Not working anymore, but search the addon ycs-continued is a continuation of the original YCS and works perfectly.

Works well, but with certain videos it just makes Firefox freeze completely

This add-on is awesome. At first I didn't like it because I thought you couldn't reply to comments but you can by clicking on the date of the comment. The only thing I don't like about it is that it can take very long to load all comments if there are a lot of comments. This is something I would donate to.

EDIT: Stopped working recently. Please fix.