diff --git a/js/src/gc/GCRuntime.h b/js/src/gc/GCRuntime.h
index a4ad292cce20..c2ff7fe72bb9 100644
--- a/js/src/gc/GCRuntime.h
+++ b/js/src/gc/GCRuntime.h
@@ -937,7 +937,8 @@ class GCRuntime
     void startTask(GCParallelTask& task, gcstats::PhaseKind phase, AutoLockHelperThreadState& locked);
     void joinTask(GCParallelTask& task, gcstats::PhaseKind phase, AutoLockHelperThreadState& locked);
 
-  private:
+    // Delete an empty zone group after its contents have been merged.
+    void deleteEmptyZoneGroup(ZoneGroup* group);
 
   private:
     enum IncrementalResult
@@ -1096,7 +1097,10 @@ class GCRuntime
     UnprotectedData<ZoneGroup*> systemZoneGroup;
 
     // List of all zone groups (protected by the GC lock).
-    ActiveThreadOrGCTaskData<ZoneGroupVector> groups;
+  private:
+    ActiveThreadOrGCTaskData<ZoneGroupVector> groups_;
+  public:
+    ZoneGroupVector& groups() { return groups_.ref(); }
 
     // The unique atoms zone, which has no zone group.
     WriteOnceData<Zone*> atomsZone;
diff --git a/js/src/gc/Zone.cpp b/js/src/gc/Zone.cpp
index 42fb083cec47..343864a1fdd7 100644
--- a/js/src/gc/Zone.cpp
+++ b/js/src/gc/Zone.cpp
@@ -73,6 +73,8 @@ JS::Zone::Zone(JSRuntime* rt, ZoneGroup* group)
 
 Zone::~Zone()
 {
+    MOZ_ASSERT(compartments_.ref().empty());
+
     JSRuntime* rt = runtimeFromAnyThread();
     if (this == rt->gc.systemZone)
         rt->gc.systemZone = nullptr;
@@ -87,7 +89,8 @@ Zone::~Zone()
 #endif
 }
 
-bool Zone::init(bool isSystemArg)
+bool
+Zone::init(bool isSystemArg)
 {
     isSystem = isSystemArg;
     return uniqueIds().init() &&
@@ -376,6 +379,21 @@ Zone::addTypeDescrObject(JSContext* cx, HandleObject obj)
     return true;
 }
 
+void
+Zone::deleteEmptyCompartment(JSCompartment* comp)
+{
+    MOZ_ASSERT(comp->zone() == this);
+    MOZ_ASSERT(arenas.checkEmptyArenaLists());
+    for (auto& i : compartments()) {
+        if (i == comp) {
+            compartments().erase(&i);
+            comp->destroy(runtimeFromActiveCooperatingThread()->defaultFreeOp());
+            return;
+        }
+    }
+    MOZ_CRASH("Compartment not found");
+}
+
 ZoneList::ZoneList()
   : head(nullptr), tail(nullptr)
 {}
diff --git a/js/src/gc/Zone.h b/js/src/gc/Zone.h
index 6a9b37ebf985..17f884f03062 100644
--- a/js/src/gc/Zone.h
+++ b/js/src/gc/Zone.h
@@ -161,6 +161,7 @@ struct Zone : public JS::shadow::Zone,
     explicit Zone(JSRuntime* rt, js::ZoneGroup* group);
     ~Zone();
     MOZ_MUST_USE bool init(bool isSystem);
+    void destroy(js::FreeOp *fop);
 
   private:
     js::ZoneGroup* const group_;
@@ -622,6 +623,9 @@ struct Zone : public JS::shadow::Zone,
         keepShapeTables_ = b;
     }
 
+    // Delete an empty compartment after its contents have been merged.
+    void deleteEmptyCompartment(JSCompartment* comp);
+
   private:
     js::ZoneGroupData<js::jit::JitZone*> jitZone_;
 
@@ -654,8 +658,8 @@ class ZoneGroupsIter
 
   public:
     explicit ZoneGroupsIter(JSRuntime* rt) : iterMarker(&rt->gc) {
-        it = rt->gc.groups.ref().begin();
-        end = rt->gc.groups.ref().end();
+        it = rt->gc.groups().begin();
+        end = rt->gc.groups().end();
 
         if (!done() && (*it)->usedByHelperThread)
             next();
diff --git a/js/src/gc/ZoneGroup.cpp b/js/src/gc/ZoneGroup.cpp
index 11e53204100a..7ecd7927b607 100644
--- a/js/src/gc/ZoneGroup.cpp
+++ b/js/src/gc/ZoneGroup.cpp
@@ -130,4 +130,20 @@ ZoneGroup::ionLazyLinkListAdd(jit::IonBuilder* builder)
     ionLazyLinkListSize_++;
 }
 
