It is needed for sites, that hide links, disguising them so they may look like a plain text, for example.
+ All the sites will have the same decoration for unvisited, hovered and visited links, so you don't need to guess if it's a link or it's not - you'll know it for sure.

This style works on Nightly (aka Firefox 6) and may not work on Firefox 5 (which is now beta) and earlier (including Firefox's current stable version) yet.
Why? Because the new -moz-text-decoration-color CSS property was added just recently. Later it might become a part of the stable/beta version of Firefox, but currently it may be not (though I didn't bother myself running tests).

How the style works?
1. It makes all links underlined.
2. It paints this line in the different colors regarding the link type (regular, visited, hovered).