--- a/widget/headless/HeadlessWidget.cpp
+++ b/widget/headless/HeadlessWidget.cpp
@@ -472,10 +472,35 @@ HeadlessWidget::SynthesizeNativeMouseEve
   if (msg == eMouseDown) {
     event.mClickCount = 1;
   }
   event.AssignEventTime(WidgetEventTime());
   DispatchInputEvent(&event);
   return NS_OK;
 }
 
+nsresult
+HeadlessWidget::SynthesizeNativeMouseScrollEvent(mozilla::LayoutDeviceIntPoint aPoint,
+                                                 uint32_t aNativeMessage,
+                                                 double aDeltaX,
+                                                 double aDeltaY,
+                                                 double aDeltaZ,
+                                                 uint32_t aModifierFlags,
+                                                 uint32_t aAdditionalFlags,
+                                                 nsIObserver* aObserver)
+{
+  AutoObserverNotifier notifier(aObserver, "mousescrollevent");
+  // The various platforms seem to handle scrolling deltas differently,
+  // but the following seems to emulate it well enough.
+  WidgetWheelEvent event(true, eWheel, this);
+  event.mDeltaMode = nsIDOMWheelEvent::DOM_DELTA_LINE;
+  event.mIsNoLineOrPageDelta = true;
+  event.mDeltaX = -aDeltaX * MOZ_HEADLESS_SCROLL_MULTIPLIER;
+  event.mDeltaY = -aDeltaY * MOZ_HEADLESS_SCROLL_MULTIPLIER;
+  event.mDeltaZ = -aDeltaZ * MOZ_HEADLESS_SCROLL_MULTIPLIER;
+  event.mRefPoint = aPoint - WidgetToScreenOffset();
+  event.AssignEventTime(WidgetEventTime());
+  DispatchInputEvent(&event);
+  return NS_OK;
+}
+
 } // namespace widget
 } // namespace mozilla

--- a/widget/headless/HeadlessWidget.h
+++ b/widget/headless/HeadlessWidget.h
@@ -12,32 +12,37 @@
 #include "CompositorWidget.h"
 
 // The various synthesized event values are hardcoded to avoid pulling
 // in the platform specific widget code.
 #if defined(MOZ_WIDGET_GTK)
 #define MOZ_HEADLESS_MOUSE_MOVE 3 // GDK_MOTION_NOTIFY
 #define MOZ_HEADLESS_MOUSE_DOWN 4 // GDK_BUTTON_PRESS
 #define MOZ_HEADLESS_MOUSE_UP   7 // GDK_BUTTON_RELEASE
+#define MOZ_HEADLESS_SCROLL_MULTIPLIER 3
 #elif defined(XP_WIN)
 #define MOZ_HEADLESS_MOUSE_MOVE 1 // MOUSEEVENTF_MOVE
 #define MOZ_HEADLESS_MOUSE_DOWN 2 // MOUSEEVENTF_LEFTDOWN
 #define MOZ_HEADLESS_MOUSE_UP   4 // MOUSEEVENTF_LEFTUP
+#define MOZ_HEADLESS_SCROLL_MULTIPLIER 1
 #elif defined(XP_MACOSX)
 #define MOZ_HEADLESS_MOUSE_MOVE 5 // NSMouseMoved
 #define MOZ_HEADLESS_MOUSE_DOWN 1 // NSLeftMouseDown
 #define MOZ_HEADLESS_MOUSE_UP   2 // NSLeftMouseUp
+#define MOZ_HEADLESS_SCROLL_MULTIPLIER 1
 #elif defined(ANDROID)
 #define MOZ_HEADLESS_MOUSE_MOVE 7 // ACTION_HOVER_MOVE
 #define MOZ_HEADLESS_MOUSE_DOWN 5 // ACTION_POINTER_DOWN
 #define MOZ_HEADLESS_MOUSE_UP   6 // ACTION_POINTER_UP
+#define MOZ_HEADLESS_SCROLL_MULTIPLIER 1
 #else
 #define MOZ_HEADLESS_MOUSE_MOVE -1
 #define MOZ_HEADLESS_MOUSE_DOWN -1
 #define MOZ_HEADLESS_MOUSE_UP   -1
+#define MOZ_HEADLESS_SCROLL_MULTIPLIER = -1
 #endif
 
 namespace mozilla {
 namespace widget {
 
 class HeadlessWidget : public nsBaseWidget
 {
 public:
@@ -119,16 +124,25 @@ public:
   virtual nsresult SynthesizeNativeMouseEvent(LayoutDeviceIntPoint aPoint,
                                               uint32_t aNativeMessage,
                                               uint32_t aModifierFlags,
                                               nsIObserver* aObserver) override;
   virtual nsresult SynthesizeNativeMouseMove(LayoutDeviceIntPoint aPoint,
                                              nsIObserver* aObserver) override
                    { return SynthesizeNativeMouseEvent(aPoint, MOZ_HEADLESS_MOUSE_MOVE, 0, aObserver); };
 
+  virtual nsresult SynthesizeNativeMouseScrollEvent(LayoutDeviceIntPoint aPoint,
+                                                    uint32_t aNativeMessage,
+                                                    double aDeltaX,
+                                                    double aDeltaY,
+                                                    double aDeltaZ,
+                                                    uint32_t aModifierFlags,
+                                                    uint32_t aAdditionalFlags,
+                                                    nsIObserver* aObserver) override;
+
 private:
   ~HeadlessWidget();
   bool mEnabled;
   bool mVisible;
   bool mDestroyed;
   nsIWidget* mTopLevel;
   HeadlessCompositorWidget* mCompositorWidget;
   // The size mode before entering fullscreen mode.