There is a lot of work underway in this area. The Webextensions project is the big long-term effort. But regarding e10s, it is true that e10s is an all-or-nothing proposition. However, as of Firefox 51, extensions do not disable e10s unless they are explicitly marked as incompatible with e10s. The rollout was deliberately done slowly in this area to favor stability over getting e10s enabled quickly, I think that was pretty clearly the right decision.