That article is more than 1 year old. And Mozilla is going beyond the Chrome APIs, they are implementing shims and additional APIs to port almost everything over.
http://www.downthemall.net/downthemall-3-0-5/
“After a rather long long time, with many new things to address such as Australis, e10s, limited time we could spend on the project, a little lack of motivation after mozilla required mostly pointless signatures and announced to kill XUL add-ons…” - Nils, September 2016
"Just to clarify: We did not port it to the WebExtensions API, nor do we think it is possible to port it in any significant manner to the WebExtensions API..."
https://addons.mozilla.org/En-us/firefox/addon/downthemall/r...
I communicated to the mozilla people who got in touch with me that there will not be a crippled DownThemAll!, at least not from me.