Link Style is a Firefox extension that overrides any site-specific CSS changes to the appearance of links in HTML documents and dims them to make it clear which links have been visited.
Previously, overriding styles in usercontent.css would suffice, but this option was significantly limited due to privacy concerns. See https://hacks.mozilla.org/2010/03/privacy-related-changes-coming-to-css-vistited/ for more information and whether the potential privacy issue is a concern to you.
The resolution to a currently open bug report may end up solving this problem in a more elegant manner: https://bugzilla.mozilla.org/show_bug.cgi?id=693164