.
+NS_IMETHODIMP
+RemoteStreamGetter::Cancel(nsresult aStatus) {
+  if (mCanceled) {
+    return NS_OK;
+  }
+
+  mCanceled = true;
+  mStatus = aStatus;
+
+  if (mPump) {
+    mPump->Cancel(aStatus);
+    mPump = nullptr;
+  }
+
+  return NS_OK;
+}
+
+// static
+void RemoteStreamGetter::CancelRequest(nsIStreamListener* aListener,
+                                       nsIChannel* aChannel, nsresult aResult) {
+  MOZ_ASSERT(aListener);
+  MOZ_ASSERT(aChannel);
+
+  aListener->OnStartRequest(aChannel);
+  aListener->OnStopRequest(aChannel, aResult);
+  aChannel->CancelWithReason(NS_BINDING_ABORTED,
+                             "RemoteStreamGetter::CancelRequest"_ns);
+}
+
+// Handle an input stream sent from the parent.
+void RemoteStreamGetter::OnStream(const Maybe<RemoteStreamInfo>& aStreamInfo) {
+  MOZ_ASSERT(IsNeckoChild());
+  MOZ_ASSERT(mChannel);
+  MOZ_ASSERT(mListener);
+
+  nsCOMPtr<nsIChannel> channel = std::move(mChannel);
+
+  // We must keep an owning reference to the listener until we pass it on
+  // to AsyncRead.
+  nsCOMPtr<nsIStreamListener> listener = mListener.forget();
+
+  if (aStreamInfo.isNothing()) {
+    // The parent didn't send us back a stream.
+    CancelRequest(listener, channel, NS_ERROR_FILE_ACCESS_DENIED);
+    return;
+  }
+
+  if (mCanceled) {
+    // The channel that has created this stream getter has been canceled.
+    CancelRequest(listener, channel, mStatus);
+    return;
+  }
+
+  nsCOMPtr<nsIInputStream> stream = std::move(aStreamInfo.ref().inputStream());
+  if (!stream) {
+    // We somehow failed to get a stream, so just cancel the request.
+    CancelRequest(listener, channel, mStatus);
+    return;
+  }
+
+  nsCOMPtr<nsIInputStreamPump> pump;
+  nsresult rv =
+      NS_NewInputStreamPump(getter_AddRefs(pump), stream.forget(), 0, 0, false,
+                            GetMainThreadSerialEventTarget());
+  if (NS_FAILED(rv)) {
+    CancelRequest(listener, channel, rv);
+    return;
+  }
+
+  channel->SetContentType(aStreamInfo.ref().contentType());
+  channel->SetContentLength(aStreamInfo.ref().contentLength());
+
+  rv = pump->AsyncRead(listener);
+  if (NS_FAILED(rv)) {
+    CancelRequest(listener, channel, rv);
+    return;
+  }
+
+  mPump = pump;
+}
+
+}  // namespace net
+}  // namespace mozilla
diff --git a/netwerk/protocol/res/RemoteStreamGetter.h b/netwerk/protocol/res/RemoteStreamGetter.h
new file mode 100644
index 0000000000..5a4dae43a4
--- /dev/null
+++ b/netwerk/protocol/res/RemoteStreamGetter.h
@@ -0,0 +1,68 @@
+/* -*- 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 RemoteStreamGetter_h___
+#define RemoteStreamGetter_h___
+
+#include "nsIChannel.h"
+#include "nsIInputStreamPump.h"
+#include "nsIStreamListener.h"
+#include "nsIInputStream.h"
+#include "nsICancelable.h"
+#include "SimpleChannel.h"
+#include "mozilla/net/NeckoChannelParams.h"
+#include "mozilla/net/NeckoChild.h"
+#include "mozilla/Maybe.h"
+
+class nsILoadInfo;
+
+namespace mozilla {
+namespace net {
+
+using RemoteStreamPromise =
+    mozilla::MozPromise<RemoteStreamInfo, nsresult, false>;
+using Method = RefPtr<
+    MozPromise<Maybe<RemoteStreamInfo>, ipc::ResponseRejectReason, true>> (
+    PNeckoChild::*)(nsIURI*, const Maybe<LoadInfoArgs>&);
+
+/**
+ * Helper class used with SimpleChannel to asynchronously obtain an input
+ * stream and metadata from the parent for a remote protocol load from the
+ * child.
+ */
+class RemoteStreamGetter final : public nsICancelable {
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSICANCELABLE
+
+ public:
+  RemoteStreamGetter(nsIURI* aURI, nsILoadInfo* aLoadInfo);
+
+  // Get an input stream from the parent asynchronously.
+  RequestOrReason GetAsync(nsIStreamListener* aListener, nsIChannel* aChannel,
+                           Method aMethod);
+
+  // Handle an input stream being returned from the parent
+  void OnStream(const Maybe<RemoteStreamInfo>& aStreamInfo);
+
+  static void CancelRequest(nsIStreamListener* aListener, nsIChannel* aChannel,
+                            nsresult aResult);
+
+ private:
+  ~RemoteStreamGetter() = default;
+
+  nsCOMPtr<nsIURI> mURI;
+  nsCOMPtr<nsILoadInfo> mLoadInfo;
+  nsCOMPtr<nsIStreamListener> mListener;
+  nsCOMPtr<nsIChannel> mChannel;
+  nsCOMPtr<nsIInputStreamPump> mPump;
+  bool mCanceled{false};
+  nsresult mStatus{NS_OK};
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* RemoteStreamGetter_h___ */
diff --git a/netwerk/protocol/res/SubstitutingJARURI.h b/netwerk/protocol/res/SubstitutingJARURI.h
new file mode 100644
index 0000000000..fe9208750c
--- /dev/null
+++ b/netwerk/protocol/res/SubstitutingJARURI.h
@@ -0,0 +1,161 @@
+/* -*- 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 SubstitutingJARURI_h
+#define SubstitutingJARURI_h
+
+#include "nsIURL.h"
+#include "nsJARURI.h"
+#include "nsISerializable.h"
+
+namespace mozilla {
+namespace net {
+
+#define NS_SUBSTITUTINGJARURI_IMPL_CID               \
+  { /* 8f8c54ed-aba7-4ebf-ba6f-e58aec0aba4c */       \
+    0x8f8c54ed, 0xaba7, 0x4ebf, {                    \
+      0xba, 0x6f, 0xe5, 0x8a, 0xec, 0x0a, 0xba, 0x4c \
+    }                                                \
+  }
+
+// Provides a Substituting URI for resource://-like substitutions which
+// allows consumers to access the underlying jar resource.
+class SubstitutingJARURI : public nsIJARURI,
+                           public nsIStandardURL,
+                           public nsISerializable {
+ protected:
+  // Contains the resource://-like URI to be mapped. nsIURI and nsIURL will
+  // forward to this.
+  nsCOMPtr<nsIURL> mSource;
+  // Contains the resolved jar resource, nsIJARURI forwards to this to let
+  // consumer acccess the underlying jar resource.
