It is possible to write extensions (aka WebExtensions Experiments) which are pretty much as powerful as the old Addons; the cost is that one can run them only with Developer/Beta/Aurora release of Firefox and there are no guarantees for stability of internal Firefox APIs [0][1][2].
So I managed to write a proof of concept extesion [3] which exposes API to create sequence keyboard shortcuts (eg. Ctrl+x Ctrl+c in emacs) and creates bindings to Awesome Bar prefixes for filtering suggestions [4]. Although it is heavy experimental by now.
I suppose it may be pretty easy from here to add a feature for managing all the built-in shortcuts and commands of the browser.
Besides I think there are some effords to improve shortcuts stuff in Firefox [5].