+void
+ZoneGroup::deleteEmptyZone(Zone* zone)
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime));
+    MOZ_ASSERT(zone->group() == this);
+    MOZ_ASSERT(zone->compartments().empty());
+    for (auto& i : zones()) {
+        if (i == zone) {
+            zones().erase(&i);
+            zone->destroy(runtime->defaultFreeOp());
+            return;
+        }
+    }
+    MOZ_CRASH("Zone not found");
+}
+
 } // namespace js
diff --git a/js/src/gc/ZoneGroup.h b/js/src/gc/ZoneGroup.h
index de49b5dcf0f8..7901793d7973 100644
--- a/js/src/gc/ZoneGroup.h
+++ b/js/src/gc/ZoneGroup.h
@@ -76,6 +76,9 @@ class ZoneGroup
     // See the useExclusiveLocking field above.
     void setUseExclusiveLocking() { useExclusiveLocking = true; }
 
+    // Delete an empty zone after its contents have been merged.
+    void deleteEmptyZone(Zone* zone);
+
 #ifdef DEBUG
   private:
     // The number of possible bailing places encounters before forcefully bailing
diff --git a/js/src/jscompartment.h b/js/src/jscompartment.h
index 714462b91c80..6e07cef4e4a6 100644
--- a/js/src/jscompartment.h
+++ b/js/src/jscompartment.h
@@ -863,6 +863,7 @@ struct JSCompartment
     ~JSCompartment();
 
     MOZ_MUST_USE bool init(JSContext* maybecx);
+    void destroy(js::FreeOp* fop);
 
     MOZ_MUST_USE inline bool wrap(JSContext* cx, JS::MutableHandleValue vp);
 
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index c3ca45bcda39..6a187c53e09d 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1220,11 +1220,12 @@ GCRuntime::finish()
         for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
             for (CompartmentsInZoneIter comp(zone); !comp.done(); comp.next())
                 js_delete(comp.get());
+            zone->compartments().clear();
             js_delete(zone.get());
         }
     }
 
-    groups.ref().clear();
+    groups().clear();
 
     FreeChunkPool(rt, fullChunks_.ref());
     FreeChunkPool(rt, availableChunks_.ref());
@@ -3483,6 +3484,25 @@ JS::Zone::sweepUniqueIds(js::FreeOp* fop)
     uniqueIds().sweep();
 }
 
+void
+JSCompartment::destroy(FreeOp* fop)
+{
+    JSRuntime* rt = fop->runtime();
+    if (auto callback = rt->destroyCompartmentCallback)
+        callback(fop, this);
+    if (principals())
+        JS_DropPrincipals(TlsContext.get(), principals());
+    fop->delete_(this);
+    rt->gc.stats().sweptCompartment();
+}
+
+void
+Zone::destroy(FreeOp* fop)
+{
+    fop->delete_(this);
+    fop->runtime()->gc.stats().sweptZone();
+}
+
 /*
  * It's simpler if we preserve the invariant that every zone has at least one
  * compartment. If we know we're deleting the entire zone, then
@@ -3495,8 +3515,9 @@ JS::Zone::sweepUniqueIds(js::FreeOp* fop)
 void
 Zone::sweepCompartments(FreeOp* fop, bool keepAtleastOne, bool destroyingRuntime)
 {
-    JSRuntime* rt = runtimeFromActiveCooperatingThread();
-    JSDestroyCompartmentCallback callback = rt->destroyCompartmentCallback;
+    MOZ_ASSERT(!compartments().empty());
+
+    mozilla::DebugOnly<JSRuntime*> rt = runtimeFromActiveCooperatingThread();
 
