The interview article is mainly for addon developers, this is the reason of the answer for the question. In past days I was a "people pleaser" and I didn't say "no" from my frailty, then my old addon became too fat, and finally dead. I hope that addon authors publish long-living small addons instead of short-live too much addons.

The sentence in the interview means that now there is a boundary of responsibility between addon and Firefox. If a feature request is impossible to be implemented due to API restrictions, it have to be solved by Firefox itself. Thus if I have strong motivation to implement a new feature requiring new API, I still file a proposal of new API, and I think other addon authors also should do it based on his demand.