I would have been interested to see a Firefox OS in the same vein as Chrome OS. A clean, functional Linux distro that works out of the box with a simple (yet maybe hidden) switch to open it up to running native apps. With XUL extensions they were actually in a get position years ago to provide more powerful extensions than Chrome could. But they are killing off XUL in the future to be some kind of Chrome-copy. Unfortunately I feel the world is going to be Webkit oriented in the future with Chrome, Safari and even Edge being mostly compatible (extension wise, apparently, time will tell).
I understand them wanting to make it in mobile space but the bar to entry in the mobile market is so damn high not even Microsoft can do it.
The biggest reason I use Firefox over Chrome is because of DownThemAll; an extension that would stop working when turn off XUL support. Trying to follow market trends is whats gonna kill Firefox.
Eliminating XUL isn't being done for the fun of it—there are significant benefits. This comment also presupposes that there's no way to implement DownThemAll in the Web Extensions model.
I understand them wanting to make it in mobile space but the bar to entry in the mobile market is so damn high not even Microsoft can do it.