From MozillaZine Knowledge Base

No longer works in Firefox 3.6a1pre (Minefield), changes are no longer restricted to the unvisited tab so can no longer distinguish between a visited tab and an unvisited tab.

See Bug 487242 -- Regression: in userChrome.css no longer possible to distinguish between unvisited tabs, visited tabs, and the selected tab

The style I created making use of this MZ article is Style 9023 -- Tab Color Underscoring active/read/unread | userstyles.org -- DMcRitchie 21:48, 27 June 2009 (UTC)