--- a/dom/workers/ServiceWorkerClients.cpp
+++ b/dom/workers/ServiceWorkerClients.cpp
@@ -3,16 +3,17 @@
 /* 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 "ServiceWorkerClients.h"
 
 #include "mozilla/dom/Promise.h"
 #include "mozilla/dom/PromiseWorkerProxy.h"
+#include "mozilla/JSObjectHolder.h"
 
 #include "ServiceWorkerClient.h"
 #include "ServiceWorkerManager.h"
 #include "ServiceWorkerPrivate.h"
 #include "ServiceWorkerWindowClient.h"
 
 #include "WorkerPrivate.h"
 #include "WorkerRunnable.h"
@@ -665,16 +666,19 @@ private:
   }
 
   nsresult
   OpenWindow(nsPIDOMWindowOuter** aWindow)
   {
     MOZ_DIAGNOSTIC_ASSERT(aWindow);
     WorkerPrivate* workerPrivate = mPromiseProxy->GetWorkerPrivate();
 
+    nsCOMPtr<nsIPrincipal> triggeringPrincipal = workerPrivate->GetPrincipal();
+    MOZ_DIAGNOSTIC_ASSERT(triggeringPrincipal);
+
     // [[1. Let url be the result of parsing url with entry settings object's API
     //   base URL.]]
     nsCOMPtr<nsIURI> uri;
     WorkerPrivate::LocationInfo& info = workerPrivate->GetLocationInfo();
 
     nsCOMPtr<nsIURI> baseURI;
     nsresult rv = NS_NewURI(getter_AddRefs(baseURI), info.mHref);
     if (NS_WARN_IF(NS_FAILED(rv))) {
@@ -689,16 +693,35 @@ private:
     // [[6.1 Open Window]]
     nsCOMPtr<nsIWindowMediator> wm = do_GetService(NS_WINDOWMEDIATOR_CONTRACTID,
                                                    &rv);
     if (NS_WARN_IF(NS_FAILED(rv))) {
       return rv;
     }
 
     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_ASSERT(xpc, "This should never be null!");
+
+      JS::Rooted<JSObject*> sandbox(cx);
+      rv = xpc->CreateSandbox(cx, triggeringPrincipal, sandbox.address());
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        return rv;
+      }
+
+      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);
@@ -749,19 +772,16 @@ private:
 
     nsCOMPtr<nsIBrowserDOMWindow> bwin;
     chromeWin->GetBrowserDOMWindow(getter_AddRefs(bwin));
 
     if (NS_WARN_IF(!bwin)) {
       return NS_ERROR_FAILURE;
     }
 
-    nsCOMPtr<nsIPrincipal> triggeringPrincipal = workerPrivate->GetPrincipal();
-    MOZ_DIAGNOSTIC_ASSERT(triggeringPrincipal);
-
     nsCOMPtr<mozIDOMWindowProxy> win;
     rv = bwin->OpenURI(uri, nullptr,
                        nsIBrowserDOMWindow::OPEN_DEFAULTWINDOW,
                        nsIBrowserDOMWindow::OPEN_NEW,
                        triggeringPrincipal,
                        getter_AddRefs(win));
     if (NS_WARN_IF(NS_FAILED(rv))) {
       return rv;