Simple script that mark all visited links as custom color.
If you want to customize color and site exceptions, you need to edit Visited Lite after install it.
var p_color_visited = "YOURCOLOR";
var p_except = "YOURURL1,YOURURL2,YOURURL3";
Visited full version is here. https://addons.mozilla.org/en-US/firefox/addon/visited/
Installation Note: Please install a user script manager first.