diff --git a/dom/clients/manager/ClientIPCTypes.ipdlh b/dom/clients/manager/ClientIPCTypes.ipdlh
index 39aa310b257d..5a439fa782ab 100644
--- a/dom/clients/manager/ClientIPCTypes.ipdlh
+++ b/dom/clients/manager/ClientIPCTypes.ipdlh
@@ -100,6 +100,9 @@ struct ClientGetInfoAndStateArgs
 
 struct ClientOpenWindowArgs
 {
+  PrincipalInfo principalInfo;
+  nsCString url;
+  nsCString baseURL;
 };
 
 union ClientOpConstructorArgs
@@ -109,6 +112,7 @@ union ClientOpConstructorArgs
   ClientMatchAllArgs;
   ClientClaimArgs;
   ClientGetInfoAndStateArgs;
+  ClientOpenWindowArgs;
 };
 
 struct ClientList
diff --git a/dom/clients/manager/ClientManager.cpp b/dom/clients/manager/ClientManager.cpp
index f366608c28fc..a66d2aa92568 100644
--- a/dom/clients/manager/ClientManager.cpp
+++ b/dom/clients/manager/ClientManager.cpp
@@ -293,5 +293,14 @@ ClientManager::Navigate(const ClientNavigateArgs& aArgs,
   return mgr->StartOp(aArgs, aSerialEventTarget);
 }
 
+// static
+RefPtr<ClientOpPromise>
+ClientManager::OpenWindow(const ClientOpenWindowArgs& aArgs,
+                          nsISerialEventTarget* aSerialEventTarget)
+{
+  RefPtr<ClientManager> mgr = GetOrCreateForCurrentThread();
+  return mgr->StartOp(aArgs, aSerialEventTarget);
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/clients/manager/ClientManager.h b/dom/clients/manager/ClientManager.h
index 0c9bdaa18f27..b6ae1144e0a9 100644
--- a/dom/clients/manager/ClientManager.h
+++ b/dom/clients/manager/ClientManager.h
@@ -26,6 +26,7 @@ class ClientManagerChild;
 class ClientMatchAllArgs;
 class ClientNavigateArgs;
 class ClientOpConstructorArgs;
+class ClientOpenWindowArgs;
 class ClientSource;
 enum class ClientType : uint8_t;
 
@@ -111,6 +112,10 @@ public:
   Navigate(const ClientNavigateArgs& aArgs,
            nsISerialEventTarget* aSerialEventTarget);
 
+  static RefPtr<ClientOpPromise>
+  OpenWindow(const ClientOpenWindowArgs& aArgs,
+             nsISerialEventTarget* aSerialEventTarget);
+
   NS_INLINE_DECL_REFCOUNTING(mozilla::dom::ClientManager)
 };
 
diff --git a/dom/clients/manager/ClientManagerOpParent.cpp b/dom/clients/manager/ClientManagerOpParent.cpp
index e7de7ed3a680..108effafe779 100644
--- a/dom/clients/manager/ClientManagerOpParent.cpp
+++ b/dom/clients/manager/ClientManagerOpParent.cpp
@@ -7,10 +7,13 @@
 #include "ClientManagerOpParent.h"
 
 #include "ClientManagerService.h"
+#include "mozilla/ipc/BackgroundParent.h"
 
 namespace mozilla {
 namespace dom {
 
+using mozilla::ipc::BackgroundParent;
+
 template <typename Method, typename... Args>
 void
 ClientManagerOpParent::DoServiceOp(Method aMethod, Args&&... aArgs)
@@ -71,6 +74,14 @@ ClientManagerOpParent::Init(const ClientOpConstructorArgs& aArgs)
                   aArgs.get_ClientGetInfoAndStateArgs());
       break;
     }
+    case ClientOpConstructorArgs::TClientOpenWindowArgs:
+    {
+      RefPtr<ContentParent> contentParent =
+        BackgroundParent::GetContentParent(Manager()->Manager());
+      DoServiceOp(&ClientManagerService::OpenWindow,
+                  aArgs.get_ClientOpenWindowArgs(), contentParent.forget());
+      break;
+    }
     default:
     {
       MOZ_ASSERT_UNREACHABLE("Unknown Client operation!");
diff --git a/dom/clients/manager/ClientManagerService.cpp b/dom/clients/manager/ClientManagerService.cpp
index f81865e4f904..953b254ca4a9 100644
--- a/dom/clients/manager/ClientManagerService.cpp
+++ b/dom/clients/manager/ClientManagerService.cpp
@@ -8,12 +8,17 @@
 
