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 WebExtensions technology is designed for cross-browser compatibility. The API is largely 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.