I see a little message that says "Keep hovering to play" -- but the autoplay block actually prevents anything from playing for me. ([[Allow or block media autoplay in Firefox]]) I don't think there is a built-in way to prevents YouTube from reacting to having your mouse over a video thumbnail. You probably would need an add-on for that, if there is one with that feature. When I searched, it seemed to be the opposite: more ways to play previews. Maybe you'll have better luck over on the Mozilla Add-ons site than I did: https://addons.mozilla.org/firefox/