Google changed some things again, and so Youtube is broken again. It's been fixed in this version but only for Chrome. Unfortunately it doesn't work on Firefox, there is some strange things happening with extension user-scripts and content-security-policies which are blocking Youtube's token generator scripts. I'm working on figuring out why and fixing it, but it may take a while.
Note that on Chrome it now requires a new permission to work on Youtube. You have to set it up manually. See the wiki page for how you can set it up.