+  nsCOMPtr<nsIJARURI> mResolved;
+  virtual ~SubstitutingJARURI() = default;
+
+ public:
+  SubstitutingJARURI(nsIURL* source, nsIJARURI* resolved);
+  // For deserializing
+  explicit SubstitutingJARURI() = default;
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSISERIALIZABLE
+
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_SUBSTITUTINGJARURI_IMPL_CID)
+
+  NS_FORWARD_SAFE_NSIURL(mSource)
+  NS_FORWARD_SAFE_NSIJARURI(mResolved)
+
+  // enum used in a few places to specify how .ref attribute should be handled
+  enum RefHandlingEnum { eIgnoreRef, eHonorRef };
+
+  // We cannot forward Equals* methods to |mSource| so we override them here
+  NS_IMETHOD EqualsExceptRef(nsIURI* aOther, bool* aResult) override;
+  NS_IMETHOD Equals(nsIURI* aOther, bool* aResult) override;
+
+  // Helper to share code between Equals methods.
+  virtual nsresult EqualsInternal(nsIURI* aOther,
+                                  RefHandlingEnum aRefHandlingMode,
+                                  bool* aResult);
+
+  // Forward the rest of nsIURI to mSource
+  NS_IMETHOD GetSpec(nsACString& aSpec) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetSpec(aSpec);
+  }
+  NS_IMETHOD GetPrePath(nsACString& aPrePath) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetPrePath(aPrePath);
+  }
+  NS_IMETHOD GetScheme(nsACString& aScheme) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetScheme(aScheme);
+  }
+  NS_IMETHOD GetUserPass(nsACString& aUserPass) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetUserPass(aUserPass);
+  }
+  NS_IMETHOD GetUsername(nsACString& aUsername) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetUsername(aUsername);
+  }
+  NS_IMETHOD GetPassword(nsACString& aPassword) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetPassword(aPassword);
+  }
+  NS_IMETHOD GetHostPort(nsACString& aHostPort) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetHostPort(aHostPort);
+  }
+  NS_IMETHOD GetHost(nsACString& aHost) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetHost(aHost);
+  }
+  NS_IMETHOD GetPort(int32_t* aPort) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetPort(aPort);
+  }
+  NS_IMETHOD GetPathQueryRef(nsACString& aPathQueryRef) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetPathQueryRef(aPathQueryRef);
+  }
+  NS_IMETHOD SchemeIs(const char* scheme, bool* _retval) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->SchemeIs(scheme, _retval);
+  }
+  NS_IMETHOD Resolve(const nsACString& relativePath,
+                     nsACString& _retval) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->Resolve(relativePath, _retval);
+  }
+  NS_IMETHOD GetAsciiSpec(nsACString& aAsciiSpec) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetAsciiSpec(aAsciiSpec);
+  }
+  NS_IMETHOD GetAsciiHostPort(nsACString& aAsciiHostPort) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetAsciiHostPort(aAsciiHostPort);
+  }
+  NS_IMETHOD GetAsciiHost(nsACString& aAsciiHost) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetAsciiHost(aAsciiHost);
+  }
+  NS_IMETHOD GetRef(nsACString& aRef) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetRef(aRef);
+  }
+  NS_IMETHOD GetSpecIgnoringRef(nsACString& aSpecIgnoringRef) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetSpecIgnoringRef(aSpecIgnoringRef);
+  }
+  NS_IMETHOD GetHasRef(bool* aHasRef) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetHasRef(aHasRef);
+  }
+  NS_IMETHOD GetFilePath(nsACString& aFilePath) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetFilePath(aFilePath);
+  }
+  NS_IMETHOD GetQuery(nsACString& aQuery) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->GetQuery(aQuery);
+  }
+  NS_IMETHOD GetDisplayHost(nsACString& aDisplayHost) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetDisplayHost(aDisplayHost);
+  }
+  NS_IMETHOD GetDisplayHostPort(nsACString& aDisplayHostPort) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetDisplayHostPort(aDisplayHostPort);
+  }
+  NS_IMETHOD GetDisplaySpec(nsACString& aDisplaySpec) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetDisplaySpec(aDisplaySpec);
+  }
+  NS_IMETHOD GetDisplayPrePath(nsACString& aDisplayPrePath) override {
+    return !mSource ? NS_ERROR_NULL_POINTER
+                    : mSource->GetDisplayPrePath(aDisplayPrePath);
+  }
+  NS_IMETHOD Mutate(nsIURIMutator** _retval) override {
+    return !mSource ? NS_ERROR_NULL_POINTER : mSource->Mutate(_retval);
+  }
+  NS_IMETHOD_(void) Serialize(mozilla::ipc::URIParams& aParams) override {
+    MOZ_ASSERT(mSource);
+    mSource->Serialize(aParams);
+  }
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(SubstitutingJARURI,
+                              NS_SUBSTITUTINGJARURI_IMPL_CID)
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* SubstitutingJARURI_h */
diff --git a/netwerk/protocol/res/SubstitutingProtocolHandler.cpp b/netwerk/protocol/res/SubstitutingProtocolHandler.cpp
new file mode 100644
index 0000000000..aaab858605
--- /dev/null
+++ b/netwerk/protocol/res/SubstitutingProtocolHandler.cpp
@@ -0,0 +1,622 @@
+/* -*- 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/ModuleUtils.h"
+#include "mozilla/Unused.h"
+#include "mozilla/chrome/RegistryMessageUtils.h"
+#include "mozilla/dom/ContentParent.h"
+
+#include "SubstitutingProtocolHandler.h"
+#include "SubstitutingURL.h"
+#include "SubstitutingJARURI.h"
+#include "nsIChannel.h"
+#include "nsIIOService.h"
+#include "nsIFile.h"
+#include "nsNetCID.h"
+#include "nsNetUtil.h"
+#include "nsReadableUtils.h"
+#include "nsURLHelper.h"
+#include "nsEscape.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIClassInfoImpl.h"
+
+using mozilla::dom::ContentParent;
+
+namespace mozilla {
+namespace net {
+
+// Log module for Substituting Protocol logging. We keep the pre-existing module
+// name of "nsResProtocol" to avoid disruption.
+static LazyLogModule gResLog("nsResProtocol");
+
+static NS_DEFINE_CID(kSubstitutingJARURIImplCID,
+                     NS_SUBSTITUTINGJARURI_IMPL_CID);
+
+//---------------------------------------------------------------------------------
+// SubstitutingURL : overrides nsStandardURL::GetFile to provide nsIFile
+// resolution
+//---------------------------------------------------------------------------------
+
+// The list of interfaces should be in sync with nsStandardURL
+// Queries this list of interfaces. If none match, it queries mURI.
+NS_IMPL_NSIURIMUTATOR_ISUPPORTS(SubstitutingURL::Mutator, nsIURISetters,
+                                nsIURIMutator, nsIStandardURLMutator,
+                                nsIURLMutator, nsIFileURLMutator,
+                                nsISerializable)
+
+NS_IMPL_CLASSINFO(SubstitutingURL, nullptr, nsIClassInfo::THREADSAFE,
+                  NS_SUBSTITUTINGURL_CID)
+// Empty CI getter. We only need nsIClassInfo for Serialization
+NS_IMPL_CI_INTERFACE_GETTER0(SubstitutingURL)
+
+NS_IMPL_ADDREF_INHERITED(SubstitutingURL, nsStandardURL)
+NS_IMPL_RELEASE_INHERITED(SubstitutingURL, nsStandardURL)
+NS_IMPL_QUERY_INTERFACE_CI_INHERITED0(SubstitutingURL, nsStandardURL)
+
+nsresult SubstitutingURL::EnsureFile() {
+  nsAutoCString ourScheme;
+  nsresult rv = GetScheme(ourScheme);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Get the handler associated with this scheme. It would be nice to just
+  // pass this in when constructing SubstitutingURLs, but we need a generic
+  // factory constructor.
