Firefox already ships with some (most?) of these features.
edit: http://kangax.github.com/es5-compat-table/es6/ considering the sea of red, it's rather clear that both Firefox and Chrome only implement some ES6 features (note: this crashes Chrome 22 after enabling "experimental javascript", which may explain the big flag, and the latest Firefox 16 improves quite a bit on the listed Firefox 13: adds const, default params, rest params, proxies and all the new Number functions)