--- a/gfx/layers/AnimationInfo.cpp
+++ b/gfx/layers/AnimationInfo.cpp
@@ -38,21 +38,16 @@ AnimationInfo::AddAnimation()
   // Here generates a new id when the first animation is added and
   // this id is used to represent the animations in this layer.
   EnsureAnimationsId();
 
   MOZ_ASSERT(!mPendingAnimations, "should have called ClearAnimations first");
 
   Animation* anim = mAnimations.AppendElement();
 
-  if (mManager->AsWebRenderLayerManager()) {
-    mManager->AsWebRenderLayerManager()->
-      KeepCompositorAnimationsIdAlive(mCompositorAnimationsId);
-  }
-
   mMutated = true;
 
   return anim;
 }
 
 Animation*
 AnimationInfo::AddAnimationForNextTransaction()
 {
@@ -68,21 +63,16 @@ void
 AnimationInfo::ClearAnimations()
 {
   mPendingAnimations = nullptr;
 
   if (mAnimations.IsEmpty() && mAnimationData.IsEmpty()) {
     return;
   }
 
-  if (mManager->AsWebRenderLayerManager()) {
-    mManager->AsWebRenderLayerManager()->
-      AddCompositorAnimationsIdForDiscard(mCompositorAnimationsId);
-  }
-
   mAnimations.Clear();
   mAnimationData.Clear();
 
   mMutated = true;
 }
 
 void
 AnimationInfo::ClearAnimationsForNextTransaction()

--- a/gfx/layers/wr/WebRenderLayerManager.cpp
+++ b/gfx/layers/wr/WebRenderLayerManager.cpp
@@ -104,16 +104,22 @@ WebRenderLayerManager::DoDestroy(bool aI
     // Just clear ImageKeys, they are deleted during WebRenderAPI destruction.
     mImageKeysToDeleteLater.Clear();
     mImageKeysToDelete.Clear();
     // CompositorAnimations are cleared by WebRenderBridgeParent.
     mDiscardedCompositorAnimationsIds.Clear();
     WrBridge()->Destroy(aIsSync);
   }
 
+  // Clear this before calling RemoveUnusedAndResetWebRenderUserData(),
+  // otherwise that function might destroy some WebRenderAnimationData instances
+  // which will put stuff back into mDiscardedCompositorAnimationsIds. If
+  // mActiveCompositorAnimationIds is empty that won't happen.
+  mActiveCompositorAnimationIds.clear();
+
   mLastCanvasDatas.Clear();
   RemoveUnusedAndResetWebRenderUserData();
 
   if (mTransactionIdAllocator) {
     // Make sure to notify the refresh driver just in case it's waiting on a
     // pending transaction. Do this at the top of the event loop so we don't
     // cause a paint to occur during compositor shutdown.
     RefPtr<TransactionIdAllocator> allocator = mTransactionIdAllocator;
@@ -951,25 +957,40 @@ WebRenderLayerManager::DiscardImages()
     resources.DeleteImage(key);
   }
   mImageKeysToDeleteLater.Clear();
   mImageKeysToDelete.Clear();
   WrBridge()->UpdateResources(resources);
 }
 
 void
-WebRenderLayerManager::AddCompositorAnimationsIdForDiscard(uint64_t aId)
+WebRenderLayerManager::AddActiveCompositorAnimationId(uint64_t aId)
 {
-  mDiscardedCompositorAnimationsIds.AppendElement(aId);
+  // In layers-free mode we track the active compositor animation ids on the
+  // client side so that we don't try to discard the same animation id multiple
+  // times. We could just ignore the multiple-discard on the parent side, but
+  // checking on the content side reduces IPC traffic.
+  MOZ_ASSERT(IsLayersFreeTransaction());
+  mActiveCompositorAnimationIds.insert(aId);
 }
 
 void
-WebRenderLayerManager::KeepCompositorAnimationsIdAlive(uint64_t aId)
+WebRenderLayerManager::AddCompositorAnimationsIdForDiscard(uint64_t aId)
 {
-  mDiscardedCompositorAnimationsIds.RemoveElement(aId);
+  if (!IsLayersFreeTransaction()) {
+    // For layers-full we don't track the active animation id in
+    // mActiveCompositorAnimationIds, we just call this on layer destruction and
+    // don't need to worry about discarding the same id multiple times.
+    mDiscardedCompositorAnimationsIds.AppendElement(aId);
+  } else if (mActiveCompositorAnimationIds.erase(aId)) {
+    // For layers-free ensure we don't try to discard an animation id that wasn't
+    // active. We also remove it from mActiveCompositorAnimationIds so we don't
+    // discard it again unless it gets re-activated.
+    mDiscardedCompositorAnimationsIds.AppendElement(aId);
+  }
 }
 
 void
 WebRenderLayerManager::DiscardCompositorAnimations()
 {
   if (WrBridge()->IPCOpen() &&
       !mDiscardedCompositorAnimationsIds.IsEmpty()) {
     WrBridge()->

