--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -8751,16 +8751,35 @@ nsContentUtils::GetReferrerPolicyFromHea
       referrerPolicy = policy;
     }
   }
   return referrerPolicy;
 }
 
 // static
 bool
+nsContentUtils::PromiseRejectionEventsEnabled(JSContext* aCx, JSObject* aObj)
+{
+  if (NS_IsMainThread()) {
+    return Preferences::GetBool("dom.promise_rejection_events.enabled", false);
+  }
+
+  using namespace workers;
+
+  // Otherwise, check the pref via the WorkerPrivate
+  WorkerPrivate* workerPrivate = GetWorkerPrivateFromContext(aCx);
+  if (!workerPrivate) {
+    return false;
+  }
+
+  return workerPrivate->PromiseRejectionEventsEnabled();
+}
+
+// static
+bool
 nsContentUtils::PushEnabled(JSContext* aCx, JSObject* aObj)
 {
   if (NS_IsMainThread()) {
     return Preferences::GetBool("dom.push.enabled", false);
   }
 
   using namespace workers;
 

--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2710,25 +2710,27 @@ public:
    *
    * https://w3c.github.io/webappsec/specs/referrer-policy/#determine-requests-referrer
    */
   static nsresult SetFetchReferrerURIWithPolicy(nsIPrincipal* aPrincipal,
                                                 nsIDocument* aDoc,
                                                 nsIHttpChannel* aChannel,
                                                 mozilla::net::ReferrerPolicy aReferrerPolicy);
 
-    /*
+  /*
    * Parse a referrer policy from a Referrer-Policy header
    * https://www.w3.org/TR/referrer-policy/#parse-referrer-policy-from-header
    *
    * @param aHeader the response's Referrer-Policy header to parse
    * @return referrer policy from the response header.
    */
   static mozilla::net::ReferrerPolicy GetReferrerPolicyFromHeader(const nsAString& aHeader);
 
+  static bool PromiseRejectionEventsEnabled(JSContext* aCx, JSObject* aObj);
+
   static bool PushEnabled(JSContext* aCx, JSObject* aObj);
 
   static bool IsNonSubresourceRequest(nsIChannel* aChannel);
 
   static uint32_t CookiesBehavior()
   {
     return sCookiesBehavior;
   }

new file mode 100644
--- /dev/null
+++ b/dom/webidl/PromiseRejectionEvent.webidl
@@ -0,0 +1,20 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+[Constructor(DOMString type, PromiseRejectionEventInit eventInitDict),
+ Exposed=(Window,Worker),
+ Func="nsContentUtils::PromiseRejectionEventsEnabled"]
+interface PromiseRejectionEvent : Event
+{
+  [BinaryName="rejectedPromise"]
+  readonly attribute Promise<any> promise;
+  readonly attribute any reason;
+};
+
+dictionary PromiseRejectionEventInit : EventInit {
+  required Promise<any> promise;
+           any reason;
+};

--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -1077,16 +1077,17 @@ GENERATED_EVENTS_WEBIDL_FILES = [
     'PageTransitionEvent.webidl',
     'PerformanceEntryEvent.webidl',
     'PluginCrashedEvent.webidl',
     'PopStateEvent.webidl',
     'PopupBlockedEvent.webidl',
     'PresentationConnectionAvailableEvent.webidl',
     'PresentationConnectionCloseEvent.webidl',
     'ProgressEvent.webidl',
+    'PromiseRejectionEvent.webidl',
     'RecordErrorEvent.webidl',
     'ScrollViewChangeEvent.webidl',
     'StyleRuleChangeEvent.webidl',
     'StyleSheetApplicableStateChangeEvent.webidl',
     'StyleSheetChangeEvent.webidl',
     'TCPServerSocketEvent.webidl',
     'TCPSocketErrorEvent.webidl',
     'TCPSocketEvent.webidl',

