Works, even if you have a script blocker installed. I have NoScript disabled on youtube to run JavaScript, so there were no issues with the NonStop addon.

For testing I had a large playlist with music albums running in background while I was playing a game. When the message from youtube popped up, there was a short stop in the music, but then it continued immediately.
When I tabbed into Firefox I saw on the youtube video tab a message in the bottom left corner which said "Thank you for confirming".
Proof enough for me that this addon works.
So, big thanks to the developer for sharing this useful addon with the world. :)