--- a/gfx/layers/wr/WebRenderLayerManager.h
+++ b/gfx/layers/wr/WebRenderLayerManager.h
@@ -1,16 +1,17 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef GFX_WEBRENDERLAYERMANAGER_H
 #define GFX_WEBRENDERLAYERMANAGER_H
 
+#include <unordered_set>
 #include <vector>
 
 #include "gfxPrefs.h"
 #include "Layers.h"
 #include "mozilla/MozPromise.h"
 #include "mozilla/layers/APZTestData.h"
 #include "mozilla/layers/FocusTarget.h"
 #include "mozilla/layers/StackingContextHelper.h"
@@ -166,22 +167,21 @@ public:
   { return mPaintedLayerCallbackData; }
 
   // adds an imagekey to a list of keys that will be discarded on the next
   // transaction or destruction
   void AddImageKeyForDiscard(wr::ImageKey);
   void DiscardImages();
   void DiscardLocalImages();
 
-  // Before destroying a layer with animations, add its compositorAnimationsId
-  // to a list of ids that will be discarded on the next transaction
+  // Methods to manage the compositor animation ids. Active animations are still
+  // going, and when they end we discard them and remove them from the active
+  // list.
+  void AddActiveCompositorAnimationId(uint64_t aId);
   void AddCompositorAnimationsIdForDiscard(uint64_t aId);
-  // If the animations are valid and running on the compositor,
-  // we should keep the compositorAnimationsId alive on the compositor side.
-  void KeepCompositorAnimationsIdAlive(uint64_t aId);
   void DiscardCompositorAnimations();
 
   WebRenderBridgeChild* WrBridge() const { return mWrChild; }
 
   virtual void Mutated(Layer* aLayer) override;
   virtual void MutatedSimple(Layer* aLayer) override;
 
   void Hold(Layer* aLayer);
@@ -292,16 +292,22 @@ private:
 private:
   nsIWidget* MOZ_NON_OWNING_REF mWidget;
   nsTArray<wr::ImageKey> mImageKeysToDelete;
   // TODO - This is needed because we have some code that creates image keys
   // and enqueues them for deletion right away which is bad not only because
   // of poor texture cache usage, but also because images end up deleted before
   // they are used. This should hopfully be temporary.
   nsTArray<wr::ImageKey> mImageKeysToDeleteLater;
+
+  // Set of compositor animation ids for which there are active animations (as
+  // of the last transaction) on the compositor side.
+  std::unordered_set<uint64_t> mActiveCompositorAnimationIds;
+  // Compositor animation ids for animations that are done now and that we want
+  // the compositor to discard information for.
   nsTArray<uint64_t> mDiscardedCompositorAnimationsIds;
 
   /* PaintedLayer callbacks; valid at the end of a transaciton,
    * while rendering */
   DrawPaintedLayerCallback mPaintedLayerCallback;
   void *mPaintedLayerCallbackData;
 
   RefPtr<WebRenderBridgeChild> mWrChild;

--- a/gfx/layers/wr/WebRenderUserData.cpp
+++ b/gfx/layers/wr/WebRenderUserData.cpp
@@ -215,16 +215,28 @@ WebRenderFallbackData::SetGeometry(nsAut
 
 WebRenderAnimationData::WebRenderAnimationData(WebRenderLayerManager* aWRManager, nsDisplayItem* aItem,
                                                WebRenderUserDataRefTable* aTable)
   : WebRenderUserData(aWRManager, aItem, aTable)
   , mAnimationInfo(aWRManager)
 {
 }
 
+WebRenderAnimationData::~WebRenderAnimationData()
+{
+  // It may be the case that nsDisplayItem that created this WebRenderUserData
+  // gets destroyed without getting a chance to discard the compositor animation
+  // id, so we should do it as part of cleanup here.
+  uint64_t animationId = mAnimationInfo.GetCompositorAnimationsId();
+  // animationId might be 0 if mAnimationInfo never held any active animations.
+  if (animationId) {
+    mWRManager->AddCompositorAnimationsIdForDiscard(animationId);
+  }
+}
+
 WebRenderCanvasData::WebRenderCanvasData(WebRenderLayerManager* aWRManager, nsDisplayItem* aItem,
                                          WebRenderUserDataRefTable* aTable)
   : WebRenderUserData(aWRManager, aItem, aTable)
 {
 }
 
