diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 35bc0df9233..e9e7b97ab3f 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -107,6 +107,10 @@ const size_t gStackSize = 8192;
 
 #define NS_FULL_GC_DELAY            60000 // ms
 
+// The amount of time to wait from the user being idle to starting a shrinking
+// GC.
+#define NS_SHRINKING_GC_DELAY       15000 // ms
+
 // Maximum amount of time that should elapse between incremental GC slices
 #define NS_INTERSLICE_GC_DELAY      100 // ms
 
@@ -148,6 +152,7 @@ static const uint32_t kMaxICCDuration = 2000; // ms
 
 static nsITimer *sGCTimer;
 static nsITimer *sShrinkGCBuffersTimer;
+static nsITimer *sShrinkingGCTimer;
 static nsITimer *sCCTimer;
 static nsITimer *sICCTimer;
 static nsITimer *sFullGCTimer;
@@ -212,6 +217,7 @@ static nsIScriptSecurityManager *sSecurityManager;
 // the appropriate pref is set.
 
 static bool sGCOnMemoryPressure;
+static bool sCompactOnUserInactive;
 
 // In testing, we call RunNextCollectorTimer() to ensure that the collectors are run more
 // aggressively than they would be in regular browsing. sExpensiveCollectorPokes keeps
@@ -234,6 +240,7 @@ static void
 KillTimers()
 {
   nsJSContext::KillGCTimer();
+  nsJSContext::KillShrinkingGCTimer();
   nsJSContext::KillShrinkGCBuffersTimer();
   nsJSContext::KillCCTimer();
   nsJSContext::KillICCTimer();
@@ -266,22 +273,30 @@ NS_IMETHODIMP
 nsJSEnvironmentObserver::Observe(nsISupports* aSubject, const char* aTopic,
                                  const char16_t* aData)
 {
-  if (sGCOnMemoryPressure && !nsCRT::strcmp(aTopic, "memory-pressure")) {
-    if(StringBeginsWith(nsDependentString(aData),
-                        NS_LITERAL_STRING("low-memory-ongoing"))) {
-      // Don't GC/CC if we are in an ongoing low-memory state since its very
-      // slow and it likely won't help us anyway.
-      return NS_OK;
-    }
-    nsJSContext::GarbageCollectNow(JS::gcreason::MEM_PRESSURE,
-                                   nsJSContext::NonIncrementalGC,
-                                   nsJSContext::ShrinkingGC);
-    nsJSContext::CycleCollectNow();
-    if (NeedsGCAfterCC()) {
+  if (!nsCRT::strcmp(aTopic, "memory-pressure")) {
+    if (sGCOnMemoryPressure) {
+      if(StringBeginsWith(nsDependentString(aData),
+                          NS_LITERAL_STRING("low-memory-ongoing"))) {
+        // Don't GC/CC if we are in an ongoing low-memory state since its very
+        // slow and it likely won't help us anyway.
+        return NS_OK;
+      }
       nsJSContext::GarbageCollectNow(JS::gcreason::MEM_PRESSURE,
                                      nsJSContext::NonIncrementalGC,
                                      nsJSContext::ShrinkingGC);
+      nsJSContext::CycleCollectNow();
+      if (NeedsGCAfterCC()) {
+        nsJSContext::GarbageCollectNow(JS::gcreason::MEM_PRESSURE,
+                                       nsJSContext::NonIncrementalGC,
+                                       nsJSContext::ShrinkingGC);
+      }
     }
+  } else if (!nsCRT::strcmp(aTopic, "user-interaction-inactive")) {
+    if (sCompactOnUserInactive) {
+      nsJSContext::PokeShrinkingGC();
+    }
+  } else if (!nsCRT::strcmp(aTopic, "user-interaction-active")) {
+    nsJSContext::KillShrinkingGCTimer();
   } else if (!nsCRT::strcmp(aTopic, "quit-application") ||
              !nsCRT::strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID)) {
     sShuttingDown = true;
@@ -1284,12 +1299,11 @@ nsJSContext::GarbageCollectNow(JS::gcreason::Reason aReason,
     return;
   }
 
