Firefox link color system settings

[Eben King <eben@gmx.us>]

In Firefox Settings on 12/Bookworm XFCE, there's

Contrast Control
  Websites have a variety of foreground and background colors. Configure
  Firefox to use the same colors across websites for improved
  readability.

  ( ) Automatic (use system settings)
  ( ) Off
  (x) Custom

Well, that's how it looks now.  I wanted to change the color of the 
visited links, but that changes text and background colors too, which 
makes many web sites somewhere between "really messed up" and 
"unreadable".  I would like to change the link colors _only_ and leave 
the other colors unset.  I don't see a way to do that in "Custom". 
Where are the "system settings" made?  Or, is there a way to do what I 
want in about:config?  Thanks.