For youtube, you need to load the video through an invidious instance as Youtube uses a proprietary loading method. https://inv.nadeko.net (Look for the .m3u8 file )

For facebook, I tried and could correctly download an .mp4 video and preview it. Which page exactly did you try on?

I don't get notfied upon review edits so I recommend you send me an email (click the heart button at the bottom of the add-on) so I can look into it in more detail! Thanks!