--- a/dom/animation/AnimationEffectReadOnly.h
+++ b/dom/animation/AnimationEffectReadOnly.h
@@ -46,20 +46,16 @@ public:
   {
     return nullptr;
   }
 
   nsISupports* GetParentObject() const { return mDocument; }
 
   bool IsCurrent() const;
   bool IsInEffect() const;
-  bool IsActiveDurationZero() const
-  {
-    return !SpecifiedTiming().ActiveDuration();
-  }
 
   already_AddRefed<AnimationEffectTimingReadOnly> Timing();
   const TimingParams& SpecifiedTiming() const
   {
     return mTiming->AsTimingParams();
   }
   void SetSpecifiedTiming(const TimingParams& aTiming);