+  nsCOMPtr<nsIIOService> io = do_GetIOService(&rv);
+  nsCOMPtr<nsIProtocolHandler> handler;
+  rv = io->GetProtocolHandler(ourScheme.get(), getter_AddRefs(handler));
+  NS_ENSURE_SUCCESS(rv, rv);
+  nsCOMPtr<nsISubstitutingProtocolHandler> substHandler =
+      do_QueryInterface(handler);
+  MOZ_ASSERT(substHandler);
+
+  nsAutoCString spec;
+  rv = substHandler->ResolveURI(this, spec);
+  if (NS_FAILED(rv)) return rv;
+
+  nsAutoCString scheme;
+  rv = net_ExtractURLScheme(spec, scheme);
+  if (NS_FAILED(rv)) return rv;
+
+  // Bug 585869:
+  // In most cases, the scheme is jar if it's not file.
+  // Regardless, net_GetFileFromURLSpec should be avoided
+  // when the scheme isn't file.
+  if (!scheme.EqualsLiteral("file")) return NS_ERROR_NO_INTERFACE;
+
+  return net_GetFileFromURLSpec(spec, getter_AddRefs(mFile));
+}
+
+/* virtual */
+nsStandardURL* SubstitutingURL::StartClone() {
+  SubstitutingURL* clone = new SubstitutingURL();
+  return clone;
+}
+
+void SubstitutingURL::Serialize(ipc::URIParams& aParams) {
+  nsStandardURL::Serialize(aParams);
+  aParams.get_StandardURLParams().isSubstituting() = true;
+}
+
+// SubstitutingJARURI
+
+SubstitutingJARURI::SubstitutingJARURI(nsIURL* source, nsIJARURI* resolved)
+    : mSource(source), mResolved(resolved) {}
+
+// SubstitutingJARURI::nsIURI
+
+NS_IMETHODIMP
+SubstitutingJARURI::Equals(nsIURI* aOther, bool* aResult) {
+  return EqualsInternal(aOther, eHonorRef, aResult);
+}
+
+NS_IMETHODIMP
+SubstitutingJARURI::EqualsExceptRef(nsIURI* aOther, bool* aResult) {
+  return EqualsInternal(aOther, eIgnoreRef, aResult);
+}
+
+nsresult SubstitutingJARURI::EqualsInternal(nsIURI* aOther,
+                                            RefHandlingEnum aRefHandlingMode,
+                                            bool* aResult) {
+  *aResult = false;
+  if (!aOther) {
+    return NS_OK;
+  }
+
+  nsresult rv;
+  RefPtr<SubstitutingJARURI> other;
+  rv =
+      aOther->QueryInterface(kSubstitutingJARURIImplCID, getter_AddRefs(other));
+  if (NS_FAILED(rv)) {
+    return NS_OK;
+  }
+
+  // We only need to check the source as the resolved URI is the same for a
+  // given source
+  return aRefHandlingMode == eHonorRef
+             ? mSource->Equals(other->mSource, aResult)
+             : mSource->EqualsExceptRef(other->mSource, aResult);
+}
+
+// SubstitutingJARURI::nsISerializable
+
+NS_IMETHODIMP
+SubstitutingJARURI::Read(nsIObjectInputStream* aStream) {
+  MOZ_ASSERT(!mSource);
+  MOZ_ASSERT(!mResolved);
+  NS_ENSURE_ARG_POINTER(aStream);
+
+  nsresult rv;
+  nsCOMPtr<nsISupports> source;
+  rv = aStream->ReadObject(true, getter_AddRefs(source));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mSource = do_QueryInterface(source, &rv);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsISupports> resolved;
+  rv = aStream->ReadObject(true, getter_AddRefs(resolved));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mResolved = do_QueryInterface(resolved, &rv);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SubstitutingJARURI::Write(nsIObjectOutputStream* aStream) {
+  NS_ENSURE_ARG_POINTER(aStream);
+
+  nsresult rv;
+  rv = aStream->WriteCompoundObject(mSource, NS_GET_IID(nsISupports), true);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = aStream->WriteCompoundObject(mResolved, NS_GET_IID(nsISupports), true);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+NS_IMPL_CLASSINFO(SubstitutingJARURI, nullptr, nsIClassInfo::MAIN_THREAD_ONLY,
+                  NS_SUBSTITUTINGJARURI_CID)
+
+NS_IMPL_ADDREF(SubstitutingJARURI)
+NS_IMPL_RELEASE(SubstitutingJARURI)
+
+NS_INTERFACE_MAP_BEGIN(SubstitutingJARURI)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIURI)
+  NS_INTERFACE_MAP_ENTRY(nsIJARURI)
+  NS_INTERFACE_MAP_ENTRY(nsIURL)
+  NS_INTERFACE_MAP_ENTRY(nsIStandardURL)
+  NS_INTERFACE_MAP_ENTRY(nsISerializable)
+  if (aIID.Equals(kSubstitutingJARURIImplCID)) {
+    foundInterface = static_cast<nsIURI*>(this);
+  } else
+    NS_INTERFACE_MAP_ENTRY(nsIURI)
+  NS_IMPL_QUERY_CLASSINFO(SubstitutingJARURI)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CI_INTERFACE_GETTER(SubstitutingJARURI, nsIURI, nsIJARURI, nsIURL,
+                            nsIStandardURL, nsISerializable)
+
+SubstitutingProtocolHandler::SubstitutingProtocolHandler(const char* aScheme,
+                                                         bool aEnforceFileOrJar)
+    : mScheme(aScheme),
+      mSubstitutionsLock("SubstitutingProtocolHandler::mSubstitutions"),
+      mSubstitutions(16),
+      mEnforceFileOrJar(aEnforceFileOrJar) {
+  ConstructInternal();
+}
+
+void SubstitutingProtocolHandler::ConstructInternal() {
+  nsresult rv;
+  mIOService = do_GetIOService(&rv);
+  MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv) && mIOService);
+}
+
+//
+// IPC marshalling.