     JSCompartment** read = compartments().begin();
     JSCompartment** end = compartments().end();
@@ -3512,12 +3533,7 @@ Zone::sweepCompartments(FreeOp* fop, bool keepAtleastOne, bool destroyingRuntime
          */
         bool dontDelete = read == end && !foundOne && keepAtleastOne;
         if ((!comp->marked && !dontDelete) || destroyingRuntime) {
-            if (callback)
-                callback(fop, comp);
-            if (comp->principals())
-                JS_DropPrincipals(TlsContext.get(), comp->principals());
-            js_delete(comp);
-            rt->gc.stats().sweptCompartment();
+            comp->destroy(fop);
         } else {
             *write++ = comp;
             foundOne = true;
@@ -3530,6 +3546,8 @@ Zone::sweepCompartments(FreeOp* fop, bool keepAtleastOne, bool destroyingRuntime
 void
 GCRuntime::sweepZones(FreeOp* fop, ZoneGroup* group, bool destroyingRuntime)
 {
+    MOZ_ASSERT(!group->zones().empty());
+
     Zone** read = group->zones().begin();
     Zone** end = group->zones().end();
     Zone** write = read;
@@ -3558,8 +3576,7 @@ GCRuntime::sweepZones(FreeOp* fop, ZoneGroup* group, bool destroyingRuntime)
                 zone->sweepCompartments(fop, false, destroyingRuntime);
                 MOZ_ASSERT(zone->compartments().empty());
                 MOZ_ASSERT_IF(arenasEmptyAtShutdown, zone->typeDescrObjects().empty());
-                fop->delete_(zone);
-                stats().sweptZone();
+                zone->destroy(fop);
                 continue;
             }
             zone->sweepCompartments(fop, true, destroyingRuntime);
@@ -3580,8 +3597,8 @@ GCRuntime::sweepZoneGroups(FreeOp* fop, bool destroyingRuntime)
 
     assertBackgroundSweepingFinished();
 
-    ZoneGroup** read = groups.ref().begin();
-    ZoneGroup** end = groups.ref().end();
+    ZoneGroup** read = groups().begin();
+    ZoneGroup** end = groups().end();
     ZoneGroup** write = read;
 
     while (read < end) {
@@ -3595,7 +3612,7 @@ GCRuntime::sweepZoneGroups(FreeOp* fop, bool destroyingRuntime)
             *write++ = group;
         }
     }
-    groups.ref().shrinkTo(write - groups.ref().begin());
+    groups().shrinkTo(write - groups().begin());
 }
 
 #ifdef DEBUG
@@ -7367,7 +7384,7 @@ js::NewCompartment(JSContext* cx, JSPrincipals* principals,
     }
 
     if (groupHolder) {
-        if (!rt->gc.groups.ref().append(group)) {
+        if (!rt->gc.groups().append(group)) {
             ReportOutOfMemory(cx);
             return nullptr;
         }
@@ -7397,6 +7414,10 @@ gc::MergeCompartments(JSCompartment* source, JSCompartment* target)
     MOZ_ASSERT(source->creationOptions().addonIdOrNull() ==
                target->creationOptions().addonIdOrNull());
 
+    MOZ_ASSERT(!source->hasBeenEntered());
+    MOZ_ASSERT(source->zone()->compartments().length() == 1);
+    MOZ_ASSERT(source->zone()->group()->zones().length() == 1);
+
     JSContext* cx = source->runtimeFromActiveCooperatingThread()->activeContextFromOwnThread();
 
     MOZ_ASSERT(!source->zone()->wasGCStarted());
@@ -7489,6 +7510,32 @@ gc::MergeCompartments(JSCompartment* source, JSCompartment* target)
 
         source->scriptNameMap->clear();
     }
+
+    // The source compartment is now completely empty, and is the only
+    // compartment in its zone, which is the only zone in its group. Delete
+    // compartment, zone and group without waiting for this to be cleaned up by
+    // a full GC.
+
+    Zone* sourceZone = source->zone();
+    ZoneGroup* sourceGroup = sourceZone->group();
+    sourceZone->deleteEmptyCompartment(source);
+    sourceGroup->deleteEmptyZone(sourceZone);
+    cx->runtime()->gc.deleteEmptyZoneGroup(sourceGroup);
+}
+
+void
+GCRuntime::deleteEmptyZoneGroup(ZoneGroup* group)
+{
+    MOZ_ASSERT(group->zones().empty());
+    MOZ_ASSERT(groups().length() > 1);
+    for (auto& i : groups()) {
+        if (i == group) {
+            groups().erase(&i);
+            js_delete(group);
+            return;
+        }
+    }
+    MOZ_CRASH("ZoneGroup not found");
 }
 
 void
diff --git a/js/src/vm/HelperThreads.cpp b/js/src/vm/HelperThreads.cpp
index 4fd85ba48a17..af56832ab007 100644
--- a/js/src/vm/HelperThreads.cpp
+++ b/js/src/vm/HelperThreads.cpp
@@ -1598,9 +1598,10 @@ GlobalHelperThreadState::mergeParseTaskCompartment(JSContext* cx, ParseTask* par
     JS::AutoAssertNoGC nogc(cx);
 
     LeaveParseTaskZone(cx->runtime(), parseTask);
-    AutoCompartment ac(cx, parseTask->parseGlobal);
 
     {
+        AutoCompartment ac(cx, parseTask->parseGlobal);
+
         // Generator functions don't have Function.prototype as prototype but a
         // different function object, so the IdentifyStandardPrototype trick
         // below won't work.  Just special-case it.