Most websites use their own colors for links, and you may sometimes encounter websites that use color schemes that makes is hard for you to easily differentiate between the visited and not visited links. To fix this, you can set Firefox to override the color style used by those websites. There are a number of ways to make it happens.
You can change the link color via Firefox options, userContent.css, or by using extensions.
Firefox options offers a way to make sure that any website you visit will use your color schemes instead of their own. Open Firefox options, go to Content, hit the Colors button, choose any color that you like, and change the override setting to Always. It works, but the result might be not what you expected as it changes the color of text and background as well, which looks pretty ugly on certain sites.
The second choice is to use userContent.css. Open your Firefox profile folder, create a new folder and name it chrome, and then create userContent.css file within the new folder. After that, open the css file, type the following codes and save it:
color: red !important;
Or, use an extension.
As for Visited, some users reported that it no longer works, but there is a workaround.
For new users: after installation, open the extension’s option, clear the site exception items, and then restart the browser. After that you can add sites to the exception list again if you need to.
For those who already install it: remove the extension, reinstall, and then apply the above workaround.
The reason why I like Visited much is not the exception list, though, but its ability to mark visited images in addition to links.
On the other hand, Visited Link Enabler provides a much simpler solution. It only changes visited link color on all websites. After installing the extension, Open the Content page of Firefox options, choose a visited link color and hit the OK button.