> There are a lot of sites (e.g. Twitter embeds) where turning off autoplay also seems to break manually clicking play as well

Pretty sure this is by design, as Twitter wants that content to auto-play. Forcing a poor experience for people who use blockers and making us white-list their domain seems a suitable strategy.

It's something like Facebook chat in mobile mode. Facebook wants its cattle to install that spyware-ridden messenger, so first they disable chat/messenger in the mobile site (not sure they already killed it, last time I checked there was only a warning; need to create a new bunch of test accounts to check again). As people will fight the change by using desktop-mode in mobile (one of the best features of Firefox for Android), they have a incentive to make the desktop site intentionally unusable in touch devices.