 #include "ClientManagerParent.h"
 #include "ClientNavigateOpParent.h"
+#include "ClientOpenWindowOpParent.h"
+#include "ClientOpenWindowUtils.h"
 #include "ClientSourceParent.h"
+#include "mozilla/dom/ContentParent.h"
 #include "mozilla/ipc/BackgroundParent.h"
 #include "mozilla/ipc/PBackgroundSharedTypes.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/SystemGroup.h"
 #include "nsIAsyncShutdown.h"
+#include "nsIXULRuntime.h"
+#include "nsProxyRelease.h"
 
 namespace mozilla {
 namespace dom {
@@ -551,5 +556,90 @@ ClientManagerService::GetInfoAndState(const ClientGetInfoAndStateArgs& aArgs)
   return source->StartOp(aArgs);
 }
 
+namespace {
+
+class OpenWindowRunnable final : public Runnable
+{
+  RefPtr<ClientOpPromise::Private> mPromise;
+  const ClientOpenWindowArgs mArgs;
+  RefPtr<ContentParent> mSourceProcess;
+
+  ~OpenWindowRunnable()
+  {
+    NS_ReleaseOnMainThreadSystemGroup(mSourceProcess.forget());
+  }
+
+public:
+  OpenWindowRunnable(ClientOpPromise::Private* aPromise,
+                     const ClientOpenWindowArgs& aArgs,
+                     already_AddRefed<ContentParent> aSourceProcess)
+    : Runnable("ClientManagerService::OpenWindowRunnable")
+    , mPromise(aPromise)
+    , mArgs(aArgs)
+    , mSourceProcess(aSourceProcess)
+  {
+    MOZ_DIAGNOSTIC_ASSERT(mPromise);
+  }
+
+  NS_IMETHOD
+  Run() override
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+
+    if (!BrowserTabsRemoteAutostart()) {
+      RefPtr<ClientOpPromise> p = ClientOpenWindowInCurrentProcess(mArgs);
+      p->ChainTo(mPromise.forget(), __func__);
+      return NS_OK;
+    }
+
+    RefPtr<ContentParent> targetProcess;
+
+    // Possibly try to open the window in the same process that called
+    // openWindow().  This is a temporary compat setting until the
+    // multi-e10s service worker refactor is complete.
+    if (Preferences::GetBool("dom.clients.openwindow_favors_same_process",
+                             false)) {
+      targetProcess = mSourceProcess;
+    }
+
+    // Otherwise, use our normal remote process selection mechanism for
+    // opening the window.  This will start a process if one is not
+    // present.
+    if (!targetProcess) {
+      targetProcess =
+        ContentParent::GetNewOrUsedBrowserProcess(NS_LITERAL_STRING(DEFAULT_REMOTE_TYPE),
+                                                  ContentParent::GetInitialProcessPriority(nullptr),
+                                                  nullptr);
+    }
+
+    ClientOpenWindowOpParent* actor =
+      new ClientOpenWindowOpParent(mArgs, mPromise);
+
+    // If this fails the actor will be automatically destroyed which will
+    // reject the promise.
+    Unused << targetProcess->SendPClientOpenWindowOpConstructor(actor, mArgs);
+
+    return NS_OK;
+  }
+};
+
+} // anonymous namespace
+
+RefPtr<ClientOpPromise>
+ClientManagerService::OpenWindow(const ClientOpenWindowArgs& aArgs,
+                                 already_AddRefed<ContentParent> aSourceProcess)
+{
+  RefPtr<ClientOpPromise::Private> promise =
+    new ClientOpPromise::Private(__func__);
+
+  nsCOMPtr<nsIRunnable> r = new OpenWindowRunnable(promise, aArgs,
+                                                   Move(aSourceProcess));
+  MOZ_ALWAYS_SUCCEEDS(SystemGroup::Dispatch(TaskCategory::Other,
+                                            r.forget()));
+
+  RefPtr<ClientOpPromise> ref = promise;
+  return ref.forget();
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/clients/manager/ClientManagerService.h b/dom/clients/manager/ClientManagerService.h
index 340ecc14a886..825900701a56 100644
--- a/dom/clients/manager/ClientManagerService.h
+++ b/dom/clients/manager/ClientManagerService.h
@@ -71,6 +71,10 @@ public:
   RefPtr<ClientOpPromise>
   GetInfoAndState(const ClientGetInfoAndStateArgs& aArgs);
 