+//
+
+nsresult SubstitutingProtocolHandler::CollectSubstitutions(
+    nsTArray<SubstitutionMapping>& aMappings) {
+  AutoReadLock lock(mSubstitutionsLock);
+  for (const auto& substitutionEntry : mSubstitutions) {
+    const SubstitutionEntry& entry = substitutionEntry.GetData();
+    nsCOMPtr<nsIURI> uri = entry.baseURI;
+    SerializedURI serialized;
+    if (uri) {
+      nsresult rv = uri->GetSpec(serialized.spec);
+      NS_ENSURE_SUCCESS(rv, rv);
+    }
+    SubstitutionMapping substitution = {mScheme,
+                                        nsCString(substitutionEntry.GetKey()),
+                                        serialized, entry.flags};
+    aMappings.AppendElement(substitution);
+  }
+
+  return NS_OK;
+}
+
+nsresult SubstitutingProtocolHandler::SendSubstitution(const nsACString& aRoot,
+                                                       nsIURI* aBaseURI,
+                                                       uint32_t aFlags) {
+  if (GeckoProcessType_Content == XRE_GetProcessType()) {
+    return NS_OK;
+  }
+
+  nsTArray<ContentParent*> parents;
+  ContentParent::GetAll(parents);
+  if (!parents.Length()) {
+    return NS_OK;
+  }
+
+  SubstitutionMapping mapping;
+  mapping.scheme = mScheme;
+  mapping.path = aRoot;
+  if (aBaseURI) {
+    nsresult rv = aBaseURI->GetSpec(mapping.resolvedURI.spec);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+  mapping.flags = aFlags;
+
+  for (uint32_t i = 0; i < parents.Length(); i++) {
+    Unused << parents[i]->SendRegisterChromeItem(mapping);
+  }
+
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------------
+// nsIProtocolHandler
+//----------------------------------------------------------------------------
+
+nsresult SubstitutingProtocolHandler::GetScheme(nsACString& result) {
+  result = mScheme;
+  return NS_OK;
+}
+
+nsresult SubstitutingProtocolHandler::NewURI(const nsACString& aSpec,
+                                             const char* aCharset,
+                                             nsIURI* aBaseURI,
+                                             nsIURI** aResult) {
+  // unescape any %2f and %2e to make sure nsStandardURL coalesces them.
+  // Later net_GetFileFromURLSpec() will do a full unescape and we want to
+  // treat them the same way the file system will. (bugs 380994, 394075)
+  nsresult rv;
+  nsAutoCString spec;
+  const char* src = aSpec.BeginReading();
+  const char* end = aSpec.EndReading();
+  const char* last = src;
+
+  spec.SetCapacity(aSpec.Length() + 1);
+  for (; src < end; ++src) {
+    if (*src == '%' && (src < end - 2) && *(src + 1) == '2') {
+      char ch = '\0';
+      if (*(src + 2) == 'f' || *(src + 2) == 'F') {
+        ch = '/';
+      } else if (*(src + 2) == 'e' || *(src + 2) == 'E') {
+        ch = '.';
+      }
+
+      if (ch) {
+        if (last < src) {
+          spec.Append(last, src - last);
+        }
+        spec.Append(ch);
+        src += 2;
+        last = src + 1;  // src will be incremented by the loop
+      }
+    }
+    if (*src == '?' || *src == '#') {
+      break;  // Don't escape %2f and %2e in the query or ref parts of the URI
+    }
+  }
+
+  if (last < end) {
+    spec.Append(last, end - last);
+  }
+
+  nsCOMPtr<nsIURI> base(aBaseURI);
+  nsCOMPtr<nsIURL> uri;
+  rv =
+      NS_MutateURI(new SubstitutingURL::Mutator())
+          .Apply(&nsIStandardURLMutator::Init, nsIStandardURL::URLTYPE_STANDARD,
+                 -1, spec, aCharset, base, nullptr)
+          .Finalize(uri);
+  if (NS_FAILED(rv)) return rv;
+
+  nsAutoCString host;
+  rv = uri->GetHost(host);
+  if (NS_FAILED(rv)) return rv;
+
+  // "android" is the only root that would return the RESOLVE_JAR_URI flag
+  // see nsResProtocolHandler::GetSubstitutionInternal
+  if (MustResolveJAR(host)) {
+    return ResolveJARURI(uri, aResult);
+  }
+
+  uri.forget(aResult);
+  return NS_OK;
+}
+
+nsresult SubstitutingProtocolHandler::ResolveJARURI(nsIURL* aURL,
+                                                    nsIURI** aResult) {
+  nsAutoCString spec;
+  nsresult rv = ResolveURI(aURL, spec);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIURI> resolvedURI;
+  rv = NS_NewURI(getter_AddRefs(resolvedURI), spec);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIURI> innermostURI = NS_GetInnermostURI(resolvedURI);
+  nsAutoCString scheme;
+  innermostURI->GetScheme(scheme);
+
+  // We only ever want to resolve to a local jar.
+  NS_ENSURE_TRUE(scheme.EqualsLiteral("file"), NS_ERROR_UNEXPECTED);
+
+  nsCOMPtr<nsIJARURI> jarURI(do_QueryInterface(resolvedURI));
+  if (!jarURI) {
+    // This substitution does not resolve to a jar: URL, so we just
+    // return the plain SubstitutionURL
+    nsCOMPtr<nsIURI> url = aURL;
+    url.forget(aResult);
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIJARURI> result = new SubstitutingJARURI(aURL, jarURI);
+  result.forget(aResult);
+
+  return rv;
+}
+
+nsresult SubstitutingProtocolHandler::NewChannel(nsIURI* uri,
+                                                 nsILoadInfo* aLoadInfo,
+                                                 nsIChannel** result) {
+  NS_ENSURE_ARG_POINTER(uri);
+  NS_ENSURE_ARG_POINTER(aLoadInfo);
+
+  nsAutoCString spec;
+  nsresult rv = ResolveURI(uri, spec);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIURI> newURI;
+  rv = NS_NewURI(getter_AddRefs(newURI), spec);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // We don't want to allow the inner protocol handler to modify the result
+  // principal URI since we want either |uri| or anything pre-set by upper
+  // layers to prevail.
+  nsCOMPtr<nsIURI> savedResultPrincipalURI;
+  rv =
+      aLoadInfo->GetResultPrincipalURI(getter_AddRefs(savedResultPrincipalURI));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = NS_NewChannelInternal(result, newURI, aLoadInfo);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = aLoadInfo->SetResultPrincipalURI(savedResultPrincipalURI);
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = (*result)->SetOriginalURI(uri);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return SubstituteChannel(uri, aLoadInfo, result);
+}
+
+nsresult SubstitutingProtocolHandler::AllowPort(int32_t port,
+                                                const char* scheme,
+                                                bool* _retval) {
+  // don't override anything.
+  *_retval = false;
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------------
+// nsISubstitutingProtocolHandler
+//----------------------------------------------------------------------------
+
+nsresult SubstitutingProtocolHandler::SetSubstitution(const nsACString& root,
+                                                      nsIURI* baseURI) {
+  // Add-ons use this API but they should not be able to make anything
+  // content-accessible.
+  return SetSubstitutionWithFlags(root, baseURI, 0);
+}
+
+nsresult SubstitutingProtocolHandler::SetSubstitutionWithFlags(
+    const nsACString& origRoot, nsIURI* baseURI, uint32_t flags) {
+  nsAutoCString root;
+  ToLowerCase(origRoot, root);
+
+  if (!baseURI) {
+    {
+      AutoWriteLock lock(mSubstitutionsLock);
+      mSubstitutions.Remove(root);
+    }
+
+    return SendSubstitution(root, baseURI, flags);
+  }
+
+  // If baseURI isn't a same-scheme URI, we can set the substitution
+  // immediately.
