XrayWrapper::defineProperty is stricter than ordinary Object.defineProperty about what sort of redefinition is permitted. But it doesn't report failure in any way when the rules are broken; it just silently doesn't define anything. There's even a comment: // We should technically report non-configurability in strict mode, but // doing that via JSAPI is a lot of trouble. With bug 1113369, it's not much trouble anymore. (In fact it's trivial, but it should be tested somehow.)