Unfortunately this extension doesn't seem to work in using with the "YouTube Classic" extension: https://addons.mozilla.org/en-US/firefox/addon/youtube-classic/ It does work but only by disabling the YouTube Classic
Wow somehow classic YouTube support hadn't even crossed my mind. I'll see what I can do :)
Edit: It actually seems to be working on YouTube classic for me. Which video were you trying it with? Do you mind seeing if it works on this video? https://www.youtube.com/watch?v=MlztVaIStuM
