Firefox is going to have some restrictions on :visisted elements:
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:
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.