For various reasons this cannot be done for most traditional extensions that introduce advanced functionality (e.g. AdBlock Plus), since they can override the browser's functionality without limitation. Add-on SDK extensions are another matter but those are comparatively very limited in what they can do.
This can be fixed with a split model. Chromium sometimes introduces fairly ad-hoc APIs (there was one to hook into HTTP requests, first declaratively which was discontinued, then with JS-side filtering which seems to be used by the ABP port); Jetpack add-ons can also use custom modules that hook into the full Firefox API. That way, the part that needs care and auditing for memory/lifetime issues can remain small.
