I'm using this addon now for around 1 year.
And from my experience it is the best youtube downloader that supports the Addon DownThemAll

It works perfectly fine, has a nice design that looks good even in Dark-Mode in Youtube and I can't complain about the download speed (even for 1080p downloads, and yes, I donated.)

I'm not using it to download music so I can't say anything about that,
but I hope it will support Firefox 56.0.2 for a long time.