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It disables autoplay and enables click to play.
< Σχολιασμός για τον κώδικα YouTube Click To Play
Yes, it does works on Firefox.
So this script is mainly for Chrome users, but it could also help other browser users with its autoplay switch feature and so on.
(Actually, my main browser is Firefox, but I use Chrome for some sites including YouTube)
I guess it may be useful for users who only want autoplay disabled specifically for YouTube
(Firefox) about:config -- > media.autoplay.default --> 0