Visited links are by default displayed in a different color to distinguish them from the unvisited ones so that you know if the links have been clicked the moment you see them. The problem arises when there are sites that change the colors that makes it hard to differentiate between visited and unvisited links, which leads you to accidentally open the pages that you’ve visited. If this happens more than often to you, then you may be interested in using this Firefox add-on called as Visited.
The extension automatically sets the color of the visited links to gold. It can also mark clicked images in custom color but the option has to be enabled first. After enabled, you will see gold borders around the images that you have opened or clicked. This is particularly useful for online galleries with many thumbnails on the page to prevent you from opening the same images twice unless you intend to.
The default colors can be changed using the color palette available in Visited’s Option page. If the color that you want to use is unavailable in the palette, go to about:config?filter=extensions.visited, double-click on it, and enter the color name or color code.
You can also change the color randomly with hotkey, which is pretty helpful if you land on pages where the normal links have the same color as your visited link color, as this hotkey (Alt+Shift+V) enables you to apply a different color quickly.
Or, you can press Alt + V to disable the custom link color. This will also automatically restore the color used in the sites. And then, if necessary, you can enter the sites into the whitelist for permanent exclusion.
As a bonus, Visited can also apply opacity effect on images when you hover mouse over them.
Visited is a restartless extension, so you don’t need to restart Firefox to activate it.