Have you given Tridactyl a shot?
We started it [1] but lost interest because we think Tridactyl works mostly fine without it (press ctrl+, anywhere to get back to Tridactyl) and I have always found more urgent things to improve.
I'd be happy to help anyone who wanted to pick it up, but a lot of the political work of persuading Mozilla that such an API would be useful would need to be restarted since most of the people we persuaded in the past have now left Mozilla.