Indeed, HTML5 Video Everywhere is one of the best add-ons. Personaly, as a free/libre software activist myself, I would recommend one of the following combinations:
* GNU LibreJS (to block only non-free software that is silently installed by the site makers on your browser. LibreJS also provides ways to cantact the site makers. Note that LibreJS might use some of your computer's power to evaluate which site software is free or not) + GreaseMonkey (to run software you want to in some page) + ViewTube (install it using GreaseMonkey. Allows you to view YouTube videos (and some from other sites) your way, and using only free/libre software). This is my preffered method. * The above, but replace GNU LibreJS with NoScript, which blocks **all** site software, everywhere.
