--- a/js/public/GCAPI.h
+++ b/js/public/GCAPI.h
@@ -147,16 +147,19 @@ extern JS_FRIEND_API(bool)
 IsIncrementalGCEnabled(JSRuntime *rt);
 
 JS_FRIEND_API(bool)
 IsIncrementalGCInProgress(JSRuntime *rt);
 
 extern JS_FRIEND_API(void)
 DisableIncrementalGC(JSRuntime *rt);
 
+extern JS_FRIEND_API(void)
+DisableGenerationalGC(JSRuntime *rt);
+
 extern JS_FRIEND_API(bool)
 IsIncrementalBarrierNeeded(JSRuntime *rt);
 
 extern JS_FRIEND_API(bool)
 IsIncrementalBarrierNeeded(JSContext *cx);
 
 extern JS_FRIEND_API(void)
 IncrementalReferenceBarrier(void *ptr, JSGCTraceKind kind);

--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -777,16 +777,17 @@ JSRuntime::JSRuntime(JSUseHelperThreads 
     gcAbortSweepAfterCurrentGroup(false),
     gcArenasAllocatedDuringSweep(NULL),
 #ifdef DEBUG
     gcMarkingValidator(NULL),
 #endif
     gcInterFrameGC(0),
     gcSliceBudget(SliceBudget::Unlimited),
     gcIncrementalEnabled(true),
+    gcGenerationalEnabled(true),
     gcManipulatingDeadZones(false),
     gcObjectsMarkedInDeadZones(0),
     gcPoke(false),
     heapState(Idle),
 #ifdef JSGC_GENERATIONAL
     gcNursery(),
     gcStoreBuffer(&gcNursery),
 #endif
@@ -2884,19 +2885,16 @@ JS_SetGCParameter(JSRuntime *rt, JSGCPar
         rt->gcDynamicMarkSlice = value;
         break;
       case JSGC_ANALYSIS_PURGE_TRIGGER:
         rt->analysisPurgeTriggerBytes = value * 1024 * 1024;
         break;
       case JSGC_ALLOCATION_THRESHOLD:
         rt->gcAllocationThreshold = value * 1024 * 1024;
         break;
-      case JSGC_ENABLE_GENERATIONAL:
-        rt->gcGenerationalEnabled = bool(value);
-        break;
       default:
         JS_ASSERT(key == JSGC_MODE);
         rt->gcMode = JSGCMode(value);
         JS_ASSERT(rt->gcMode == JSGC_MODE_GLOBAL ||
                   rt->gcMode == JSGC_MODE_COMPARTMENT ||
                   rt->gcMode == JSGC_MODE_INCREMENTAL);
         return;
     }

--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -2909,19 +2909,16 @@ typedef enum JSGCParamKey {
     /* If true, high-frequency GCs will use a longer mark slice. */
     JSGC_DYNAMIC_MARK_SLICE = 18,
 
     /* Number of megabytes of analysis data to allocate before purging. */
     JSGC_ANALYSIS_PURGE_TRIGGER = 19,
 
     /* Lower limit after which we limit the heap growth. */
     JSGC_ALLOCATION_THRESHOLD = 20,
-
-    /* Enable the generational GC. */
-    JSGC_ENABLE_GENERATIONAL = 21
 } JSGCParamKey;
 
 typedef enum JSGCMode {
     /* Perform only global GCs. */
     JSGC_MODE_GLOBAL = 0,
 
     /* Perform per-compartment GCs until too much garbage has accumulated. */
     JSGC_MODE_COMPARTMENT = 1,

--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -841,16 +841,25 @@ JS::IsIncrementalGCInProgress(JSRuntime 
 }
 
 JS_FRIEND_API(void)
 JS::DisableIncrementalGC(JSRuntime *rt)
 {
     rt->gcIncrementalEnabled = false;
 }
 
+extern JS_FRIEND_API(void)
+JS::DisableGenerationalGC(JSRuntime *rt)
+{
+    rt->gcGenerationalEnabled = false;
+#ifdef JSGC_GENERATIONAL
+    rt->gcStoreBuffer.disable();
+#endif
+}
+
 JS_FRIEND_API(bool)
 JS::IsIncrementalBarrierNeeded(JSRuntime *rt)
 {
     return (rt->gcIncrementalState == gc::MARK && !rt->isHeapBusy());
 }
 
 JS_FRIEND_API(bool)
 JS::IsIncrementalBarrierNeeded(JSContext *cx)

--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -5251,17 +5251,18 @@ main(int argc, char **argv, char **envp)
     if (!rt)
         return 1;
     gTimeoutFunc = NullValue();
     if (!JS_AddNamedValueRootRT(rt, &gTimeoutFunc, "gTimeoutFunc"))
         return 1;
 
     JS_SetGCParameter(rt, JSGC_MAX_BYTES, 0xffffffff);
 #ifdef JSGC_GENERATIONAL
-    JS_SetGCParameter(rt, JSGC_ENABLE_GENERATIONAL, op.getBoolOption("ggc"));
+    if (!op.getBoolOption("ggc"))
+        JS::DisableGenerationalGC(rt);
 #endif
 
     JS_SetTrustedPrincipals(rt, &shellTrustedPrincipals);
     JS_SetSecurityCallbacks(rt, &securityCallbacks);
 
     JS_SetNativeStackQuota(rt, gMaxStackSize);
 
     if (!InitWatchdog(rt))