--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -1250,16 +1250,31 @@ Event::GetShadowRelatedTarget(nsIContent
     }
 
     relatedTarget = ancestorShadow->GetHost();
   }
 
   return nullptr;
 }
 
+NS_IMETHODIMP
+Event::GetCancelBubble(bool* aCancelBubble)
+{
+  NS_ENSURE_ARG_POINTER(aCancelBubble);
+  *aCancelBubble = CancelBubble();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+Event::SetCancelBubble(bool aCancelBubble)
+{
+  mEvent->mFlags.mPropagationStopped = aCancelBubble;
+  return NS_OK;
+}
+
 } // namespace dom
 } // namespace mozilla
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 already_AddRefed<Event>
 NS_NewDOMEvent(EventTarget* aOwner,

--- a/dom/events/Event.h
+++ b/dom/events/Event.h
@@ -171,16 +171,21 @@ public:
     return mEvent->mFlags.mCancelable;
   }
 
   bool Composed() const
   {
     return mEvent->mFlags.mComposed;
   }
 
+  bool CancelBubble() const
+  {
+    return mEvent->PropagationStopped();
+  }
+
   // xpidl implementation
   // void PreventDefault();
 
   // You MUST NOT call PreventDefault(JSContext*, CallerType) from C++ code.  A
   // call of this method always sets Event.defaultPrevented true for web
   // contents.  If default action handler calls this, web applications see wrong
   // defaultPrevented value.
   virtual void PreventDefault(JSContext* aCx, CallerType aCallerType);
@@ -378,17 +383,19 @@ private:
   NS_IMETHOD GetExplicitOriginalTarget(nsIDOMEventTarget** aExplicitOriginalTarget) override { return _to GetExplicitOriginalTarget(aExplicitOriginalTarget); } \
   NS_IMETHOD GetPreventDefault(bool* aRetval) override { return _to GetPreventDefault(aRetval); } \
   NS_IMETHOD GetIsTrusted(bool* aIsTrusted) override { return _to GetIsTrusted(aIsTrusted); } \
   NS_IMETHOD SetTarget(nsIDOMEventTarget* aTarget) override { return _to SetTarget(aTarget); } \
   NS_IMETHOD_(bool) IsDispatchStopped(void) override { return _to IsDispatchStopped(); } \
   NS_IMETHOD_(WidgetEvent*) WidgetEventPtr(void) override { return _to WidgetEventPtr(); } \
   NS_IMETHOD_(void) SetTrusted(bool aTrusted) override { _to SetTrusted(aTrusted); } \
   NS_IMETHOD_(void) SetOwner(EventTarget* aOwner) override { _to SetOwner(aOwner); } \
-  NS_IMETHOD_(Event*) InternalDOMEvent() override { return _to InternalDOMEvent(); }
+  NS_IMETHOD_(Event*) InternalDOMEvent() override { return _to InternalDOMEvent(); } \
+  NS_IMETHOD GetCancelBubble(bool* aCancelBubble) override { return _to GetCancelBubble(aCancelBubble); } \
+  NS_IMETHOD SetCancelBubble(bool aCancelBubble) override { return _to SetCancelBubble(aCancelBubble); }
 
 #define NS_FORWARD_TO_EVENT_NO_SERIALIZATION_NO_DUPLICATION \
   NS_FORWARD_NSIDOMEVENT_NO_SERIALIZATION_NO_DUPLICATION(Event::) \
   virtual void PreventDefault(JSContext* aCx, CallerType aCallerType) override { Event::PreventDefault(aCx, aCallerType); }
 
 inline nsISupports*
 ToSupports(mozilla::dom::Event* e)
 {

--- a/dom/events/UIEvent.cpp
+++ b/dom/events/UIEvent.cpp
@@ -283,31 +283,16 @@ UIEvent::RangeOffset() const
     return 0;
   }
 
   nsPoint pt = nsLayoutUtils::GetEventCoordinatesRelativeTo(mEvent,
                                                             targetFrame);
   return targetFrame->GetContentOffsetsFromPoint(pt).offset;
 }
 
-NS_IMETHODIMP
-UIEvent::GetCancelBubble(bool* aCancelBubble)
-{
-  NS_ENSURE_ARG_POINTER(aCancelBubble);
-  *aCancelBubble = CancelBubble();
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-UIEvent::SetCancelBubble(bool aCancelBubble)
-{
-  mEvent->mFlags.mPropagationStopped = aCancelBubble;
-  return NS_OK;
-}
-
 nsIntPoint
 UIEvent::GetLayerPoint() const
 {
   if (!mEvent ||
       (mEvent->mClass != eMouseEventClass &&
        mEvent->mClass != eMouseScrollEventClass &&
        mEvent->mClass != eWheelEventClass &&
        mEvent->mClass != ePointerEventClass &&

--- a/dom/events/UIEvent.h
+++ b/dom/events/UIEvent.h
@@ -88,21 +88,16 @@ public:
                "Mouse events should override Which()");
     return 0;
   }
 
   already_AddRefed<nsINode> GetRangeParent();
 
   int32_t RangeOffset() const;
 
