I think I would introduce the new APIs through Jetpack (which has a formal separation between the extension proper and the APIs it uses, but ships the implementation for both). Once an extension uses only stabilised APIs (APIs taken from chromium could stabilise first, assuming there aren't too many abstraction leaks), label it as Forward-Compatible on AMO (similar to restartless addons). Help the top extensions port, and start breaking internal APIs more aggressively. Maybe start measuring the latency introduced by cross-process wrappers (assuming the extension that uses them can be determined at runtime), and warn about extensions that use them. The perception that Firefox is slow largely comes from that kind of compatibility hack, and people would probably make an informed choice to speed up their browser.