+  nsAutoCString scheme;
+  nsresult rv = baseURI->GetScheme(scheme);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (!scheme.Equals(mScheme)) {
+    if (mEnforceFileOrJar && !scheme.EqualsLiteral("file") &&
+        !scheme.EqualsLiteral("jar") && !scheme.EqualsLiteral("app") &&
+        !scheme.EqualsLiteral("resource")) {
+      NS_WARNING("Refusing to create substituting URI to non-file:// target");
+      return NS_ERROR_INVALID_ARG;
+    }
+
+    {
+      AutoWriteLock lock(mSubstitutionsLock);
+      mSubstitutions.InsertOrUpdate(root, SubstitutionEntry{baseURI, flags});
+    }
+
+    return SendSubstitution(root, baseURI, flags);
+  }
+
+  // baseURI is a same-type substituting URI, let's resolve it first.
+  nsAutoCString newBase;
+  rv = ResolveURI(baseURI, newBase);
+  if (NS_FAILED(rv)) return rv;
+
+  nsCOMPtr<nsIURI> newBaseURI;
+  rv =
+      mIOService->NewURI(newBase, nullptr, nullptr, getter_AddRefs(newBaseURI));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  {
+    AutoWriteLock lock(mSubstitutionsLock);
+    mSubstitutions.InsertOrUpdate(root, SubstitutionEntry{newBaseURI, flags});
+  }
+
+  return SendSubstitution(root, newBaseURI, flags);
+}
+
+nsresult SubstitutingProtocolHandler::GetSubstitution(
+    const nsACString& origRoot, nsIURI** result) {
+  NS_ENSURE_ARG_POINTER(result);
+
+  nsAutoCString root;
+  ToLowerCase(origRoot, root);
+
+  {
+    AutoReadLock lock(mSubstitutionsLock);
+    SubstitutionEntry entry;
+    if (mSubstitutions.Get(root, &entry)) {
+      nsCOMPtr<nsIURI> baseURI = entry.baseURI;
+      baseURI.forget(result);
+      return NS_OK;
+    }
+  }
+
+  uint32_t flags;
+  return GetSubstitutionInternal(root, result, &flags);
+}
+
+nsresult SubstitutingProtocolHandler::GetSubstitutionFlags(
+    const nsACString& root, uint32_t* flags) {
+#ifdef DEBUG
+  nsAutoCString lcRoot;
+  ToLowerCase(root, lcRoot);
+  MOZ_ASSERT(root.Equals(lcRoot),
+             "GetSubstitutionFlags should never receive mixed-case root name");
+#endif
+
+  *flags = 0;
+
+  {
+    AutoReadLock lock(mSubstitutionsLock);
+
+    SubstitutionEntry entry;
+    if (mSubstitutions.Get(root, &entry)) {
+      *flags = entry.flags;
+      return NS_OK;
+    }
+  }
+
+  nsCOMPtr<nsIURI> baseURI;
+  return GetSubstitutionInternal(root, getter_AddRefs(baseURI), flags);
+}
+
+nsresult SubstitutingProtocolHandler::HasSubstitution(
+    const nsACString& origRoot, bool* result) {
+  NS_ENSURE_ARG_POINTER(result);
+
+  nsAutoCString root;
+  ToLowerCase(origRoot, root);
+
+  *result = HasSubstitution(root);
+  return NS_OK;
+}
+
+nsresult SubstitutingProtocolHandler::ResolveURI(nsIURI* uri,
+                                                 nsACString& result) {
+  nsresult rv;
+
+  nsAutoCString host;
+  nsAutoCString path;
+  nsAutoCString pathname;
+
+  nsCOMPtr<nsIURL> url = do_QueryInterface(uri);
+  if (!url) {
+    return NS_ERROR_MALFORMED_URI;
+  }
+
+  rv = uri->GetAsciiHost(host);
+  if (NS_FAILED(rv)) return rv;
+
+  rv = uri->GetPathQueryRef(path);
+  if (NS_FAILED(rv)) return rv;
+
+  rv = url->GetFilePath(pathname);
+  if (NS_FAILED(rv)) return rv;
+
+  if (ResolveSpecialCases(host, path, pathname, result)) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIURI> baseURI;
+  rv = GetSubstitution(host, getter_AddRefs(baseURI));
+  if (NS_FAILED(rv)) return rv;
+
+  // Unescape the path so we can perform some checks on it.
+  NS_UnescapeURL(pathname);
+  if (pathname.FindChar('\\') != -1) {
+    return NS_ERROR_MALFORMED_URI;
+  }
+
+  // Some code relies on an empty path resolving to a file rather than a
+  // directory.
+  NS_ASSERTION(path.CharAt(0) == '/', "Path must begin with '/'");
+  if (path.Length() == 1) {
+    rv = baseURI->GetSpec(result);
+  } else {
+    // Make sure we always resolve the path as file-relative to our target URI.
+    // When the baseURI is a nsIFileURL, and the directory it points to doesn't
+    // exist, it doesn't end with a /. In that case, a file-relative resolution
+    // is going to pick something in the parent directory, so we resolve using
+    // an absolute path derived from the full path in that case.
+    nsCOMPtr<nsIFileURL> baseDir = do_QueryInterface(baseURI);
+    if (baseDir) {
+      nsAutoCString basePath;
+      rv = baseURI->GetFilePath(basePath);
+      if (NS_SUCCEEDED(rv) && !StringEndsWith(basePath, "/"_ns)) {
+        // Cf. the assertion above, path already starts with a /, so prefixing
+        // with a string that doesn't end with one will leave us wit the right
+        // amount of /.
+        path.Insert(basePath, 0);
+      } else {
+        // Allow to fall through below.
+        baseDir = nullptr;
+      }
+    }
+    if (!baseDir) {
+      path.Insert('.', 0);
+    }
+    rv = baseURI->Resolve(path, result);
+  }
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (MOZ_LOG_TEST(gResLog, LogLevel::Debug)) {
+    nsAutoCString spec;
+    uri->GetAsciiSpec(spec);
+    MOZ_LOG(gResLog, LogLevel::Debug,
+            ("%s\n -> %s\n", spec.get(), PromiseFlatCString(result).get()));
+  }
+  return rv;
+}
+
+}  // namespace net
+}  // namespace mozilla
diff --git a/netwerk/protocol/res/SubstitutingProtocolHandler.h b/netwerk/protocol/res/SubstitutingProtocolHandler.h
new file mode 100644
index 0000000000..6bdb27f38a
--- /dev/null
+++ b/netwerk/protocol/res/SubstitutingProtocolHandler.h
@@ -0,0 +1,129 @@
+/* -*- 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 SubstitutingProtocolHandler_h___
+#define SubstitutingProtocolHandler_h___
+
+#include "nsISubstitutingProtocolHandler.h"
+
+#include "nsTHashMap.h"
+#include "nsStandardURL.h"
+#include "nsJARURI.h"
+#include "mozilla/chrome/RegistryMessageUtils.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/RWLock.h"
+
+class nsIIOService;
+
+namespace mozilla {
+namespace net {
+
+//
+// Base class for resource://-like substitution protocols.
+//
+// If you add a new protocol, make sure to change nsChromeRegistryChrome
+// to properly invoke CollectSubstitutions at the right time.
