Auto play should be configurable in the browser. In Firefox: Settings->Search "permissions". Autoplay is one of the options and can be controlled on a per-site basis (I have it disabled globally).
I have autoplay turned off in Safari, but the esteemed minds at YouTube have somehow found a way to circumvent that setting. Like the person you're replying to, I had to resort to an extension to get the site to stop auto playing videos.