> And this is the kind of thinking I'm talking about.

The kind of thinking that says "security-sensitive software with no active maintenance and regular updates is a security hole waiting to happen"?

Letting users install and run unmaintained software is dangerous. They'll get exploited. (And they'll blame the browser for that, and rightfully so.)

You can still do what you want; it's entirely possible to install local untrusted addons. You just have to poke some settings that are intentionally hidden from most end users, to make it somewhat more likely that users doing so understand the implications. And similarly, you can make your addon available to others willing to go through the same steps. They'll balk at doing so, and rightfully so.