Rated 4 out of 5 stars
The only free addon that lets you easily save full (non-protected) videos from Youtube (even Dash formats).
It's also quickly updated after each YT change.
The current version (5.7.36), however, has still some problems:
sometimes the download fails without warning or the program doesn't mux the audio.
Example to manually join video+audio via command line:
ffmpeg.exe -i "video.mp4" -i "audio.m4a" -vcodec copy -acodec copy "1output.mp4"