This is what is going to happen to most extensions. Mozilla is bonkers, doing this. They're going to regret it.
Web Extensions are designed in such a way that these issues can go away.
Apparently Mozilla weighs the complaints with regards to missing multiprocess support and addon security issues higher than they weigh the complaints about the old extensions going away.
I wouldn't call this decision "bonkers" either as, clearly, adding security is more important than keeping the platform stuck in the past in order to keep addons used by a minority of users working.