diff --git a/js/src/xpconnect/src/nsXPConnect.cpp b/js/src/xpconnect/src/nsXPConnect.cpp
index ae20148a7107..f94c56b78aec 100644
--- a/js/src/xpconnect/src/nsXPConnect.cpp
+++ b/js/src/xpconnect/src/nsXPConnect.cpp
@@ -1785,7 +1785,8 @@ nsXPConnect::RestoreWrappedNativePrototype(JSContext * aJSContext,
     if(si && si->GetFlags().DontSharePrototype())
         return UnexpectedFailure(NS_ERROR_INVALID_ARG);
 
-    ClassInfo2WrappedNativeProtoMap* map = scope->GetWrappedNativeProtoMap();
+    ClassInfo2WrappedNativeProtoMap* map =
+        scope->GetWrappedNativeProtoMap(proto->ClassIsMainThreadOnly());
     XPCLock* lock = GetRuntime()->GetMapLock();
 
     {   // scoped lock
diff --git a/js/src/xpconnect/src/xpcprivate.h b/js/src/xpconnect/src/xpcprivate.h
index d0060f33846c..5dad1589c53e 100644
--- a/js/src/xpconnect/src/xpcprivate.h
+++ b/js/src/xpconnect/src/xpcprivate.h
@@ -1234,7 +1234,10 @@ public:
     GetWrapperMap() const {return mWrapperMap;}
 
     ClassInfo2WrappedNativeProtoMap*
-    GetWrappedNativeProtoMap() const {return mWrappedNativeProtoMap;}
+    GetWrappedNativeProtoMap(JSBool aMainThreadOnly) const
+        {return aMainThreadOnly ?
+                mMainThreadWrappedNativeProtoMap :
+                mWrappedNativeProtoMap;}
 
     nsXPCComponents*
     GetComponents() const {return mComponents;}
@@ -1340,6 +1343,7 @@ private:
     XPCJSRuntime*                    mRuntime;
     Native2WrappedNativeMap*         mWrappedNativeMap;
     ClassInfo2WrappedNativeProtoMap* mWrappedNativeProtoMap;
+    ClassInfo2WrappedNativeProtoMap* mMainThreadWrappedNativeProtoMap;
     WrappedNative2WrapperMap*        mWrapperMap;
     nsXPCComponents*                 mComponents;
     XPCWrappedNativeScope*           mNext;
diff --git a/js/src/xpconnect/src/xpcwrappednativeproto.cpp b/js/src/xpconnect/src/xpcwrappednativeproto.cpp
index 28bc4bb1d25d..948a96d0d491 100644
--- a/js/src/xpconnect/src/xpcwrappednativeproto.cpp
+++ b/js/src/xpconnect/src/xpcwrappednativeproto.cpp
@@ -167,7 +167,7 @@ XPCWrappedNativeProto::JSProtoObjectFinalized(JSContext *cx, JSObject *obj)
     {
         // Only remove this proto from the map if it is the one in the map.
         ClassInfo2WrappedNativeProtoMap* map = 
-            GetScope()->GetWrappedNativeProtoMap();
+            GetScope()->GetWrappedNativeProtoMap(ClassIsMainThreadOnly());
         if(map->Find(mClassInfo) == this)
             map->Remove(mClassInfo);
     }
@@ -244,8 +244,9 @@ XPCWrappedNativeProto::GetNewOrUsed(XPCCallContext& ccx,
 
     if(shared)
     {
-        map = Scope->GetWrappedNativeProtoMap();
-        lock = Scope->GetRuntime()->GetMapLock();
+        JSBool mainThreadOnly = !!(ciFlags & nsIClassInfo::MAIN_THREAD_ONLY);
+        map = Scope->GetWrappedNativeProtoMap(mainThreadOnly);
+        lock = mainThreadOnly ? nsnull : Scope->GetRuntime()->GetMapLock();
         {   // scoped lock
             XPCAutoLock al(lock);
             proto = map->Find(ClassInfo);
diff --git a/js/src/xpconnect/src/xpcwrappednativescope.cpp b/js/src/xpconnect/src/xpcwrappednativescope.cpp
index c33e67aac5e9..c9d6cc8325f9 100644
--- a/js/src/xpconnect/src/xpcwrappednativescope.cpp
+++ b/js/src/xpconnect/src/xpcwrappednativescope.cpp
@@ -138,6 +138,7 @@ XPCWrappedNativeScope::XPCWrappedNativeScope(XPCCallContext& ccx,
     :   mRuntime(ccx.GetRuntime()),
         mWrappedNativeMap(Native2WrappedNativeMap::newMap(XPC_NATIVE_MAP_SIZE)),
         mWrappedNativeProtoMap(ClassInfo2WrappedNativeProtoMap::newMap(XPC_NATIVE_PROTO_MAP_SIZE)),
+        mMainThreadWrappedNativeProtoMap(ClassInfo2WrappedNativeProtoMap::newMap(XPC_NATIVE_PROTO_MAP_SIZE)),
         mWrapperMap(WrappedNative2WrapperMap::newMap(XPC_WRAPPER_MAP_SIZE)),
         mComponents(nsnull),
         mNext(nsnull),
@@ -319,6 +320,12 @@ XPCWrappedNativeScope::~XPCWrappedNativeScope()
         delete mWrappedNativeProtoMap;
     }
 