+  RefPtr<ClientOpPromise>
+  OpenWindow(const ClientOpenWindowArgs& aArgs,
+             already_AddRefed<ContentParent> aSourceProcess);
+
   NS_INLINE_DECL_REFCOUNTING(mozilla::dom::ClientManagerService)
 };
 
diff --git a/dom/clients/manager/ClientOpenWindowOpChild.cpp b/dom/clients/manager/ClientOpenWindowOpChild.cpp
index c1cfaef62b0a..84b4c7c5b064 100644
--- a/dom/clients/manager/ClientOpenWindowOpChild.cpp
+++ b/dom/clients/manager/ClientOpenWindowOpChild.cpp
@@ -5,18 +5,38 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "ClientOpenWindowOpChild.h"
+#include "ClientOpenWindowUtils.h"
+#include "mozilla/SystemGroup.h"
 
 namespace mozilla {
 namespace dom {
 
+already_AddRefed<ClientOpPromise>
+ClientOpenWindowOpChild::DoOpenWindow(const ClientOpenWindowArgs& aArgs)
+{
+  RefPtr<ClientOpPromise> ref =
+    ClientOpenWindowInCurrentProcess(aArgs);
+  return ref.forget();
+}
+
 void
 ClientOpenWindowOpChild::ActorDestroy(ActorDestroyReason aReason)
 {
+  mPromiseRequestHolder.DisconnectIfExists();
 }
 
 void
 ClientOpenWindowOpChild::Init(const ClientOpenWindowArgs& aArgs)
 {
+  RefPtr<ClientOpPromise> promise = DoOpenWindow(aArgs);
+  promise->Then(SystemGroup::EventTargetFor(TaskCategory::Other), __func__,
+    [this] (const ClientOpResult& aResult) {
+      mPromiseRequestHolder.Complete();
+      PClientOpenWindowOpChild::Send__delete__(this, aResult);
+    }, [this] (nsresult aResult) {
+      mPromiseRequestHolder.Complete();
+      PClientOpenWindowOpChild::Send__delete__(this, aResult);
+  })->Track(mPromiseRequestHolder);
 }
 
 } // namespace dom
diff --git a/dom/clients/manager/ClientOpenWindowOpChild.h b/dom/clients/manager/ClientOpenWindowOpChild.h
index 3aaf79fbdb9d..acc021ea5312 100644
--- a/dom/clients/manager/ClientOpenWindowOpChild.h
+++ b/dom/clients/manager/ClientOpenWindowOpChild.h
@@ -7,12 +7,18 @@
 #define _mozilla_dom_ClientOpenWindowOpChild_h
 
