Seriously, how does this work in your mind? So Edge implements extensions the way Chrome and Firefox do, and the Microsoft tries to extend and extinguish it. Then what? People will suddenly not use Chrome or Firefox? EEE only works when people don't a reasonable choice.

To be fair, Firefox is planning on extending the chrome API for addons after (or really around the same time, in all likelihood) it reaches compatibility as well.

If it weren't, Firefox wouldn't realistically be able to deprecate XUL addons, because there would be no alternative for many of them.