Owning the code that runs on your machine is your right as a user of open-source software, but there's no right that says that Mozilla must be the one to maintain that code. And nobody's asking you to maintain your own personal fork either: find all the other people who care about unlimited (if fragile) browser extensibility via unrestricted addons and form a community (or hell, a corporation) to collectively make the browser you want, starting from a fork of Firefox (or Chromium, or Midori).

If you believe that there are enough people in your position to make such a collaboration successful, then you're all set. And if you don't believe that there are enough people out there who care about unrestricted addons to make such a project viable, then--while I sympathize with your plight--maybe it makes business sense for Mozilla to drop support for unrestricted addons, especially due to the ways that this feature hinders all the other features that Firefox users are clamoring for: speed, reduced memory usage, speed, HTML5 adoption, speed, ES6+ adoption, speed, speed, security, and speed.