--- a/dom/workers/WorkerPrefs.h
+++ b/dom/workers/WorkerPrefs.h
@@ -30,16 +30,17 @@ WORKER_SIMPLE_PREF("dom.caches.testing.e
 WORKER_SIMPLE_PREF("dom.performance.enable_user_timing_logging", PerformanceLoggingEnabled, PERFORMANCE_LOGGING_ENABLED)
 WORKER_SIMPLE_PREF("dom.webnotifications.enabled", DOMWorkerNotificationEnabled, DOM_WORKERNOTIFICATION)
 WORKER_SIMPLE_PREF("dom.webnotifications.serviceworker.enabled", DOMServiceWorkerNotificationEnabled, DOM_SERVICEWORKERNOTIFICATION)
 WORKER_SIMPLE_PREF("dom.webnotifications.requireinteraction.enabled", DOMWorkerNotificationRIEnabled, DOM_WORKERNOTIFICATIONRI)
 WORKER_SIMPLE_PREF("dom.serviceWorkers.enabled", ServiceWorkersEnabled, SERVICEWORKERS_ENABLED)
 WORKER_SIMPLE_PREF("dom.serviceWorkers.testing.enabled", ServiceWorkersTestingEnabled, SERVICEWORKERS_TESTING_ENABLED)
 WORKER_SIMPLE_PREF("dom.serviceWorkers.openWindow.enabled", OpenWindowEnabled, OPEN_WINDOW_ENABLED)
 WORKER_SIMPLE_PREF("dom.storageManager.enabled", StorageManagerEnabled, STORAGEMANAGER_ENABLED)
+WORKER_SIMPLE_PREF("dom.promise_rejection_events.enabled", PromiseRejectionEventsEnabled, PROMISE_REJECTION_EVENTS_ENABLED)
 WORKER_SIMPLE_PREF("dom.push.enabled", PushEnabled, PUSH_ENABLED)
 WORKER_SIMPLE_PREF("dom.requestcontext.enabled", RequestContextEnabled, REQUESTCONTEXT_ENABLED)
 WORKER_SIMPLE_PREF("gfx.offscreencanvas.enabled", OffscreenCanvasEnabled, OFFSCREENCANVAS_ENABLED)
 WORKER_SIMPLE_PREF("dom.webkitBlink.dirPicker.enabled", WebkitBlinkDirectoryPickerEnabled, DOM_WEBKITBLINK_DIRPICKER_WEBKITBLINK)
 WORKER_SIMPLE_PREF("dom.netinfo.enabled", NetworkInformationEnabled, NETWORKINFORMATION_ENABLED)
 WORKER_SIMPLE_PREF("dom.fetchController.enabled", FetchControllerEnabled, FETCHCONTROLLER_ENABLED)
 WORKER_SIMPLE_PREF("dom.fetchObserver.enabled", FetchObserverEnabled, FETCHOBSERVER_ENABLED)
 WORKER_PREF("intl.accept_languages", PrefLanguagesChanged)

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4940,16 +4940,21 @@ pref("dom.w3c_pointer_events.enabled", f
 // messages.
 #if defined(XP_WIN)
 pref("dom.w3c_pointer_events.dispatch_by_pointer_messages", false);
 #endif
 
 // W3C pointer events draft
 pref("dom.w3c_pointer_events.implicit_capture", false);
 
+// WHATWG promise rejection events. See
+// https://html.spec.whatwg.org/multipage/webappapis.html#promiserejectionevent
+// TODO: Enable the event interface once actually firing it (bug 1362272).
+pref("dom.promise_rejection_events.enabled", false);
+
 // W3C draft ImageCapture API
 pref("dom.imagecapture.enabled", false);
 
 // W3C MediaDevices devicechange event
 pref("media.ondevicechange.enabled", true);
 
 // W3C MediaDevices devicechange fake event
 pref("media.ondevicechange.fakeDeviceChangeEvent.enabled", false);