+  JSGCInvocationKind gckind = aShrinking == ShrinkingGC ? GC_SHRINK : GC_NORMAL;
   JS::PrepareForFullGC(sRuntime);
   if (aIncremental == IncrementalGC) {
-    MOZ_ASSERT(aShrinking == NonShrinkingGC);
-    JS::StartIncrementalGC(sRuntime, GC_NORMAL, aReason, aSliceMillis);
+    JS::StartIncrementalGC(sRuntime, gckind, aReason, aSliceMillis);
   } else {
-    JSGCInvocationKind gckind = aShrinking == ShrinkingGC ? GC_SHRINK : GC_NORMAL;
     JS::GCForReason(sRuntime, gckind, aReason);
   }
 }
@@ -1798,6 +1812,16 @@ ShrinkGCBuffersTimerFired(nsITimer *aTimer, void *aClosure)
   nsJSContext::ShrinkGCBuffersNow();
 }
 
+// static
+void
+ShrinkingGCTimerFired(nsITimer* aTimer, void* aClosure)
+{
+  nsJSContext::KillShrinkingGCTimer();
+  nsJSContext::GarbageCollectNow(JS::gcreason::USER_INACTIVE,
+                                 nsJSContext::IncrementalGC,
+                                 nsJSContext::ShrinkingGC);
+}
+
 static bool
 ShouldTriggerCC(uint32_t aSuspected)
 {
@@ -2030,6 +2054,26 @@ nsJSContext::PokeShrinkGCBuffers()
                                               nsITimer::TYPE_ONE_SHOT);
 }
 
+// static
+void
+nsJSContext::PokeShrinkingGC()
+{
+  if (sShrinkingGCTimer || sShuttingDown) {
+    return;
+  }
+
+  CallCreateInstance("@mozilla.org/timer;1", &sShrinkingGCTimer);
+
+  if (!sShrinkingGCTimer) {
+    // Failed to create timer (probably because we're in XPCOM shutdown)
+    return;
+  }
+
+  sShrinkingGCTimer->InitWithFuncCallback(ShrinkingGCTimerFired, nullptr,
+                                          NS_SHRINKING_GC_DELAY,
+                                          nsITimer::TYPE_ONE_SHOT);
+}
+
 // static
 void
 nsJSContext::MaybePokeCC()
@@ -2091,6 +2135,16 @@ nsJSContext::KillShrinkGCBuffersTimer()
   }
 }
 
+//static
+void
+nsJSContext::KillShrinkingGCTimer()
+{
+  if (sShrinkingGCTimer) {
+    sShrinkingGCTimer->Cancel();
+    NS_RELEASE(sShrinkingGCTimer);
+  }
+}
+
 //static
 void
 nsJSContext::KillCCTimer()
@@ -2282,7 +2336,7 @@ void
 mozilla::dom::StartupJSEnvironment()
 {
   // initialize all our statics, so that we can restart XPCOM
-  sGCTimer = sFullGCTimer = sCCTimer = sICCTimer = nullptr;
+  sGCTimer = sShrinkingGCTimer = sFullGCTimer = sCCTimer = sICCTimer = nullptr;
   sCCLockedOut = false;
   sCCLockedOutTime = 0;
   sLastCCEndTime = TimeStamp();
@@ -2687,8 +2741,14 @@ nsJSContext::EnsureStatics()
                                "javascript.options.gc_on_memory_pressure",
                                true);
 
+  Preferences::AddBoolVarCache(&sCompactOnUserInactive,
+                               "javascript.options.compact_on_user_inactive",
+                               true);
+
   nsIObserver* observer = new nsJSEnvironmentObserver();
   obs->AddObserver(observer, "memory-pressure", false);
+  obs->AddObserver(observer, "user-interaction-inactive", false);
+  obs->AddObserver(observer, "user-interaction-active", false);
   obs->AddObserver(observer, "quit-application", false);
   obs->AddObserver(observer, NS_XPCOM_SHUTDOWN_OBSERVER_ID, false);
 
diff --git a/dom/base/nsJSEnvironment.h b/dom/base/nsJSEnvironment.h
index 17bf0335a6c..3ebf50f5727 100644
--- a/dom/base/nsJSEnvironment.h
+++ b/dom/base/nsJSEnvironment.h
@@ -116,6 +116,9 @@ public:
   static void PokeShrinkGCBuffers();
   static void KillShrinkGCBuffersTimer();
 
