Oddly, I don't remember installing this extension. I had been using AdBlock Plus for a long time. At any rate, recently (August 2023), YouTube stopped functioning. Nearly every video I tried to play would simply hang, showing the spinner spinning endlessly. Yet when I tried viewing the same video in Chrome with no ad blocker running, it worked.

That's when I looked at my Firefox extensions, and realized I was running AdBlocker Ultimate. I disabled that, and re-enabled AdBlock Plus, and now YouTube is working properly.