+class SubstitutingProtocolHandler {
+ public:
+  explicit SubstitutingProtocolHandler(const char* aScheme,
+                                       bool aEnforceFileOrJar = true);
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(SubstitutingProtocolHandler);
+  NS_DECL_NON_VIRTUAL_NSIPROTOCOLHANDLER;
+  NS_DECL_NON_VIRTUAL_NSISUBSTITUTINGPROTOCOLHANDLER;
+
+  bool HasSubstitution(const nsACString& aRoot) const {
+    AutoReadLock lock(const_cast<RWLock&>(mSubstitutionsLock));
+    return mSubstitutions.Get(aRoot, nullptr);
+  }
+
+  nsresult NewURI(const nsACString& aSpec, const char* aCharset,
+                  nsIURI* aBaseURI, nsIURI** aResult);
+
+  [[nodiscard]] nsresult CollectSubstitutions(
+      nsTArray<SubstitutionMapping>& aMappings);
+
+ protected:
+  virtual ~SubstitutingProtocolHandler() = default;
+  void ConstructInternal();
+
+  [[nodiscard]] nsresult SendSubstitution(const nsACString& aRoot,
+                                          nsIURI* aBaseURI, uint32_t aFlags);
+
+  nsresult GetSubstitutionFlags(const nsACString& root, uint32_t* flags);
+
+  // Override this in the subclass to try additional lookups after checking
+  // mSubstitutions.
+  [[nodiscard]] virtual nsresult GetSubstitutionInternal(
+      const nsACString& aRoot, nsIURI** aResult, uint32_t* aFlags) {
+    *aResult = nullptr;
+    *aFlags = 0;
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // Override this in the subclass to check for special case when resolving URIs
+  // _before_ checking substitutions.
+  [[nodiscard]] virtual bool ResolveSpecialCases(const nsACString& aHost,
+                                                 const nsACString& aPath,
+                                                 const nsACString& aPathname,
+                                                 nsACString& aResult) {
+    return false;
+  }
+
+  // This method should only return true if GetSubstitutionInternal would
+  // return the RESOLVE_JAR_URI flag.
+  [[nodiscard]] virtual bool MustResolveJAR(const nsACString& aRoot) {
+    return false;
+  }
+
+  // Override this in the subclass to check for special case when opening
+  // channels.
+  [[nodiscard]] virtual nsresult SubstituteChannel(nsIURI* uri,
+                                                   nsILoadInfo* aLoadInfo,
+                                                   nsIChannel** result) {
+    return NS_OK;
+  }
+
+  nsIIOService* IOService() { return mIOService; }
+
+ private:
+  struct SubstitutionEntry {
+    nsCOMPtr<nsIURI> baseURI;
+    uint32_t flags = 0;
+  };
+
+  // Notifies all observers that a new substitution from |aRoot| to
+  // |aBaseURI| has been set/installed for this protocol handler.
+  void NotifyObservers(const nsACString& aRoot, nsIURI* aBaseURI);
+
+  nsCString mScheme;
+
+  RWLock mSubstitutionsLock;
+  nsTHashMap<nsCStringHashKey, SubstitutionEntry> mSubstitutions
+      MOZ_GUARDED_BY(mSubstitutionsLock);
+  nsCOMPtr<nsIIOService> mIOService;
+
+  // Returns a SubstitutingJARURI if |aUrl| maps to a |jar:| URI,
+  // otherwise will return |aURL|
+  nsresult ResolveJARURI(nsIURL* aURL, nsIURI** aResult);
+
+  // In general, we expect the principal of a document loaded from a
+  // substituting URI to be a content principal for that URI (rather than
+  // a principal for whatever is underneath). However, this only works if
+  // the protocol handler for the underlying URI doesn't set an explicit
+  // owner (which chrome:// does, for example). So we want to require that
+  // substituting URIs only map to other URIs of the same type, or to
+  // file:// and jar:// URIs.
+  //
+  // Enforcing this for ye olde resource:// URIs could carry compat risks, so
+  // we just try to enforce it on new protocols going forward.
+  bool mEnforceFileOrJar;
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* SubstitutingProtocolHandler_h___ */
diff --git a/netwerk/protocol/res/SubstitutingURL.h b/netwerk/protocol/res/SubstitutingURL.h
new file mode 100644
index 0000000000..b9012f1665
--- /dev/null
+++ b/netwerk/protocol/res/SubstitutingURL.h
@@ -0,0 +1,66 @@
+/* -*- 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 SubstitutingURL_h
+#define SubstitutingURL_h
+
+#include "nsStandardURL.h"
+
+class nsIIOService;
+
+namespace mozilla {
+namespace net {
+
+// SubstitutingURL : overrides nsStandardURL::GetFile to provide nsIFile
+// resolution
+class SubstitutingURL : public nsStandardURL {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  virtual nsStandardURL* StartClone() override;
+  [[nodiscard]] virtual nsresult EnsureFile() override;
+
+ private:
+  explicit SubstitutingURL() : nsStandardURL(true) {}
+  explicit SubstitutingURL(bool aSupportsFileURL) : nsStandardURL(true) {
+    MOZ_ASSERT(aSupportsFileURL);
+  }
+  virtual nsresult Clone(nsIURI** aURI) override {
+    return nsStandardURL::Clone(aURI);
+  }
+  virtual ~SubstitutingURL() = default;
+
+ public:
+  class Mutator : public TemplatedMutator<SubstitutingURL> {
+    NS_DECL_ISUPPORTS
+   public:
+    explicit Mutator() = default;
+
+   private:
+    virtual ~Mutator() = default;
+
+    SubstitutingURL* Create() override { return new SubstitutingURL(); }
+  };
+
+  NS_IMETHOD Mutate(nsIURIMutator** aMutator) override {
+    RefPtr<SubstitutingURL::Mutator> mutator = new SubstitutingURL::Mutator();
+    nsresult rv = mutator->InitFromURI(this);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    mutator.forget(aMutator);
+    return NS_OK;
+  }
+
+  NS_IMETHOD_(void) Serialize(ipc::URIParams& aParams) override;
+
+  friend BaseURIMutator<SubstitutingURL>;
+  friend TemplatedMutator<SubstitutingURL>;
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif /* SubstitutingURL_h */
diff --git a/netwerk/protocol/res/moz.build b/netwerk/protocol/res/moz.build
new file mode 100644
index 0000000000..63407eebd8
--- /dev/null
+++ b/netwerk/protocol/res/moz.build
@@ -0,0 +1,42 @@
+# -*- 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/.
