Mark Selected Links As Visited от Dima PM

Please note that extension does not colorize links, it just puts them in History. If visited and unvisited links on some site look the same, then you need some other extension to colorize links.
Also, there is an old bug in Firefox: even if some URL is removed from history, links with this URL are still displayed as visited. You need to open page with these links in another tab. Only in this new tab they will be displayed as unvisited. And sometimes it will take some time for links to become unvisited.