-  bool CancelBubble() const
-  {
-    return mEvent->PropagationStopped();
-  }
-
   bool IsChar() const;
 
 protected:
   ~UIEvent() {}
 
   // Internal helper functions
   nsIntPoint GetMovementPoint();
   nsIntPoint GetLayerPoint() const;

--- a/dom/events/test/mochitest.ini
+++ b/dom/events/test/mochitest.ini
@@ -179,8 +179,9 @@ skip-if = toolkit == 'android' #CRASH_DU
 [test_moz_mouse_pixel_scroll_event.html]
 [test_offsetxy.html]
 [test_onerror_handler_args.html]
 [test_passive_listeners.html]
 [test_paste_image.html]
 [test_wheel_default_action.html]
 [test_bug687787.html]
 [test_bug1305458.html]
+[test_bug1298970.html]

new file mode 100644
--- /dev/null
+++ b/dom/events/test/test_bug1298970.html
@@ -0,0 +1,33 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1298970
+-->
+<head>
+  <title>Test for Bug 1298970</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1298970">Mozilla Bug 1298970</a>
+<p id="display"></p>
+<div id="inner"></div>
+<script class="testbody" type="text/javascript">
+
+/** Test for Bug 1298970 **/
+var target = document.getElementById("inner");
+var event = new Event("test", { bubbles: true, cancelable: true });
+
+is(event.cancelBubble, false, "Event.cancelBubble should be false by default");
+
+target.addEventListener("test", (e) => {
+  e.stopPropagation();
+  is(e.cancelBubble, true, "Event.cancelBubble should be true after stopPropagation");
+}, true);
+
+target.dispatchEvent(event);
+
+</script>
+</body>
+</html>
+

--- a/dom/interfaces/events/nsIDOMEvent.idl
+++ b/dom/interfaces/events/nsIDOMEvent.idl
@@ -200,16 +200,17 @@ interface nsIDOMEvent : nsISupports
 
   /**
    * @deprecated Use nsIDOMEvent::defaultPrevented.
    * To be removed in bug 691151.
    */
   boolean                   getPreventDefault();
 
   readonly attribute boolean isTrusted;
+  attribute boolean cancelBubble;
 
   [noscript] void duplicatePrivateData();
   [noscript] void setTarget(in nsIDOMEventTarget aTarget);
   [notxpcom] boolean IsDispatchStopped();
   [notxpcom] WidgetEvent WidgetEventPtr();
   [noscript,notxpcom] void SetTrusted(in boolean aTrusted);
   [notxpcom] void Serialize(in IPCMessagePtr aMsg,
                             in boolean aSerializeInterfaceType);

--- a/dom/interfaces/events/nsIDOMUIEvent.idl
+++ b/dom/interfaces/events/nsIDOMUIEvent.idl
@@ -44,14 +44,12 @@ interface nsIDOMUIEvent : nsISupports
   readonly attribute long               layerX;
   readonly attribute long               layerY;
   readonly attribute long               pageX;
   readonly attribute long               pageY;
   readonly attribute unsigned long      which;
   readonly attribute nsIDOMNode         rangeParent;
   readonly attribute long               rangeOffset;
 
-           attribute boolean            cancelBubble;
-
   readonly attribute boolean            isChar;
 
   [notxpcom, nostdcall] EventPtr AsEvent();
 };

--- a/dom/webidl/Event.webidl
+++ b/dom/webidl/Event.webidl
@@ -46,31 +46,31 @@ interface Event {
   readonly attribute boolean composed;
 
   [Unforgeable, Pure]
   readonly attribute boolean isTrusted;
   [Pure]
   readonly attribute DOMHighResTimeStamp timeStamp;
 
   void initEvent(DOMString type, boolean bubbles, boolean cancelable);
+  attribute boolean cancelBubble;
 };
 
 // Mozilla specific legacy stuff.
 partial interface Event {
   const long ALT_MASK     = 0x00000001;
   const long CONTROL_MASK = 0x00000002;
   const long SHIFT_MASK   = 0x00000004;
   const long META_MASK    = 0x00000008;
 
   readonly attribute EventTarget? originalTarget;
   readonly attribute EventTarget? explicitOriginalTarget;
   [ChromeOnly] readonly attribute EventTarget? composedTarget;
   [ChromeOnly] readonly attribute boolean multipleActionsPrevented;
   [ChromeOnly] readonly attribute boolean isSynthesized;
-
   boolean getPreventDefault();
 };
 
 dictionary EventInit {
   boolean bubbles = false;
   boolean cancelable = false;
   boolean composed = false;
 };

--- a/dom/webidl/UIEvent.webidl
+++ b/dom/webidl/UIEvent.webidl
@@ -29,17 +29,16 @@ partial interface UIEvent {
 
   readonly attribute long          layerX;
   readonly attribute long          layerY;
   readonly attribute long          pageX;
   readonly attribute long          pageY;
   readonly attribute unsigned long which;
   readonly attribute Node?         rangeParent;
   readonly attribute long          rangeOffset;
-           attribute boolean       cancelBubble;
   readonly attribute boolean       isChar;
 };
 
 dictionary UIEventInit : EventInit
 {
   Window? view = null;
   long    detail = 0;
 };