You can't use it currently if you have a release version of Firefox after 68, yes. The API is buggy and in fact quite a few extensions don't work, even if you force-install them. It's still unclear if they will ever whitelist stuff that's outside of their "recommended extensions" program, and presumably the best chance it would have of getting whitelisted is if it were actively maintained and bugs encountered with the new FFA could be worked on in coordination with the developer.