>In a sandbox even I, as the owner of the computer, can't let them out of. That's not cool.
Good software doesn't easily let you shoot yourself in the foot. It turns out if you allow it, people will do it. There exists software that does let you shoot yourself in the foot, but Firefox is designed for the mass market, not a specific niche. It's the only serious competitor to Chrome.
NPAPI is also gone. Not behind a flag, the code to support it is gone. It was broken, and so was the old extension API. It's about time for the majority of users to move on.
>In the sense that it's no longer possible to do the things one could do with a Firefox codebase.
This doesn't make any sense. You can do the same things you could do with the Firefox codebase before: download it, modify it, redistribute it. The runtime is what you're thinking of, and you were never guaranteed any abilities with regards to the runtime; you just got used to them, then they changed. This happened a few times with Firefox before, so it's not really clear how this update is much different.
>I own my computer, and my browser; Mozilla have taken away the ability for me to extend my computing environment (without reïmplementing it all myself).
Firefox is not your "computing environment." You're free to remove Firefox from your computing environment, or use an ESR release of Firefox and still get security updates with your old extensions, or use a fork of Firefox, of which there are a few to choose from.
Even so, it's depressing that every time a breaking change happens, even one like this that's 2 years in the making, the same responses happen: weird arguments about entitlement and a lot of looking toward the past.
Why not look toward the future? If you relied on an extension and now it's not possible with WebExtensions, it would help a lot of people out if bugs were filed and features were requested to bridge the gap. It's clear that there's overwhelming reasons to continue down the WebExtensions path, and whether people like it or not it's definitely going to be the only option in the browser you use in a few short years. So it's in our best interests to push for WebExtensions to meet the needs of extensions rather than make pointless arguments for decisions that were made a long time ago and have no chance of being reverted.