You can see the latest proposals on the tc39/ecma262 GitHub repository.
A channel for feedback on ECMAScript standards is es-discuss.
These ES2016 / ES7 features are implemented:
- Generators and generator methods are no longer constructable (Firefox 43)
- Proxy enumerate handler removed (Firefox 47)
- Exponentiation operator (Firefox Nightly, see bug 1243858 for remaining work)
These ES2017 / ES8 features are implemented:
Experimental new features
The following features are already implemented, but only available in the Firefox Nightly channel and not yet included in a draft edition of an ECMAScript specification.