The visited style has been removed from Firefox (and most other browsers) in recent versions due to a security issue.
The problem is that a malicious website could use the browsing history using it - they set the color visited , create a load of URLs (even hidden ones, so that the user does not know about it), and check their color. This caused quite a stir in browser security circles a couple of years ago.
The visited function can be re-enabled in Firefox by going to the security settings, but disabled by default, and most users will be disabled.
For more information about the problem and how Firefox fixed it, see here: http://blog.mozilla.com/security/2010/03/31/plugging-the-css-history-leak/
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