+
+XPIDL_SOURCES += [
+    "nsIResProtocolHandler.idl",
+    "nsISubstitutingProtocolHandler.idl",
+]
+
+XPIDL_MODULE = "necko_res"
+
+EXPORTS.mozilla.net += [
+    "ExtensionProtocolHandler.h",
+    "PageThumbProtocolHandler.h",
+    "RemoteStreamGetter.h",
+    "SubstitutingJARURI.h",
+    "SubstitutingProtocolHandler.h",
+    "SubstitutingURL.h",
+]
+
+EXPORTS += [
+    "nsResProtocolHandler.h",
+]
+
+UNIFIED_SOURCES += [
+    "ExtensionProtocolHandler.cpp",
+    "nsResProtocolHandler.cpp",
+    "PageThumbProtocolHandler.cpp",
+    "RemoteStreamGetter.cpp",
+    "SubstitutingProtocolHandler.cpp",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "/netwerk/base",
+    "/xpcom/base",
+]
diff --git a/netwerk/protocol/res/nsIResProtocolHandler.idl b/netwerk/protocol/res/nsIResProtocolHandler.idl
new file mode 100644
index 0000000000..7046f2f1d4
--- /dev/null
+++ b/netwerk/protocol/res/nsIResProtocolHandler.idl
@@ -0,0 +1,15 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 "nsISubstitutingProtocolHandler.idl"
+
+/**
+ * Protocol handler interface for the resource:// protocol
+ */
+[scriptable, uuid(241d34ac-9ed5-46d7-910c-7a9d914aa0c5)]
+interface nsIResProtocolHandler : nsISubstitutingProtocolHandler
+{
+  boolean allowContentToAccess(in nsIURI url);
+};
diff --git a/netwerk/protocol/res/nsISubstitutingProtocolHandler.idl b/netwerk/protocol/res/nsISubstitutingProtocolHandler.idl
new file mode 100644
index 0000000000..cf2e0d42ab
--- /dev/null
+++ b/netwerk/protocol/res/nsISubstitutingProtocolHandler.idl
@@ -0,0 +1,62 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIProtocolHandler.idl"
+
+
+/**
+ * Protocol handler superinterface for a protocol which performs substitutions
+ * from URIs of its scheme to URIs of another scheme.
+ */
+[scriptable, uuid(154c64fd-a69e-4105-89f8-bd7dfe621372)]
+interface nsISubstitutingProtocolHandler : nsIProtocolHandler
+{
+  /**
+   * Content script may access files in this package.
+   */
+  const short ALLOW_CONTENT_ACCESS = 1 << 0;
+  /**
+   * This substitution exposes nsIJARURI instead of a nsIFileURL.  By default
+   * NewURI will always return a nsIFileURL even when the URL is jar:
+   */
+  const short RESOLVE_JAR_URI = 1 << 1;
+
+  /**
+   * Sets the substitution for the root key:
+   *   resource://root/path ==> baseURI.resolve(path)
+   *
+   * A null baseURI removes the specified substitution.
+   *
+   * The root key will be converted to lower-case to conform to
+   * case-insensitive URI hostname matching behavior.
+   */
+  [must_use] void setSubstitution(in ACString root, in nsIURI baseURI);
+
+  /**
+   * Same as setSubstitution, but with specific flags.
+   */
+  [must_use] void setSubstitutionWithFlags(in ACString root, in nsIURI baseURI, in uint32_t flags);
+
+  /**
+   * Gets the substitution for the root key.
+   *
+   * @throws NS_ERROR_NOT_AVAILABLE if none exists.
+   */
+  [must_use] nsIURI getSubstitution(in ACString root);
+
+  /**
+   * Returns TRUE if the substitution exists and FALSE otherwise.
+   */
+  [must_use] boolean hasSubstitution(in ACString root);
+
+  /**
+   * Utility function to resolve a substituted URI.  A resolved URI is not
+   * guaranteed to reference a resource that exists (ie. opening a channel to
+   * the resolved URI may fail).
+   *
+   * @throws NS_ERROR_NOT_AVAILABLE if resURI.host() is an unknown root key.
+   */
+  [must_use] AUTF8String resolveURI(in nsIURI resURI);
+};
diff --git a/netwerk/protocol/res/nsResProtocolHandler.cpp b/netwerk/protocol/res/nsResProtocolHandler.cpp
new file mode 100644
index 0000000000..208baedf2b
--- /dev/null
+++ b/netwerk/protocol/res/nsResProtocolHandler.cpp
@@ -0,0 +1,195 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/chrome/RegistryMessageUtils.h"
+#include "mozilla/dom/ContentParent.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/Unused.h"
+
+#include "nsResProtocolHandler.h"
+#include "nsNetCID.h"
+#include "nsNetUtil.h"
+#include "nsURLHelper.h"
+#include "nsEscape.h"
+
+#include "mozilla/Omnijar.h"
+
+using mozilla::LogLevel;
+using mozilla::Unused;
+using mozilla::dom::ContentParent;
+
+#define kAPP "app"
+#define kGRE "gre"
+#define kAndroid "android"
+
+mozilla::StaticRefPtr<nsResProtocolHandler> nsResProtocolHandler::sSingleton;
+
+already_AddRefed<nsResProtocolHandler> nsResProtocolHandler::GetSingleton() {
+  if (!sSingleton) {
+    RefPtr<nsResProtocolHandler> handler = new nsResProtocolHandler();
+    if (NS_WARN_IF(NS_FAILED(handler->Init()))) {
+      return nullptr;
+    }
+    sSingleton = handler;
+    ClearOnShutdown(&sSingleton);
+  }
+  return do_AddRef(sSingleton);
+}
+
+nsresult nsResProtocolHandler::Init() {
+  nsresult rv;
+  rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::APP, mAppURI);
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::GRE, mGREURI);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // mozilla::Omnijar::GetURIString always returns a string ending with /,
+  // and we want to remove it.
+  mGREURI.Truncate(mGREURI.Length() - 1);
+  if (mAppURI.Length()) {
+    mAppURI.Truncate(mAppURI.Length() - 1);
+  } else {
+    mAppURI = mGREURI;
+  }
+
+#ifdef ANDROID
+  rv = GetApkURI(mApkURI);
+#endif
+
+  // XXXbsmedberg Neil wants a resource://pchrome/ for the profile chrome dir...
+  // but once I finish multiple chrome registration I'm not sure that it is
+  // needed
+
+  // XXX dveditz: resource://pchrome/ defeats profile directory salting
+  // if web content can load it. Tread carefully.
