diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index 9bb8efd22f8..084cda85c94 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -3871,6 +3871,13 @@ nsDOMWindowUtils::SetNextPaintSyncId(int32_t aSyncId)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::CycleCollectFull()
+{
+  nsJSContext::CycleCollectFullNow();
+  return NS_OK;
+}
+
 NS_INTERFACE_MAP_BEGIN(nsTranslationNodeList)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
   NS_INTERFACE_MAP_ENTRY(nsITranslationNodeList)
diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 8d27cf3d468..c653bc86619 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1583,6 +1583,22 @@ nsJSContext::CycleCollectNow(nsICycleCollectorListener *aListener,
   gCCStats.FinishCycleCollectionSlice();
 }
 
+//static
+void
+nsJSContext::CycleCollectFullNow()
+{
+  if (!NS_IsMainThread()) {
+    return;
+  }
+
+  PROFILER_LABEL("nsJSContext", "CycleCollectFullNow",
+    js::ProfileEntry::Category::CC);
+
+  gCCStats.PrepareForCycleCollectionSlice(0);
+  nsCycleCollector_collectFull();
+  gCCStats.FinishCycleCollectionSlice();
+}
+
 //static
 void
 nsJSContext::RunCycleCollectorSlice()
diff --git a/dom/base/nsJSEnvironment.h b/dom/base/nsJSEnvironment.h
index 5b9509cf9d3..b25dd48b007 100644
--- a/dom/base/nsJSEnvironment.h
+++ b/dom/base/nsJSEnvironment.h
@@ -94,6 +94,7 @@ public:
   // called even if the previous collection was GC.
   static void CycleCollectNow(nsICycleCollectorListener *aListener = nullptr,
                               int32_t aExtraForgetSkippableCalls = 0);
+  static void CycleCollectFullNow();
 
   // Run a cycle collector slice, using a heuristic to decide how long to run it.
   static void RunCycleCollectorSlice();
diff --git a/dom/interfaces/base/nsIDOMWindowUtils.idl b/dom/interfaces/base/nsIDOMWindowUtils.idl
index 5df31008946..33533561725 100644
--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -1842,6 +1842,9 @@ interface nsIDOMWindowUtils : nsISupports {
   void forceUseCounterFlush(in nsIDOMNode aNode);
 
   void setNextPaintSyncId(in long aSyncId);
+
+  /* Force a full cycle collection, including garbage collection and freeing all SnowWhites. */
+  void cycleCollectFull();
 };
 
 [scriptable, uuid(c694e359-7227-4392-a138-33c0cc1f15a6)]
diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 0fc5051d684..bb9627a10f8 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -1239,6 +1239,7 @@ enum ccType
 {
   SliceCC,     /* If a CC is in progress, continue it. Otherwise, start a new one. */
   ManualCC,    /* Explicitly triggered. */
+  FullCC,      /* Explicitly triggered, always doing a full collection. */
   ShutdownCC   /* Shutdown CC, used for finding leaks. */
 };
 
@@ -3692,6 +3693,8 @@ nsCycleCollector::Collect(ccType aCCType,
     if (Collect(aCCType, aBudget, aManualListener)) {
       collectedAny = true;
     }
+  } else if (aCCType == FullCC && collectedAny) {
+    FreeSnowWhite(true);
   }
 
   MOZ_ASSERT_IF(aCCType != SliceCC, IsIdle());
@@ -3808,7 +3811,7 @@ nsCycleCollector::BeginCollection(ccType aCCType,
 
   // On a WantAllTraces CC, force a synchronous global GC to prevent
   // hijinks from ForgetSkippable and compartmental GCs.
-  bool forceGC = isShutdown || (mLogger && mLogger->IsAllTraces());
+  bool forceGC = isShutdown || aCCType == FullCC || (mLogger && mLogger->IsAllTraces());
 
   // BeginCycleCollectionCallback() might have started an IGC, and we need
   // to finish it before we run FixGrayBits.
@@ -4137,6 +4140,22 @@ nsCycleCollector_collect(nsICycleCollectorListener* aManualListener)
   data->mCollector->Collect(ManualCC, unlimitedBudget, aManualListener);
 }
 
+void
+nsCycleCollector_collectFull()
+{
+  CollectorData* data = sCollectorData.get();
+
+  // We should have started the cycle collector by now.
+  MOZ_ASSERT(data);
+  MOZ_ASSERT(data->mCollector);
+
+  PROFILER_LABEL("nsCycleCollector", "collectFull",
+                 js::ProfileEntry::Category::CC);
+
+  SliceBudget unlimitedBudget = SliceBudget::unlimited();
+  data->mCollector->Collect(FullCC, unlimitedBudget, NULL);
+}
+
 void
 nsCycleCollector_collectSlice(SliceBudget& budget,
                               bool aPreferShorterSlices)
diff --git a/xpcom/base/nsCycleCollector.h b/xpcom/base/nsCycleCollector.h
index 3988702e48d..747a777b724 100644
--- a/xpcom/base/nsCycleCollector.h
+++ b/xpcom/base/nsCycleCollector.h
@@ -47,6 +47,8 @@ already_AddRefed<nsICycleCollectorLogSink> nsCycleCollector_createLogSink();
 
 void nsCycleCollector_collect(nsICycleCollectorListener* aManualListener);
 
+void nsCycleCollector_collectFull();
+
 void nsCycleCollector_collectSlice(js::SliceBudget& budget,
                                    bool aPreferShorterSlices = false);