TL;DR: Firefox has already disabled everything other than Flash, Flash is currently discouraged, and Flash will be killed off in 2020.
At some point in 2018 I decided to disable the Flash plugin on my system. I figured I would end up activating it maybe once every three months. I was surprised to discover that a surprising amount of video content is still displayed via flash players. Let's hope that 2019 is the last year of that crap.