 #include "mozilla/dom/PClientOpenWindowOpChild.h"
+#include "ClientOpPromise.h"
 
 namespace mozilla {
 namespace dom {
 
 class ClientOpenWindowOpChild final : public PClientOpenWindowOpChild
 {
+  MozPromiseRequestHolder<ClientOpPromise> mPromiseRequestHolder;
+
+  already_AddRefed<ClientOpPromise>
+  DoOpenWindow(const ClientOpenWindowArgs& aArgs);
+
   // PClientOpenWindowOpChild interface
   void
   ActorDestroy(ActorDestroyReason aReason) override;
diff --git a/dom/clients/manager/ClientOpenWindowUtils.cpp b/dom/clients/manager/ClientOpenWindowUtils.cpp
new file mode 100644
index 000000000000..055ca1373985
--- /dev/null
+++ b/dom/clients/manager/ClientOpenWindowUtils.cpp
@@ -0,0 +1,463 @@
+/* -*- 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 "ClientOpenWindowUtils.h"
+
+#include "ClientInfo.h"
+#include "ClientState.h"
+#include "nsContentUtils.h"
+#include "nsIBrowserDOMWindow.h"
+#include "nsIDocShell.h"
+#include "nsIDOMChromeWindow.h"
+#include "nsIURI.h"
+#include "nsIWebProgress.h"
+#include "nsIWebProgressListener.h"
+#include "nsIWindowWatcher.h"
+#include "nsNetUtil.h"
+#include "nsPIDOMWindow.h"
+#include "nsPIWindowWatcher.h"
+
+#ifdef MOZ_WIDGET_ANDROID
+#include "FennecJNIWrappers.h"
+#endif
+
+namespace mozilla {
+namespace dom {
+
+namespace {
+
+class WebProgressListener final : public nsIWebProgressListener
+                                , public nsSupportsWeakReference
+{
+public:
+  NS_DECL_ISUPPORTS
+
+  WebProgressListener(nsPIDOMWindowOuter* aWindow,
+                      nsIURI* aBaseURI,
+                      already_AddRefed<ClientOpPromise::Private> aPromise)
+  : mPromise(aPromise)
+  , mWindow(aWindow)
+  , mBaseURI(aBaseURI)
+  {
+    MOZ_ASSERT(aWindow);
+    MOZ_ASSERT(aBaseURI);
+    MOZ_ASSERT(NS_IsMainThread());
+  }
+
+  NS_IMETHOD
+  OnStateChange(nsIWebProgress* aWebProgress,
+                nsIRequest* aRequest,
+                uint32_t aStateFlags, nsresult aStatus) override
+  {
+    if (!(aStateFlags & STATE_IS_DOCUMENT) ||
+         !(aStateFlags & (STATE_STOP | STATE_TRANSFERRING))) {
+      return NS_OK;
+    }
+
+    // Our caller keeps a strong reference, so it is safe to remove the listener
+    // from ServiceWorkerPrivate.
+    aWebProgress->RemoveProgressListener(this);
+
+    nsCOMPtr<nsIDocument> doc = mWindow->GetExtantDoc();
+    if (NS_WARN_IF(!doc)) {
+      mPromise->Reject(NS_ERROR_FAILURE, __func__);
+      mPromise = nullptr;
+      return NS_OK;
+    }
+
+    // Check same origin.
+    nsCOMPtr<nsIScriptSecurityManager> securityManager =
+      nsContentUtils::GetSecurityManager();
+    nsresult rv = securityManager->CheckSameOriginURI(doc->GetOriginalURI(),
+                                                      mBaseURI, false);
+    if (NS_FAILED(rv)) {
+      mPromise->Resolve(NS_OK, __func__);
+      mPromise = nullptr;
+      return NS_OK;
+    }
+
+    nsPIDOMWindowInner* innerWindow = doc->GetInnerWindow();
+    if (NS_WARN_IF(!innerWindow)) {
+      mPromise->Reject(NS_ERROR_FAILURE, __func__);
+      mPromise = nullptr;
+      return NS_OK;
+    }
+
+    Maybe<ClientInfo> info = innerWindow->GetClientInfo();
+    Maybe<ClientState> state = innerWindow->GetClientState();
+
+    if (NS_WARN_IF(info.isNothing() || state.isNothing())) {
+      mPromise->Reject(NS_ERROR_FAILURE, __func__);
+      mPromise = nullptr;
+      return NS_OK;
+    }
+
+    mPromise->Resolve(ClientInfoAndState(info.ref().ToIPC(), state.ref().ToIPC()),
+                      __func__);
+    mPromise = nullptr;
+
+    return NS_OK;
+  }
+
+  NS_IMETHOD
+  OnProgressChange(nsIWebProgress* aWebProgress,
+                   nsIRequest* aRequest,
+                   int32_t aCurSelfProgress,
+                   int32_t aMaxSelfProgress,
+                   int32_t aCurTotalProgress,
+                   int32_t aMaxTotalProgress) override
+  {
