AWESOME!!! It's working perfectly as advertised.
- No third party apps.
- No linking to external websites.
- Downloads (DL's) varied video resolutions based on available resolution playbacks in YouTube
- Download features video only & audio only

I'm using latest Firefox version, Win 10 x64 build 1803.

It's a "gray arrow-down inside a circle" icon beside the "Like" button for those who can't find it.
Availability of download resolutions are based on YouTube's specific video available quality.

HOWEVER, I hope and wish the developers would add a feature on download variable speed limit. I'd love that.