--- a/gfx/layers/Layers.cpp
+++ b/gfx/layers/Layers.cpp
@@ -426,35 +426,49 @@ CreateCSSValueList(const InfallibleTArra
 void
 Layer::SetAnimations(const AnimationArray& aAnimations)
 {
   MOZ_LAYERS_LOG_IF_SHADOWABLE(this, ("Layer::Mutated(%p) SetAnimations", this));
 
   mAnimations = aAnimations;
   mAnimationData.Clear();
   for (uint32_t i = 0; i < mAnimations.Length(); i++) {
+    Animation& animation = mAnimations[i];
+    // Adjust fill mode to fill forwards so that if the main thread is delayed
+    // in clearing this animation we don't introduce flicker by jumping back to
+    // the old underlying value
+    switch (static_cast<dom::FillMode>(animation.fillMode())) {
+      case dom::FillMode::None:
+        animation.fillMode() = static_cast<uint8_t>(dom::FillMode::Forwards);
+        break;
+      case dom::FillMode::Backwards:
+        animation.fillMode() = static_cast<uint8_t>(dom::FillMode::Both);
+        break;
+      default:
+        break;
+    }
+
     AnimData* data = mAnimationData.AppendElement();
     InfallibleTArray<Maybe<ComputedTimingFunction>>& functions =
       data->mFunctions;
-    const InfallibleTArray<AnimationSegment>& segments =
-      mAnimations.ElementAt(i).segments();
+    const InfallibleTArray<AnimationSegment>& segments = animation.segments();
     for (uint32_t j = 0; j < segments.Length(); j++) {
       TimingFunction tf = segments.ElementAt(j).sampleFn();
 
       Maybe<ComputedTimingFunction> ctf =
         AnimationUtils::TimingFunctionToComputedTimingFunction(tf);
       functions.AppendElement(ctf);
     }
 
     // Precompute the StyleAnimationValues that we need if this is a transform
     // animation.
     InfallibleTArray<StyleAnimationValue>& startValues = data->mStartValues;
     InfallibleTArray<StyleAnimationValue>& endValues = data->mEndValues;
-    for (uint32_t j = 0; j < mAnimations[i].segments().Length(); j++) {
-      const AnimationSegment& segment = mAnimations[i].segments()[j];
+    for (uint32_t j = 0; j < segments.Length(); j++) {
+      const AnimationSegment& segment = segments[j];
       StyleAnimationValue* startValue = startValues.AppendElement();
       StyleAnimationValue* endValue = endValues.AppendElement();
       if (segment.endState().type() == Animatable::TArrayOfTransformFunction) {
         const InfallibleTArray<TransformFunction>& startFunctions =
           segment.startState().get_ArrayOfTransformFunction();
         startValue->SetTransformValue(CreateCSSValueList(startFunctions));
 
         const InfallibleTArray<TransformFunction>& endFunctions =

--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -426,17 +426,17 @@ AddAnimationForProperty(nsIFrame* aFrame
                               ToTimeStamp(startTime.Value());
   animation->initialCurrentTime() = aAnimation->GetCurrentTime().Value()
                                     - timing.mDelay;
   animation->delay() = timing.mDelay;
   animation->duration() = computedTiming.mDuration;
   animation->iterations() = computedTiming.mIterations;
   animation->iterationStart() = computedTiming.mIterationStart;
   animation->direction() = static_cast<uint8_t>(timing.mDirection);
-  animation->fillMode() = static_cast<uint8_t>(timing.mFill);
+  animation->fillMode() = static_cast<uint8_t>(computedTiming.mFill);
   animation->property() = aProperty.mProperty;
   animation->playbackRate() = aAnimation->PlaybackRate();
   animation->data() = aData;
   animation->easingFunction() = ToTimingFunction(timing.mFunction);
   animation->iterationComposite() =
     static_cast<uint8_t>(aAnimation->GetEffect()->
                          AsKeyframeEffect()->IterationComposite());