Facebook Volume Control
This sets the default volume on all video's to 10%. So you're ears and speakers can get a break.
This extension is SUPER lightweight. It's as minimal as it gets.
This also remembers the last volume you chose and applies it to newly loaded videos. I don't set EVERY video on the page to keep with my minimal style.
This is my first extension after people on the facebook community pages requested I make something easier to share than a User Script.