diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index d2f96c26aa6..697e0501632 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -5716,14 +5716,16 @@ EventStateManager::WheelPrefs::NeedToComputeLineOrPageDelta(
          (mMultiplierY[index] != 1.0 && mMultiplierY[index] != -1.0);
 }
 
-bool
-EventStateManager::WheelPrefs::HasUserPrefsForDelta(WidgetWheelEvent* aEvent)
+void
+EventStateManager::WheelPrefs::GetUserPrefsForEvent(WidgetWheelEvent* aEvent,
+                                                    double* aOutMultiplierX,
+                                                    double* aOutMultiplierY)
 {
   Index index = GetIndexFor(aEvent);
   Init(index);
 
-  return mMultiplierX[index] != 1.0 ||
-         mMultiplierY[index] != 1.0;
+  *aOutMultiplierX = mMultiplierX[index];
+  *aOutMultiplierY = mMultiplierY[index];
 }
 
 bool
@@ -5733,10 +5735,13 @@ EventStateManager::WheelEventIsScrollAction(WidgetWheelEvent* aEvent)
          WheelPrefs::GetInstance()->ComputeActionFor(aEvent) == WheelPrefs::ACTION_SCROLL;
 }
 
-bool
-EventStateManager::WheelEventNeedsDeltaMultipliers(WidgetWheelEvent* aEvent)
+void
+EventStateManager::GetUserPrefsForWheelEvent(WidgetWheelEvent* aEvent,
+                                             double* aOutMultiplierX,
+                                             double* aOutMultiplierY)
 {
-  return WheelPrefs::GetInstance()->HasUserPrefsForDelta(aEvent);
+  WheelPrefs::GetInstance()->GetUserPrefsForEvent(
+    aEvent, aOutMultiplierX, aOutMultiplierY);
 }
 
 bool
diff --git a/dom/events/EventStateManager.h b/dom/events/EventStateManager.h
index 0575233c05e..e10e87e3303 100644
--- a/dom/events/EventStateManager.h
+++ b/dom/events/EventStateManager.h
@@ -242,9 +242,10 @@ public:
   // Returns true if the given WidgetWheelEvent will resolve to a scroll action.
   static bool WheelEventIsScrollAction(WidgetWheelEvent* aEvent);
 
-  // Returns true if user prefs for wheel deltas apply to the given
-  // WidgetWheelEvent.
-  static bool WheelEventNeedsDeltaMultipliers(WidgetWheelEvent* aEvent);
+  // Returns user-set multipliers for a wheel event.
+  static void GetUserPrefsForWheelEvent(WidgetWheelEvent* aEvent,
+                                        double* aOutMultiplierX,
+                                        double* aOutMultiplierY);
 
   // Returns whether or not a frame can be vertically scrolled with a mouse
   // wheel (as opposed to, say, a selection or touch scroll).
@@ -449,7 +450,9 @@ protected:
      * Returns whether or not ApplyUserPrefsToDelta() would change the delta
      * values of an event.
      */
-    bool HasUserPrefsForDelta(WidgetWheelEvent* aEvent);
+    void GetUserPrefsForEvent(WidgetWheelEvent* aEvent,
+                              double* aOutMultiplierX,
+                              double* aOutMultiplierY);
 
     /**
      * If ApplyUserPrefsToDelta() changed the delta values with customized
diff --git a/gfx/layers/apz/src/APZCTreeManager.cpp b/gfx/layers/apz/src/APZCTreeManager.cpp
index 4d7e7df26d5..6989933bfaf 100644
--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -626,8 +626,7 @@ WillHandleWheelEvent(WidgetWheelEvent* aEvent)
 {
   return EventStateManager::WheelEventIsScrollAction(aEvent) &&
          (aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_LINE ||
-          aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_PIXEL) &&
-         !EventStateManager::WheelEventNeedsDeltaMultipliers(aEvent);
+          aEvent->deltaMode == nsIDOMWheelEvent::DOM_DELTA_PIXEL);
 }
 
