Firefox is going to have some restrictions on :visisted elements:
http://hacks.mozilla.org/2010/03/privacy-related-changes-coming-to-css-vistited/

We're limiting the CSS properties that can be used to style visited links to color, background-color, border-*-color, and outline-color and the color parts of the fill and stroke properties.

I searched through the stylesheets of prosilver to see where this touches phpbb and just found one issue:
Links in posts won't be underlined with dots anymore:

.postlink:visited {

	color: #bdbdbd;

	border-bottom-style: dotted;

	border-bottom-color: #666666;

}

Personally, I would just remove the border-style and adjust the color so it fits prosilver-blue instead of being grey.
This would make links with [url] and without look the same, what I think should have happened long time ago.