This is what is going to happen to most extensions. Mozilla is bonkers, doing this. They're going to regret it.
Are they? The old system is holding improvements back. If they move to a superset of Chrome's they can still get Chrome add-ons and then some. If people value the old extension system that much, then it will live on in one of the FF forks. Regardless Firefox will finally get faster, more responsive, more secure, AND still have a better extension API than Blink/WebKit.
Using a fork is also bonkers. I don't trust the security of my web browsing on a fork. Most people won't, either. So Firefox will just get this nice metric saying "most people have not moved to a fork" and will be able to pat themselves in the back. But it's bogus.