WebExtensions are currently in an experimental alpha state. From Firefox 46, you can publish WebExtensions to Firefox users, just like any other add-on. We're aiming for a first stable release in Firefox 48.
WebExtensions are a cross-browser system for developing browser add-ons. To a large extent the system is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox or Microsoft Edge with just a few changes. The API is also fully compatible with multiprocess Firefox.
We're also intending to extend the APIs to support the needs of add-on developers, so if you have ideas, we'd love to hear them. You can reach us on the dev-addons mailing list or #webextensions on IRC.