+    MOZ_ASSERT(false, "Unexpected notification.");
+    return NS_OK;
+  }
+
+  NS_IMETHOD
+  OnLocationChange(nsIWebProgress* aWebProgress,
+                   nsIRequest* aRequest,
+                   nsIURI* aLocation,
+                   uint32_t aFlags) override
+  {
+    MOZ_ASSERT(false, "Unexpected notification.");
+    return NS_OK;
+  }
+
+  NS_IMETHOD
+  OnStatusChange(nsIWebProgress* aWebProgress,
+                 nsIRequest* aRequest,
+                 nsresult aStatus, const char16_t* aMessage) override
+  {
+    MOZ_ASSERT(false, "Unexpected notification.");
+    return NS_OK;
+  }
+
+  NS_IMETHOD
+  OnSecurityChange(nsIWebProgress* aWebProgress,
+                   nsIRequest* aRequest,
+                   uint32_t aState) override
+  {
+    MOZ_ASSERT(false, "Unexpected notification.");
+    return NS_OK;
+  }
+
+private:
+  ~WebProgressListener()
+  {
+    if (mPromise) {
+      mPromise->Reject(NS_ERROR_ABORT, __func__);
+      mPromise = nullptr;
+    }
+  }
+
+  RefPtr<ClientOpPromise::Private> mPromise;
+  // TODO: make window a weak ref and stop cycle collecting
+  nsCOMPtr<nsPIDOMWindowOuter> mWindow;
+  nsCOMPtr<nsIURI> mBaseURI;
+};
+
+NS_IMPL_ISUPPORTS(WebProgressListener, nsIWebProgressListener,
+                                       nsISupportsWeakReference);
+
+nsresult
+OpenWindow(const ClientOpenWindowArgs& aArgs,
+           nsPIDOMWindowOuter** aWindow)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aWindow);
+
+  // [[1. Let url be the result of parsing url with entry settings object's API
+  //   base URL.]]
+  nsCOMPtr<nsIURI> uri;
+
+  nsCOMPtr<nsIURI> baseURI;
+  nsresult rv = NS_NewURI(getter_AddRefs(baseURI), aArgs.baseURL());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return NS_ERROR_TYPE_ERR;
+  }
+
+  rv = NS_NewURI(getter_AddRefs(uri), aArgs.url(), nullptr, baseURI);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return NS_ERROR_TYPE_ERR;
+  }
+
+  nsCOMPtr<nsIPrincipal> principal =
+    PrincipalInfoToPrincipal(aArgs.principalInfo());
+  MOZ_DIAGNOSTIC_ASSERT(principal);
+
+  // [[6.1 Open Window]]
+  if (XRE_IsContentProcess()) {
+
+    // Let's create a sandbox in order to have a valid JSContext and correctly
+    // propagate the SubjectPrincipal.
+    AutoJSAPI jsapi;
+    jsapi.Init();
+
+    JSContext* cx = jsapi.cx();
+
+    nsIXPConnect* xpc = nsContentUtils::XPConnect();
+    MOZ_DIAGNOSTIC_ASSERT(xpc);
+
+    JS::Rooted<JSObject*> sandbox(cx);
+    rv = xpc->CreateSandbox(cx, principal, sandbox.address());
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return NS_ERROR_TYPE_ERR;
+    }
+
+    JSAutoCompartment ac(cx, sandbox);
+
+    // ContentProcess
+    nsCOMPtr<nsIWindowWatcher> wwatch =
+      do_GetService(NS_WINDOWWATCHER_CONTRACTID, &rv);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+    nsCOMPtr<nsPIWindowWatcher> pwwatch(do_QueryInterface(wwatch));
+    NS_ENSURE_STATE(pwwatch);
+
+    nsCString spec;
+    rv = uri->GetSpec(spec);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    nsCOMPtr<mozIDOMWindowProxy> newWindow;
+    rv = pwwatch->OpenWindow2(nullptr,
+                              spec.get(),
+                              nullptr,
+                              nullptr,
+                              false, false, true, nullptr,
+                              // Not a spammy popup; we got permission, we swear!
+                              /* aIsPopupSpam = */ false,
+                              // Don't force noopener.  We're not passing in an
+                              // opener anyway, and we _do_ want the returned
+                              // window.
+                              /* aForceNoOpener = */ false,
+                              /* aLoadInfp = */ nullptr,
+                              getter_AddRefs(newWindow));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+    nsCOMPtr<nsPIDOMWindowOuter> pwindow = nsPIDOMWindowOuter::From(newWindow);
