I find the extension rather unreliable actually. There are a lot of sites (e.g. Twitter embeds) where turning off autoplay also seems to break manually clicking play as well (forcing me to temporarily disabled the extension and reload page).
> 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.
It has indeed been entirely disabled in the mobile site, and is nearly unusable if you request the desktop site (which of course can only be done through the browser settings, they don't provide a link for it). So I stopped using it!
For Twitter, you can disable video autoplay in the account settings and whitelist twitter.com in the extension. But there are other sites where this doesn't work.