On meta I am seeing no difference in color between visited and unvisited links, either titles on the main page or other links on individual pages. In the following screen shot, I have visited the second question but not the first:
In the following screen shot, I have visited my profile but not Stéphane's, nor have I visited the MSO post in this browser (I posted that comment from elsewhere):
These links used to be visibly different. I believe the change happened within the last 24-36 hours.
I turned off Stylish to verify that no custom CSS is causing this; no change. Main looks fine.
MacOS 10.6, Firefox 27.0.1.