+    pwindow.forget(aWindow);
+    MOZ_DIAGNOSTIC_ASSERT(*aWindow);
+    return NS_OK;
+  }
+
+  // Find the most recent browser window and open a new tab in it.
+  nsCOMPtr<nsPIDOMWindowOuter> browserWindow =
+    nsContentUtils::GetMostRecentNonPBWindow();
+  if (!browserWindow) {
+    // It is possible to be running without a browser window on Mac OS, so
+    // we need to open a new chrome window.
+    // TODO(catalinb): open new chrome window. Bug 1218080
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  nsCOMPtr<nsIDOMChromeWindow> chromeWin = do_QueryInterface(browserWindow);
+  if (NS_WARN_IF(!chromeWin)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIBrowserDOMWindow> bwin;
+  chromeWin->GetBrowserDOMWindow(getter_AddRefs(bwin));
+
+  if (NS_WARN_IF(!bwin)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<mozIDOMWindowProxy> win;
+  rv = bwin->OpenURI(uri, nullptr,
+                     nsIBrowserDOMWindow::OPEN_DEFAULTWINDOW,
+                     nsIBrowserDOMWindow::OPEN_NEW,
+                     principal,
+                     getter_AddRefs(win));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+  NS_ENSURE_STATE(win);
+
+  nsCOMPtr<nsPIDOMWindowOuter> pWin = nsPIDOMWindowOuter::From(win);
+  pWin.forget(aWindow);
+  MOZ_DIAGNOSTIC_ASSERT(*aWindow);
+
+  return NS_OK;
+}
+
+void
+WaitForLoad(const ClientOpenWindowArgs& aArgs,
+            nsPIDOMWindowOuter* aOuterWindow,
+            ClientOpPromise::Private* aPromise)
+{
+  MOZ_DIAGNOSTIC_ASSERT(aOuterWindow);
+
+  RefPtr<ClientOpPromise::Private> promise = aPromise;
+
+  nsresult rv = nsContentUtils::DispatchFocusChromeEvent(aOuterWindow);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    promise->Reject(rv, __func__);
+    return;
+  }
+
+  nsCOMPtr<nsIURI> baseURI;
+  rv = NS_NewURI(getter_AddRefs(baseURI), aArgs.baseURL());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    promise->Reject(rv, __func__);
+    return;
+  }
+
+  nsCOMPtr<nsIDocShell> docShell = aOuterWindow->GetDocShell();
+  nsCOMPtr<nsIWebProgress> webProgress = do_GetInterface(docShell);
+
+  if (NS_WARN_IF(!webProgress)) {
+    promise->Reject(NS_ERROR_FAILURE, __func__);
+    return;
+  }
+
+  RefPtr<ClientOpPromise> ref = promise;
+
+  RefPtr<WebProgressListener> listener =
+    new WebProgressListener(aOuterWindow, baseURI, promise.forget());
+
+
+  rv = webProgress->AddProgressListener(listener,
+                                        nsIWebProgress::NOTIFY_STATE_DOCUMENT);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    promise->Reject(rv, __func__);
+    return;
+  }
+
+  // Hold the listener alive until the promise settles
+  ref->Then(aOuterWindow->EventTargetFor(TaskCategory::Other), __func__,
+    [listener] (const ClientOpResult& aResult) { },
+    [listener] (nsresult aResult) { });
+}
+
+#ifdef MOZ_WIDGET_ANDROID
+
+class LaunchObserver final : public nsIObserver
+{
+  RefPtr<GenericPromise::Private> mPromise;
+
+  LaunchObserver()
+    : mPromise(new GenericPromise::Private(__func__))
+  {
+  }
+
+  ~LaunchObserver() = default;
+
+  NS_IMETHOD
+  Observe(nsISupports* aSubject, const char* aTopic, const char16_t * aData) override
+  {
+    nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "BrowserChrome:Ready");
+    }
+    mPromise->Resolve(true, __func__);
+    return NS_OK;
+  }
+
+public:
+  static already_AddRefed<LaunchObserver>
+  Create()
+  {
+    nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+    if (NS_WARN_IF(!os)) {
+      return nullptr;
+    }
+
+    RefPtr<LaunchObserver> ref = new LaunchObserver();
+
+    nsresult rv = os->AddObserver(ref, "BrowserChrome:Ready", /* weakRef */ false);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return nullptr;
+    }
+
+    return ref.forget();
+  }
+
+  void
+  Cancel()
+  {
