--- a/js/public/Class.h
+++ b/js/public/Class.h
@@ -513,19 +513,19 @@ Valueify(const JSClass *c)
     return (const Class *)c;
 }
 
 /*
  * Enumeration describing possible values of the [[Class]] internal property
  * value of objects.
  */
 enum ESClassValue {
-    ESClass_Array, ESClass_Number, ESClass_String, ESClass_Boolean,
-    ESClass_RegExp, ESClass_ArrayBuffer, ESClass_Date, ESClass_Set,
-    ESClass_Map
+    ESClass_Object, ESClass_Array, ESClass_Number, ESClass_String,
+    ESClass_Boolean, ESClass_RegExp, ESClass_ArrayBuffer, ESClass_Date,
+    ESClass_Set, ESClass_Map
 };
 
 /*
  * Return whether the given object has the given [[Class]] internal property
  * value. Beware, this query says nothing about the js::Class of the JSObject
  * so the caller must not assume anything about obj's representation (e.g., obj
  * may be a proxy).
  */

--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -1021,16 +1021,17 @@ GuessArrayGCKind(size_t numSlots)
 
 inline bool
 ObjectClassIs(HandleObject obj, ESClassValue classValue, JSContext *cx)
 {
     if (MOZ_UNLIKELY(obj->is<ProxyObject>()))
         return Proxy::objectClassIs(obj, classValue, cx);
 
     switch (classValue) {
+      case ESClass_Object: return obj->is<JSObject>();
       case ESClass_Array: return obj->is<ArrayObject>();
       case ESClass_Number: return obj->is<NumberObject>();
       case ESClass_String: return obj->is<StringObject>();
       case ESClass_Boolean: return obj->is<BooleanObject>();
       case ESClass_RegExp: return obj->is<RegExpObject>();
       case ESClass_ArrayBuffer:
         return obj->is<ArrayBufferObject>() || obj->is<SharedArrayBufferObject>();
       case ESClass_Date: return obj->is<DateObject>();

--- a/js/src/vm/StructuredClone.cpp
+++ b/js/src/vm/StructuredClone.cpp
@@ -925,17 +925,17 @@ JSStructuredCloneWriter::traverseObject(
 
     /* Push obj and count to the stack. */
     if (!objs.append(ObjectValue(*obj)) || !counts.append(properties.length()))
         return false;
 
     checkStack();
 
     /* Write the header for obj. */
-    return out.writePair(obj->is<ArrayObject>() ? SCTAG_ARRAY_OBJECT : SCTAG_OBJECT_OBJECT, 0);
+    return out.writePair(ObjectClassIs(obj, ESClass_Array, context()) ? SCTAG_ARRAY_OBJECT : SCTAG_OBJECT_OBJECT, 0);
 }
 
 bool
 JSStructuredCloneWriter::traverseMap(HandleObject obj)
 {
     AutoValueVector newEntries(context());
     {
         // If there is no wrapper, the compartment munging is a no-op.
@@ -1037,17 +1037,19 @@ JSStructuredCloneWriter::startWrite(Hand
                    writeString(SCTAG_STRING, re->getSource());
         } else if (ObjectClassIs(obj, ESClass_Date, context())) {
             double d = js_DateGetMsecSinceEpoch(obj);
             return out.writePair(SCTAG_DATE_OBJECT, 0) && out.writeDouble(d);
         } else if (JS_IsTypedArrayObject(obj)) {
             return writeTypedArray(obj);
         } else if (JS_IsArrayBufferObject(obj) && JS_ArrayBufferHasData(obj)) {
             return writeArrayBuffer(obj);
-        } else if (obj->is<JSObject>() || obj->is<ArrayObject>()) {
+        } else if (ObjectClassIs(obj, ESClass_Object, context())) {
+            return traverseObject(obj);
+        } else if (ObjectClassIs(obj, ESClass_Array, context())) {
             return traverseObject(obj);
         } else if (ObjectClassIs(obj, ESClass_Boolean, context())) {
             RootedValue unboxed(context());
             if (!Unbox(context(), obj, &unboxed))
                 return false;
             return out.writePair(SCTAG_BOOLEAN_OBJECT, unboxed.toBoolean());
         } else if (ObjectClassIs(obj, ESClass_Number, context())) {
             RootedValue unboxed(context());