If you are talking about the color differences between visited and unvisited in the links on your webpages then that that would be affected by any of the following:

1. changing the link colors in your Tools > Options > Content > fonts & colors: "Advanced" for which I have blue for Unvisited and magenta for Unvisited links. I have all three items check-marked "Use system colors:, "Underline links:, and "Allow pages to choose their own colors instead of my selections above".
2. Loss of history data, such as clearing of your browsing history
   • http://img232.imageshack.us/img232/4928/clearcachew.png
3. Private browsing ("Ctrl+Shift+P") will not mark any links as visited, same would apply to permanent Private Browsing ("Always use private browsing mode") in Tools > Options > Privacy
   • http://img232.imageshack.us/img232/4928/clearcachew.png

I left out the link on the following and have added it, but since the above answered the question, I'm changing this last part to a footnote.

None of which would be directly related to updating Firefox, but at such times the localstore.rdf file in you profile may be affected (corrupted), and changes made to your toolbars and options will not stick.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Thank you. Your response gave me the exact information I needed.

Glad to have helped. I left out the link for the ending paragraph, but since it did not apply to you reduced it to a footnote upon adding the missing link.