Try harder - This is probably work tinkering with a little more.
5.6.32 tested 2015-11-15, on Pale Moon 25.7.3 (x64), on Windows 10, updated automatically.
I don't know how to use this. There is no toolbar icon available. Presumably this is a Pale Moon issue.
See also Youtube Video and Audio Downloader which seems similar.
Tools > Complete YouTube Saver > Options
[Change FFmpeg folder]
to C:\Program Files (x86)\ffmpeg(etc) where (etc) is the appropriate name. After selecting that folder, just click ok. There is no executable that you need to select.