YAD YouTube Download is a simple yad GUI for youtube-dl (included)
for latest version run:
curl -L https://yt-dl.org/downloads/latest/youtube-dl # and replace included ytd. or run "ytd -U" after install
( I chose to rename it to avoid conflicts with original older versions in the repositories of the old distributions that have different options and will not work )
youtube-dl supported sites
ffmpeg or avconv, yad
It can be used from the command line, but can also be integrated in Firefox thanks to an addon: https://addons.mozilla.org/firefox/addon/open-with/ (https://github.com/darktrojan/openwith) then run firefox, open the "open with" options and add *.desktop file from: /opt/ytd/ff_open_with_addon/ see open_with_ff_settings.png image example.