+    nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "BrowserChrome:Ready");
+    }
+    mPromise->Reject(NS_ERROR_ABORT, __func__);
+  }
+
+  GenericPromise*
+  Promise()
+  {
+    return mPromise;
+  }
+
+  NS_DECL_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(LaunchObserver, nsIObserver);
+
+#endif // MOZ_WIDGET_ANDROID
+
+} // anonymous namespace
+
+already_AddRefed<ClientOpPromise>
+ClientOpenWindowInCurrentProcess(const ClientOpenWindowArgs& aArgs)
+{
+  RefPtr<ClientOpPromise::Private> promise =
+    new ClientOpPromise::Private(__func__);
+  RefPtr<ClientOpPromise> ref = promise;
+
+#ifdef MOZ_WIDGET_ANDROID
+  // This fires an intent that will start launching Fennec and foreground it,
+  // if necessary.  We create an observer so that we can determine when
+  // the launch has completed.
+  RefPtr<LaunchObserver> launchObserver = LaunchObserver::Create();
+  java::GeckoApp::LaunchOrBringToFront();
+#endif // MOZ_WIDGET_ANDROID
+
+  nsCOMPtr<nsPIDOMWindowOuter> outerWindow;
+  nsresult rv = OpenWindow(aArgs, getter_AddRefs(outerWindow));
+
+#ifdef MOZ_WIDGET_ANDROID
+  // If we get the NOT_AVAILABLE error that means the browser is still
+  // launching on android.  Use the observer we created above to wait
+  // until the launch completes and then try to open the window again.
+  if (rv == NS_ERROR_NOT_AVAILABLE && launchObserver) {
+    RefPtr<GenericPromise> p = launchObserver->Promise();
+    p->Then(outerWindow->EventTargetFor(TaskCategory::Other), __func__,
+      [aArgs, promise] (bool aResult) {
+        nsCOMPtr<nsPIDOMWindowOuter> outerWindow;
+        nsresult rv = OpenWindow(aArgs, getter_AddRefs(outerWindow));
+        if (NS_WARN_IF(NS_FAILED(rv))) {
+          promise->Reject(rv, __func__);
+        }
+
+        WaitForLoad(aArgs, outerWindow, promise);
+      }, [promise] (nsresult aResult) {
+        promise->Reject(aResult, __func__);
+      });
+    return ref.forget();
+  }
+
+  // If we didn't get the NOT_AVAILABLE error then there is no need
+  // wait for the browser to launch.  Cancel the observer so that it
+  // will release.
+  if (launchObserver) {
+    launchObserver->Cancel();
+  }
+#endif // MOZ_WIDGET_ANDROID
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    promise->Reject(rv, __func__);
+    return ref.forget();
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(outerWindow);
+  WaitForLoad(aArgs, outerWindow, promise);
+
+  return ref.forget();
+}
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/clients/manager/ClientOpenWindowUtils.h b/dom/clients/manager/ClientOpenWindowUtils.h
new file mode 100644
index 000000000000..5e6d4c0950ed
--- /dev/null
+++ b/dom/clients/manager/ClientOpenWindowUtils.h
@@ -0,0 +1,21 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef _mozilla_dom_ClientOpenWindowUtils_h
+#define _mozilla_dom_ClientOpenWindowUtils_h
+
+#include "ClientOpPromise.h"
+#include "mozilla/dom/ClientIPCTypes.h"
+
+namespace mozilla {
+namespace dom {
+
+already_AddRefed<ClientOpPromise>
+ClientOpenWindowInCurrentProcess(const ClientOpenWindowArgs &aArgs);
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // _mozilla_dom_ClientOpenWindowUtils_h
diff --git a/dom/clients/manager/moz.build b/dom/clients/manager/moz.build
index 3ce6e0070789..00c4a588cc80 100644
--- a/dom/clients/manager/moz.build
+++ b/dom/clients/manager/moz.build
@@ -38,6 +38,7 @@ UNIFIED_SOURCES += [
   'ClientOpenWindowOpActors.cpp',
   'ClientOpenWindowOpChild.cpp',
   'ClientOpenWindowOpParent.cpp',
+  'ClientOpenWindowUtils.cpp',
   'ClientPrefs.cpp',
   'ClientSource.cpp',
   'ClientSourceChild.cpp',
diff --git a/dom/ipc/ContentParent.h b/dom/ipc/ContentParent.h
index b3b879837f8e..3151b389e101 100644
--- a/dom/ipc/ContentParent.h
+++ b/dom/ipc/ContentParent.h
@@ -647,6 +647,8 @@ public:
   virtual bool
   DeallocPClientOpenWindowOpParent(PClientOpenWindowOpParent* aActor) override;
 