 static bool
@@ -1090,8 +1089,17 @@ APZCTreeManager::ProcessWheelEvent(WidgetWheelEvent& aEvent,
                          scrollMode,
                          ScrollWheelInput::DeltaTypeForDeltaMode(aEvent.deltaMode),
                          origin,
-                         aEvent.deltaX,
-                         aEvent.deltaY);
+                         aEvent.deltaX, aEvent.deltaY);
+
+  // We add the user multiplier as a separate field, rather than premultiplying
+  // it, because if the input is converted back to a WidgetWheelEvent, then
+  // EventStateManager would apply the delta a second time. We could in theory
+  // work around this by asking ESM to customize the event much sooner, and
+  // then save the "customizedByUserPrefs" bit on ScrollWheelInput - but for
+  // now, this seems easier.
+  EventStateManager::GetUserPrefsForWheelEvent(&aEvent,
+    &input.mUserDeltaMultiplierX,
+    &input.mUserDeltaMultiplierY);
 
   nsEventStatus status = ReceiveInputEvent(input, aOutTargetGuid, aOutInputBlockId);
   aEvent.refPoint.x = input.mOrigin.x;
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
index 173ecdab1fb..c4da0148a8c 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -1604,7 +1604,18 @@ AsyncPanZoomController::GetScrollWheelDelta(const ScrollWheelInput& aEvent) cons
       MOZ_ASSERT_UNREACHABLE("unexpected scroll delta type");
   }
 
-  if (isRootContent && gfxPrefs::MouseWheelHasRootScrollDeltaOverride()) {
+  // Apply user-set multipliers.
+  delta.x *= aEvent.mUserDeltaMultiplierX;
+  delta.y *= aEvent.mUserDeltaMultiplierY;
+
+  // For the conditions under which we allow system scroll overrides, see
+  // EventStateManager::DeltaAccumulator::ComputeScrollAmountForDefaultAction
+  // and WheelTransaction::OverrideSystemScrollSpeed.
+  if (isRootContent &&
+      gfxPrefs::MouseWheelHasRootScrollDeltaOverride() &&
+      !aEvent.IsCustomizedByUserPrefs() &&
+      aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_LINE)
+  {
     // Only apply delta multipliers if we're increasing the delta.
     double hfactor = double(gfxPrefs::MouseWheelRootHScrollDeltaFactor()) / 100;
     double vfactor = double(gfxPrefs::MouseWheelRootVScrollDeltaFactor()) / 100;
diff --git a/widget/InputData.cpp b/widget/InputData.cpp
index 471717903f9..a0d0bb868e9 100644
--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -376,6 +376,8 @@ ScrollWheelInput::ScrollWheelInput(const WidgetWheelEvent& aWheelEvent) :
   mDeltaY(aWheelEvent.deltaY),
   mLineOrPageDeltaX(aWheelEvent.lineOrPageDeltaX),
   mLineOrPageDeltaY(aWheelEvent.lineOrPageDeltaY),
+  mUserDeltaMultiplierX(1.0),
+  mUserDeltaMultiplierY(1.0),
   mIsMomentum(aWheelEvent.isMomentum)
 {
   mOrigin =
@@ -415,4 +417,11 @@ ScrollWheelInput::TransformToLocal(const gfx::Matrix4x4& aTransform)
   return true;
 }
 
+bool
+ScrollWheelInput::IsCustomizedByUserPrefs() const
+{
+  return mUserDeltaMultiplierX != 1.0 ||
+         mUserDeltaMultiplierY != 1.0;
+}
+
 } // namespace mozilla
diff --git a/widget/InputData.h b/widget/InputData.h
index 0c1a4979468..331f52874a6 100644
--- a/widget/InputData.h
+++ b/widget/InputData.h
@@ -583,6 +583,8 @@ public:
      mLineOrPageDeltaX(0),
      mLineOrPageDeltaY(0),
      mScrollSeriesNumber(0),
+     mUserDeltaMultiplierX(1.0),
+     mUserDeltaMultiplierY(1.0),
      mIsMomentum(false)
   {}
 
@@ -591,6 +593,8 @@ public:
   WidgetWheelEvent ToWidgetWheelEvent(nsIWidget* aWidget) const;
   bool TransformToLocal(const gfx::Matrix4x4& aTransform);
 
+  bool IsCustomizedByUserPrefs() const;
+
   ScrollDeltaType mDeltaType;
   ScrollMode mScrollMode;
   ScreenPoint mOrigin;
@@ -620,6 +624,10 @@ public:
   // first event is 1; if not a member of a transaction, this is 0.
   uint32_t mScrollSeriesNumber;
 
+  // User-set delta multipliers.
+  double mUserDeltaMultiplierX;
+  double mUserDeltaMultiplierY;
+
   bool mIsMomentum;
 };