diff --git a/dom/animation/AnimationEffect.cpp b/dom/animation/AnimationEffect.cpp
index 6ef8c30d49..07cad770c9 100644
--- a/dom/animation/AnimationEffect.cpp
+++ b/dom/animation/AnimationEffect.cpp
@@ -108,10 +108,18 @@ ComputedTiming AnimationEffect::GetComputedTimingAt(
   // Always return the same object to benefit from return-value optimization.
   ComputedTiming result;
 
+  result.mActiveDuration = aTiming.ActiveDuration();
   if (aTiming.Duration()) {
     MOZ_ASSERT(aTiming.Duration().ref() >= zeroDuration,
                "Iteration duration should be positive");
-    result.mDuration = aTiming.Duration().ref();
+    if (result.mActiveDuration != StickyTimeDuration::Forever()) {
+      // On css animations with finite duration, set the duration & active duration to 0
+      result.mDuration = zeroDuration;
+      result.mActiveDuration = zeroDuration;
+    } else {
+      // On infinite-running animations, use the default duration
+      result.mDuration = aTiming.Duration().ref();
+    }
   }
 
   MOZ_ASSERT(aTiming.Iterations() >= 0.0 && !std::isnan(aTiming.Iterations()),
@@ -124,7 +132,6 @@ ComputedTiming AnimationEffect::GetComputedTimingAt(
              "ValidateIterationStart");
   result.mIterationStart = aTiming.IterationStart();
 
-  result.mActiveDuration = aTiming.ActiveDuration();
   result.mEndTime = aTiming.EndTime();
   result.mFill = aTiming.Fill() == dom::FillMode::Auto ? dom::FillMode::None
                                                        : aTiming.Fill();