Video DownloadHelper is non-free:
https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/license/5.6.1
Apart from some of the already listed add-ons, I use for privacy protection:
Clean Links;
ShareMeNot (but I think it is useless given that I have Privacy Badger);
Skip Redirect;
Smart Referer;
For videos:
ViewTube and ViewTube+ (two GreaseMonkey scripts);
UnPlug;
For esthetics:
GNotifier
the "Global Black for AMOLED Display" style (on top of the Stylish add-on);
the GNOME theme for Firfox.