+
+  return rv;
+}
+
+#ifdef ANDROID
+nsresult nsResProtocolHandler::GetApkURI(nsACString& aResult) {
+  nsCString::const_iterator start, iter;
+  mGREURI.BeginReading(start);
+  mGREURI.EndReading(iter);
+  nsCString::const_iterator start_iter = start;
+
+  // This is like jar:jar:file://path/to/apk/base.apk!/path/to/omni.ja!/
+  bool found = FindInReadable("!/"_ns, start_iter, iter);
+  NS_ENSURE_TRUE(found, NS_ERROR_UNEXPECTED);
+
+  // like jar:jar:file://path/to/apk/base.apk!/
+  const nsDependentCSubstring& withoutPath = Substring(start, iter);
+  NS_ENSURE_TRUE(withoutPath.Length() >= 4, NS_ERROR_UNEXPECTED);
+
+  // Let's make sure we're removing what we expect to remove
+  NS_ENSURE_TRUE(Substring(withoutPath, 0, 4).EqualsLiteral("jar:"),
+                 NS_ERROR_UNEXPECTED);
+
+  // like jar:file://path/to/apk/base.apk!/
+  aResult = ToNewCString(Substring(withoutPath, 4));
+
+  // Remove the trailing /
+  NS_ENSURE_TRUE(aResult.Length() >= 1, NS_ERROR_UNEXPECTED);
+  aResult.Truncate(aResult.Length() - 1);
+  return NS_OK;
+}
+#endif
+
+//----------------------------------------------------------------------------
+// nsResProtocolHandler::nsISupports
+//----------------------------------------------------------------------------
+
+NS_IMPL_QUERY_INTERFACE(nsResProtocolHandler, nsIResProtocolHandler,
+                        nsISubstitutingProtocolHandler, nsIProtocolHandler,
+                        nsISupportsWeakReference)
+NS_IMPL_ADDREF_INHERITED(nsResProtocolHandler, SubstitutingProtocolHandler)
+NS_IMPL_RELEASE_INHERITED(nsResProtocolHandler, SubstitutingProtocolHandler)
+
+NS_IMETHODIMP
+nsResProtocolHandler::AllowContentToAccess(nsIURI* aURI, bool* aResult) {
+  *aResult = false;
+
+  nsAutoCString host;
+  nsresult rv = aURI->GetAsciiHost(host);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  uint32_t flags;
+  rv = GetSubstitutionFlags(host, &flags);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  *aResult = flags & nsISubstitutingProtocolHandler::ALLOW_CONTENT_ACCESS;
+  return NS_OK;
+}
+
+nsresult nsResProtocolHandler::GetSubstitutionInternal(const nsACString& aRoot,
+                                                       nsIURI** aResult,
+                                                       uint32_t* aFlags) {
+  nsAutoCString uri;
+
+  if (!ResolveSpecialCases(aRoot, "/"_ns, "/"_ns, uri)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  if (aRoot.Equals(kAndroid)) {
+    *aFlags = nsISubstitutingProtocolHandler::RESOLVE_JAR_URI;
+  } else {
+    *aFlags = 0;  // No content access.
+  }
+  return NS_NewURI(aResult, uri);
+}
+
+bool nsResProtocolHandler::ResolveSpecialCases(const nsACString& aHost,
+                                               const nsACString& aPath,
+                                               const nsACString& aPathname,
+                                               nsACString& aResult) {
+  if (aHost.EqualsLiteral("") || aHost.EqualsLiteral(kAPP)) {
+    aResult.Assign(mAppURI);
+  } else if (aHost.Equals(kGRE)) {
+    aResult.Assign(mGREURI);
+#ifdef ANDROID
+  } else if (aHost.Equals(kAndroid)) {
+    aResult.Assign(mApkURI);
+#endif
+  } else {
+    return false;
+  }
+  aResult.Append(aPath);
+  return true;
+}
+
+nsresult nsResProtocolHandler::SetSubstitution(const nsACString& aRoot,
+                                               nsIURI* aBaseURI) {
+  MOZ_ASSERT(!aRoot.EqualsLiteral(""));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kAPP));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kGRE));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kAndroid));
+  return SubstitutingProtocolHandler::SetSubstitution(aRoot, aBaseURI);
+}
+
+nsresult nsResProtocolHandler::SetSubstitutionWithFlags(const nsACString& aRoot,
+                                                        nsIURI* aBaseURI,
+                                                        uint32_t aFlags) {
+  MOZ_ASSERT(!aRoot.EqualsLiteral(""));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kAPP));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kGRE));
+  MOZ_ASSERT(!aRoot.EqualsLiteral(kAndroid));
+  return SubstitutingProtocolHandler::SetSubstitutionWithFlags(aRoot, aBaseURI,
+                                                               aFlags);
+}
+
+nsresult nsResProtocolHandler::HasSubstitution(const nsACString& aRoot,
+                                               bool* aResult) {
+  if (aRoot.EqualsLiteral(kAPP) || aRoot.EqualsLiteral(kGRE)
+#ifdef ANDROID
+      || aRoot.EqualsLiteral(kAndroid)
+#endif
+  ) {
+    *aResult = true;
+    return NS_OK;
+  }
+
+  return mozilla::net::SubstitutingProtocolHandler::HasSubstitution(aRoot,
+                                                                    aResult);
+}
diff --git a/netwerk/protocol/res/nsResProtocolHandler.h b/netwerk/protocol/res/nsResProtocolHandler.h
new file mode 100644
index 0000000000..50e790a53a
--- /dev/null
+++ b/netwerk/protocol/res/nsResProtocolHandler.h
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsResProtocolHandler_h___
+#define nsResProtocolHandler_h___
+
+#include "mozilla/net/SubstitutingProtocolHandler.h"
+
+#include "nsIResProtocolHandler.h"
+#include "nsInterfaceHashtable.h"
+#include "nsWeakReference.h"
+
+struct SubstitutionMapping;
+class nsResProtocolHandler final
+    : public nsIResProtocolHandler,
+      public mozilla::net::SubstitutingProtocolHandler,
+      public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIRESPROTOCOLHANDLER
+
+  static already_AddRefed<nsResProtocolHandler> GetSingleton();
+
+  NS_FORWARD_NSIPROTOCOLHANDLER(mozilla::net::SubstitutingProtocolHandler::)
+
+  nsResProtocolHandler()
+      : mozilla::net::SubstitutingProtocolHandler(
+            "resource",
+            /* aEnforceFileOrJar = */ false) {}
+
+  NS_IMETHOD SetSubstitution(const nsACString& aRoot,
+                             nsIURI* aBaseURI) override;
+  NS_IMETHOD SetSubstitutionWithFlags(const nsACString& aRoot, nsIURI* aBaseURI,
+                                      uint32_t aFlags) override;
+  NS_IMETHOD HasSubstitution(const nsACString& aRoot, bool* aResult) override;
+
+  NS_IMETHOD GetSubstitution(const nsACString& aRoot,
+                             nsIURI** aResult) override {
+    return mozilla::net::SubstitutingProtocolHandler::GetSubstitution(aRoot,
+                                                                      aResult);
+  }
+
+  NS_IMETHOD ResolveURI(nsIURI* aResURI, nsACString& aResult) override {
+    return mozilla::net::SubstitutingProtocolHandler::ResolveURI(aResURI,
+                                                                 aResult);
+  }
+
+ protected:
+  [[nodiscard]] nsresult GetSubstitutionInternal(const nsACString& aRoot,
+                                                 nsIURI** aResult,
+                                                 uint32_t* aFlags) override;
+  virtual ~nsResProtocolHandler() = default;
+
+  [[nodiscard]] bool ResolveSpecialCases(const nsACString& aHost,
+                                         const nsACString& aPath,
+                                         const nsACString& aPathname,
+                                         nsACString& aResult) override;
+
+  [[nodiscard]] virtual bool MustResolveJAR(const nsACString& aRoot) override {
+    return aRoot.EqualsLiteral("android");
+  }
+
+ private:
+  [[nodiscard]] nsresult Init();
+  static mozilla::StaticRefPtr<nsResProtocolHandler> sSingleton;
+
+  nsCString mAppURI;
+  nsCString mGREURI;
+#ifdef ANDROID
+  // Used for resource://android URIs
+  nsCString mApkURI;
+  nsresult GetApkURI(nsACString& aResult);
+#endif
+};
+
+#endif /* nsResProtocolHandler_h___ */