It doesn't. Flashblock is one of the most popular Firefox extensions[1], and variants are available in other browsers. Even though it offers some protection aga

It doesn't. Flashblock is one of the most popular Firefox extensions[1], and variants are available in other browsers. Even though it offers some protection against hidden Flash elements and supercookies, it would still be popular just to prevent autoplaying flash presentations. The author of Flashblock even specifically mentions YouTube on the extension page[2]: Youtube videos not blocked: This is because they are now increasingly HTML5 Videos. I plan to add HTML5 blocking in the next version. Meanwhile you can try out a experimental version at: http://flashblock.mozdev.org/installation1.html#unstable http://flashblock.mozdev.org/installation1.html#unstable This indicates there is a demand for the autoplay blocking feature alone, regardless of the medium. [1] https://addons.mozilla.org/en-US/firefox/extensions/?sort=users https://addons.mozilla.org/en-US/firefox/extensions/?sort=us... [2] https://addons.mozilla.org/en-US/firefox/addon/flashblock/ https://addons.mozilla.org/en-US/firefox/addon/flashblock/