We are implementing a new extension API, called WebExtensions—largely compatible with the model used by Chrome and Opera—to make it easier to develop extensions across multiple browsers.
They're getting rind of those people who are willing to dive deep into the internals do develop something that nobody thought of before (if they did there would be high-level APIs for it).
Seems like there's still a lot of power available, if you really want to dive in.
That's the powerful parts. The high-level APIs are fairly restrictive, quite similar to chrome APIs, maybe even less powerful on their own.
And with enforced signature signing you can't even distribute it to users yourself.
So effectively it will restrict extension-development to a whitelist of APIs. Everything else will be verboten, maybe with varying degrees of enforcement.
It certainly doesn't sounds like an open system to me.
That kind of keyboard permission could be used for keyloggers though, it's not something every extension should get.