You need to copy appcomps.dll and mork.dll from mozilla 1.2b distribution to components directory and set
user_pref("browser.visited_color", "#551A8B");
in your prefs.js file. The color above is just default purple, you can set it any other color you like. Visited links on web-pages still get their designed vlink color, though.
If you don't have needed dll-s then try here:
http://aab.spin.ee/kmeleon/files/07b-visited-links/
These dll-s should work, at least they worked for me with 0.7 release.