Great extension. I recently switched to the new Firefox from Chrome where I used a similar extension. Unfortunately the Chrome extension has ceased development, and is not compatible with the new YT update.

Some suggestions:
1. The extension seriously needs a "mark as watched" button. The other reviewer said the same thing.
2. A show/hide button for subscriptions

I'm delighted to have this extension on Firefox!! Keep up the good work! Hopefully you get a chance to update the extension soon.