We’ve been working on getting WebExtensions ready for Firefox 57, and
wanted to follow up with our status regarding the W3C Community Group.
At this time Firefox implements a large part of the specification.
Deviations from the spec are being tracked in bug 1392434.
Following the specification allows developers to write extensions and
know that the core of the extension will work across browsers, and
also understand where browsers may diverge from the specification.
We believe that extensions can do more than they do today, and will
continue to expand the API set Firefox supports as we move forward. As
we extend the APIs available to developers, we’ll be marking the API
appropriately on MDN so developers know what to expect. It is our
hope that other browsers will do the same, and that we’ll collectively
grow the extension API.
This is documented on the wiki.
You can read the original email about this topic on the dev-addons archive.