Does absolutely nothing on Firefox 141, and when I emailed the author asking how to make it work (or to report the issue to him), I got an email claiming that he doesn't intend to make it work as long as YouTube has restrictions, and promoted in the email a $30/year desktop app I can use instead:

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YouTube has aggressively begun blocking our server IP addresses, leading us to rely on thousands of proxies to mitigate this issue. Unfortunately, this dependence often results in unreliable speeds, broken downloads and blockage after a few hours and as of now almost all datacenter IP's are blocked by Youtuebe and server based downloading is now a thing of the past. This is done because of the upcoming AI wars between companies

To address this limitation, I have developed a desktop app that offers several additional benefits:
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Mozilla should take this false, advertising add-on down, and DEFINITELY off their "Recommended" list.