+    if(mMainThreadWrappedNativeProtoMap)
+    {
+        NS_ASSERTION(0 == mMainThreadWrappedNativeProtoMap->Count(), "scope has non-empty map");
+        delete mMainThreadWrappedNativeProtoMap;
+    }
+
     if(mWrapperMap)
     {
         NS_ASSERTION(0 == mWrapperMap->Count(), "scope has non-empty map");
@@ -532,6 +539,7 @@ XPCWrappedNativeScope::MarkAllWrappedNativesAndProtos()
     {
         cur->mWrappedNativeMap->Enumerate(WrappedNativeMarker, nsnull);
         cur->mWrappedNativeProtoMap->Enumerate(WrappedNativeProtoMarker, nsnull);
+        cur->mMainThreadWrappedNativeProtoMap->Enumerate(WrappedNativeProtoMarker, nsnull);
     }
 
     DEBUG_TrackScopeTraversal();
@@ -564,6 +572,8 @@ XPCWrappedNativeScope::ASSERT_NoInterfaceSetsAreMarked()
             ASSERT_WrappedNativeSetNotMarked, nsnull);
         cur->mWrappedNativeProtoMap->Enumerate(
             ASSERT_WrappedNativeProtoSetNotMarked, nsnull);
+        cur->mMainThreadWrappedNativeProtoMap->Enumerate(
+            ASSERT_WrappedNativeProtoSetNotMarked, nsnull);
     }
 }
 #endif
@@ -678,6 +688,8 @@ XPCWrappedNativeScope::SystemIsBeingShutDown(JSContext* cx)
         // proto pointers in the proto map.
         cur->mWrappedNativeProtoMap->
                 Enumerate(WrappedNativeProtoShutdownEnumerator,  &data);
+        cur->mMainThreadWrappedNativeProtoMap->
+                Enumerate(WrappedNativeProtoShutdownEnumerator,  &data);
         cur->mWrappedNativeMap->
                 Enumerate(WrappedNativeShutdownEnumerator,  &data);
     }
@@ -867,6 +879,7 @@ XPCWrappedNativeScope::ClearAllWrappedNativeSecurityPolicies(XPCCallContext& ccx
     for(XPCWrappedNativeScope* cur = gScopes; cur; cur = cur->mNext)
     {
         cur->mWrappedNativeProtoMap->Enumerate(WNProtoSecPolicyClearer, nsnull);
+        cur->mMainThreadWrappedNativeProtoMap->Enumerate(WNProtoSecPolicyClearer, nsnull);
         cur->mWrappedNativeMap->Enumerate(WNSecPolicyClearer, nsnull);
     }
 
@@ -896,6 +909,8 @@ XPCWrappedNativeScope::RemoveWrappedNativeProtos()
     
     mWrappedNativeProtoMap->Enumerate(WNProtoRemover, 
         GetRuntime()->GetDetachedWrappedNativeProtoMap());
+    mMainThreadWrappedNativeProtoMap->Enumerate(WNProtoRemover, 
+        GetRuntime()->GetDetachedWrappedNativeProtoMap());
 }
 
 /***************************************************************************/
@@ -976,6 +991,17 @@ XPCWrappedNativeScope::DebugDump(PRInt16 depth)
             mWrappedNativeProtoMap->Enumerate(WrappedNativeProtoMapDumpEnumerator, &depth);
             XPC_LOG_OUTDENT();
         }
+
+        XPC_LOG_ALWAYS(("mMainThreadWrappedNativeProtoMap @ %x with %d protos(s)", \
+                         mMainThreadWrappedNativeProtoMap, \
+                         mMainThreadWrappedNativeProtoMap ? mMainThreadWrappedNativeProtoMap->Count() : 0));
+        // iterate contexts...
+        if(depth && mMainThreadWrappedNativeProtoMap && mMainThreadWrappedNativeProtoMap->Count())
+        {
+            XPC_LOG_INDENT();
+            mMainThreadWrappedNativeProtoMap->Enumerate(WrappedNativeProtoMapDumpEnumerator, &depth);
+            XPC_LOG_OUTDENT();
+        }
     XPC_LOG_OUTDENT();
 #endif
 }