 WebRenderCanvasData::~WebRenderCanvasData()
 {

--- a/gfx/layers/wr/WebRenderUserData.h
+++ b/gfx/layers/wr/WebRenderUserData.h
@@ -133,17 +133,17 @@ protected:
   bool mInvalid;
 };
 
 class WebRenderAnimationData : public WebRenderUserData
 {
 public:
   explicit WebRenderAnimationData(WebRenderLayerManager* aWRManager, nsDisplayItem* aItem,
                                   WebRenderUserDataRefTable* aTable);
-  virtual ~WebRenderAnimationData() {}
+  virtual ~WebRenderAnimationData();
 
   virtual UserDataType GetType() override { return UserDataType::eAnimation; }
   static UserDataType Type() { return UserDataType::eAnimation; }
   AnimationInfo& GetAnimationInfo() { return mAnimationInfo; }
 
 protected:
   AnimationInfo mAnimationInfo;
 };

--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -6260,27 +6260,32 @@ nsDisplayOpacity::CreateWebRenderCommand
   float* opacityForSC = &mOpacity;
 
   RefPtr<WebRenderAnimationData> animationData = aManager->CreateOrRecycleWebRenderUserData<WebRenderAnimationData>(this);
   AnimationInfo& animationInfo = animationData->GetAnimationInfo();
   AddAnimationsForProperty(Frame(), aDisplayListBuilder,
                            this, eCSSProperty_opacity,
                            animationInfo, false);
   animationInfo.StartPendingAnimations(aManager->GetAnimationReadyTime());
-  uint64_t animationsId = 0;
+
+  // Note that animationsId can be 0 (uninitialized in AnimationInfo) if there
+  // are no active animations.
+  uint64_t animationsId = animationInfo.GetCompositorAnimationsId();
 
   if (!animationInfo.GetAnimations().IsEmpty()) {
-    animationsId = animationInfo.GetCompositorAnimationsId();
     opacityForSC = nullptr;
     OptionalOpacity opacityForCompositor = mOpacity;
 
     OpAddCompositorAnimations
       anim(CompositorAnimations(animationInfo.GetAnimations(), animationsId),
            void_t(), opacityForCompositor);
     aManager->WrBridge()->AddWebRenderParentCommand(anim);
+    aManager->AddActiveCompositorAnimationId(animationsId);
+  } else if (animationsId) {
+    aManager->AddCompositorAnimationsIdForDiscard(animationsId);
   }
 
   nsTArray<mozilla::wr::WrFilterOp> filters;
   StackingContextHelper sc(aSc,
                            aBuilder,
                            aDisplayListBuilder,
                            this,
                            &mList,
@@ -8023,34 +8028,38 @@ nsDisplayTransform::CreateWebRenderComma
 
   RefPtr<WebRenderAnimationData> animationData = aManager->CreateOrRecycleWebRenderUserData<WebRenderAnimationData>(this);
 
   AnimationInfo& animationInfo = animationData->GetAnimationInfo();
   AddAnimationsForProperty(Frame(), aDisplayListBuilder,
                            this, eCSSProperty_transform,
                            animationInfo, false);
   animationInfo.StartPendingAnimations(aManager->GetAnimationReadyTime());
-  uint64_t animationsId = 0;
+
+  // Note that animationsId can be 0 (uninitialized in AnimationInfo) if there
+  // are no active animations.
+  uint64_t animationsId = animationInfo.GetCompositorAnimationsId();
 
   if (!animationInfo.GetAnimations().IsEmpty()) {
-    animationsId = animationInfo.GetCompositorAnimationsId();
-
     // Update transfrom as nullptr in stacking context if there exists
     // transform animation, the transform value will be resolved
     // after animation sampling on the compositor
     transformForSC = nullptr;
 
     // Pass default transform to compositor in case gecko fails to
     // get animated value after animation sampling.
     OptionalTransform transformForCompositor = newTransformMatrix;
 
     OpAddCompositorAnimations
       anim(CompositorAnimations(animationInfo.GetAnimations(), animationsId),
            transformForCompositor, void_t());
     aManager->WrBridge()->AddWebRenderParentCommand(anim);
+    aManager->AddActiveCompositorAnimationId(animationsId);
+  } else if (animationsId) {
+    aManager->AddCompositorAnimationsIdForDiscard(animationsId);
   }
 
   gfx::Matrix4x4Typed<LayerPixel, LayerPixel> boundTransform = ViewAs<gfx::Matrix4x4Typed<LayerPixel, LayerPixel>>(newTransformMatrix);
 
   nsTArray<mozilla::wr::WrFilterOp> filters;
   StackingContextHelper sc(aSc,
                            aBuilder,
                            aDisplayListBuilder,