+  static hal::ProcessPriority GetInitialProcessPriority(Element* aFrameElement);
+
   // Control the priority of the IPC messages for input events.
   void SetInputPriorityEventEnabled(bool aEnabled);
   bool IsInputPriorityEventEnabled()
@@ -680,8 +682,6 @@ private:
   static nsDataHashtable<nsUint32HashKey, ContentParent*> *sJSPluginContentParents;
   static StaticAutoPtr<LinkedList<ContentParent> > sContentParents;
 
-  static hal::ProcessPriority GetInitialProcessPriority(Element* aFrameElement);
-
   static ContentBridgeParent* CreateContentBridgeParent(const TabContext& aContext,
                                                         const hal::ProcessPriority& aPriority,
                                                         const TabId& aOpenerTabId,
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 4fe5831531eb..b390e6130c43 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5843,6 +5843,13 @@ pref("layers.advanced.text-layers", 2);
 // Enable lowercased response header name
 pref("dom.xhr.lowercase_header.enabled", false);
 
+// Control whether clients.openWindow() opens windows in the same process
+// that called the API vs following our normal multi-process selection
+// algorithm.  Restricting openWindow to same process improves service worker
+// web compat in the short term.  Once the SW multi-e10s refactor is complete
+// this can be removed.
+pref("dom.clients.openwindow_favors_same_process", true);
+
 // When a crash happens, whether to include heap regions of the crash context
 // in the minidump. Enabled by default on nightly and aurora.
 #ifdef RELEASE_OR_BETA