+  static void PokeShrinkingGC();
+  static void KillShrinkingGCTimer();
+
   static void MaybePokeCC();
   static void KillCCTimer();
   static void KillICCTimer();
diff --git a/js/public/GCAPI.h b/js/public/GCAPI.h
index 1c49d348737..6d67dc18ad1 100644
--- a/js/public/GCAPI.h
+++ b/js/public/GCAPI.h
@@ -99,7 +99,8 @@ namespace JS {
     D(REFRESH_FRAME)                            \
     D(FULL_GC_TIMER)                            \
     D(SHUTDOWN_CC)                              \
-    D(FINISH_LARGE_EVALUATE)
+    D(FINISH_LARGE_EVALUATE)                    \
+    D(USER_INACTIVE)
 
 namespace gcreason {
 
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index 537290f2175..096123329ae 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -364,7 +364,9 @@ js::AssertSameCompartment(JSObject *objA, JSObject *objB)
 JS_FRIEND_API(void)
 js::NotifyAnimationActivity(JSObject *obj)
 {
-    obj->compartment()->lastAnimationTime = PRMJ_Now();
+    int64_t timeNow = PRMJ_Now();
+    obj->compartment()->lastAnimationTime = timeNow;
+    obj->runtimeFromMainThread()->lastAnimationTime = timeNow;
 }
 
 JS_FRIEND_API(uint32_t)
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index c44e761c405..5995ab666a6 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1888,7 +1888,10 @@ ArenaLists::allocateFromArenaInner(JS::Zone *zone, ArenaHeader *aheader, AllocKi
 bool
 GCRuntime::shouldCompact()
 {
-    return invocationKind == GC_SHRINK && isCompactingGCEnabled();
+    // Compact on shrinking GC if enabled, but skip compacting in incremental
+    // GCs if we are currently animating.
+    return invocationKind == GC_SHRINK && isCompactingGCEnabled() &&
+        (!isIncremental || rt->lastAnimationTime + PRMJ_USEC_PER_SEC < PRMJ_Now());
 }
 
 void
diff --git a/js/src/vm/Runtime.cpp b/js/src/vm/Runtime.cpp
index 97e162e201b..a3dce47fbab 100644
--- a/js/src/vm/Runtime.cpp
+++ b/js/src/vm/Runtime.cpp
@@ -212,7 +212,8 @@ JSRuntime::JSRuntime(JSRuntime *parentRuntime)
 #endif
     largeAllocationFailureCallback(nullptr),
     oomCallback(nullptr),
-    debuggerMallocSizeOf(ReturnZeroSize)
+    debuggerMallocSizeOf(ReturnZeroSize),
+    lastAnimationTime(0)
 {
     setGCStoreBufferPtr(&gc.storeBuffer);
 
diff --git a/js/src/vm/Runtime.h b/js/src/vm/Runtime.h
index 57bcf8a29e9..ef1104787f1 100644
--- a/js/src/vm/Runtime.h
+++ b/js/src/vm/Runtime.h
@@ -1417,6 +1417,9 @@ struct JSRuntime : public JS::shadow::Runtime,
      * function to assess the size of malloc'd blocks of memory.
      */
     mozilla::MallocSizeOf debuggerMallocSizeOf;
+
+    /* Last time at which an animation was played for this runtime. */
+    int64_t lastAnimationTime;
 };
 
 namespace js {
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index f50f34c4e60..938eba86b43 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1081,6 +1081,7 @@ pref("javascript.options.mem.gc_compacting", true);
 pref("javascript.options.mem.log", false);
 pref("javascript.options.mem.notify", false);
 pref("javascript.options.gc_on_memory_pressure", true);
+pref("javascript.options.compact_on_user_inactive", true);
 
 pref("javascript.options.mem.gc_high_frequency_time_limit_ms", 1000);
 pref("javascript.options.mem.gc_high_frequency_low_limit_mb", 100);