This is all we have so far: https://wiki.mozilla.org/Browser_Extensions
I guess that for extension developers there would also be some more general informations interesting. Also like the big shaped plans of other browser vendors to see if it is already interesting to switch to Web Extension. Or future plans/road map/ coming features ... Maybe the Wikipedia page is also a nice (back) link: https://en.wikipedia.org/wiki/Browser_extension
https://wiki.mozilla.org/WebExtensions/FAQ https://wiki.mozilla.org/WebExtensions/Future https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Anatomy_of_a_WebExtension https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/ https://discourse.mozilla-community.org/t/proposal-native-js-to-embrace-extend-the-webextensions-api/3457 http://www.pro-linux.de/news/1/22679/mozilla-fragen-und-antworten-zu-webextensions.html
We got docs! https://developer.mozilla.org/en-US/Add-ons/WebExtensions, we are using dev-doc-needed on specific bugs, if there's anything else less file specific bugs for them.