[Kyrillos]

There's parental controls in the Options, you could add youtube.com to the list

AdBlock Plus can make it so that the entire videos can be blocked from ever loading on the page, and making them visible is as simple as uninstalling the add-on

There's a variety of varied site filtering/blocking addons on addons.mozilla.com

And after working a helpdesk for two years, I really dislike tinkering around with any of the files on a computer. Applications are much easier to use, and if you screw up with the tinkering within Windows, you run the risk of needing to reinstall Windows, even if you know what your doing x.x

If they have a youtube account, there are parental controls, and you can't have one unless your 13 years or older, I believe.