diff --git a/js/src/xpconnect/src/xpcprivate.h b/js/src/xpconnect/src/xpcprivate.h
index 88b2003e6d18..bf05d807f333 100644
--- a/js/src/xpconnect/src/xpcprivate.h
+++ b/js/src/xpconnect/src/xpcprivate.h
@@ -4405,7 +4405,9 @@ inline jsval
 GetRTStringByIndex(JSContext *cx, uintN index);
 
 // Wrapper for JS_NewObject to mark the new object as system when parent is
-// also a system object.
+// also a system object. If uniqueType is specified then a new type object will
+// be created which is used only by the result, so that its property types
+// will be tracked precisely.
 inline JSObject*
 xpc_NewSystemInheritingJSObject(JSContext *cx, JSClass *clasp, JSObject *proto,
                                 bool uniqueType, JSObject *parent);
diff --git a/modules/libpref/src/init/all.js b/modules/libpref/src/init/all.js
index 2217054d5c90..2a334a7f66a1 100644
--- a/modules/libpref/src/init/all.js
+++ b/modules/libpref/src/init/all.js
@@ -616,7 +616,7 @@ pref("javascript.options.methodjit.content", true);
 pref("javascript.options.methodjit.chrome",  true);
 pref("javascript.options.jitprofiling.content", true);
 pref("javascript.options.jitprofiling.chrome",  true);
-pref("javascript.options.methodjit_always", false);
+pref("javascript.options.methodjit_always", true);
 pref("javascript.options.typeinference", true);
 // This preference limits the memory usage of javascript.
 // If you want to change these values for your device,