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 new way to write browser extensions.
The technology is designed for cross-browser compatibility: to a large extent the API is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox with just a few changes. The API is also fully compatible with multiprocess Firefox.
We intend to deprecate XUL and XPCOM-based add-ons, and encourage add-on developers to use the WebExtensions API for new development.
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.