Developer seems to believe they need a service to convert something. YouTube sends split audio/video streams now. If you want the original video, you'll need to download both and convert them yourself. Of course, the developer could have done this on your local machine with ffmpeg, but decided to do it on a server instead and charge for it. The developer does not allow you to download the HD streams yourself so you can combine them locally.

Use this addon for now: https://addons.mozilla.org/en-US/firefox/addon/youtube-download-hd-download/