@ljharb See my comment https://github.com/paulmillr/es6-shim/issues/182#issuecomment-32467084 .
Firefox already implements Map/Set with iterator protocol, but instead of using `Symbol.iterator` Firefox uses '@@iterator'; Currently es6-shim rewrite Firefox Map/Set due FF allows to creating new Map/Set without `new` operator. When FF disallow this behavior es6-shim Array.from function would be broken (in the working with Iterator Protocol part).