The redirect is not done before accessing youtube so there is a request to youtube as well every single time. You should use a webRequest listener in the background and modify the url from there.

does what it says and doesn't need all possible permissions for that!

Exactly what I was looking for. And thanks for sharing the source code on GitHub!

This add-on works. Every link to Youtube is redirected to Hooktube. Even if you go to the Youtube site, you are redirected to Hooktube. So if you actually want to go to Youtube, you would have to disable it temporarily. I don't have a problem with that.