fix(firefox): darken the unreadable address-bar result highlight (task 0032)

Stylix's Nord Firefox Color mapping paints the selected address-bar result in
base04, a near-white grey, against base05 text — light on light, so the text
of the highlighted result is illegible. Override that one highlight to the Nord
selection grey (base03), derived from the same Stylix scheme, restoring
contrast without touching the rest of the theme.

modules/desktop/firefox.nix

@@ -51,6 +51,19 @@ in
           # owns the extension-settings store, overwriting runtime changes to it.
           extensions.force = true;
 
+          # Stylix's Nord mapping paints the selected address-bar result a
+          # near-white grey, leaving its light text unreadable. Darken that one
+          # highlight to the Nord selection grey, from the same scheme.
+          extensions.settings."FirefoxColor@mozilla.com".settings.theme.colors.popup_highlight =
+            let
+              c = hm.config.lib.stylix.colors;
+            in
+            lib.mkForce {
+              r = c."base03-rgb-r";
+              g = c."base03-rgb-g";
+              b = c."base03-rgb-b";
+            };
+
           settings = {
             # Auto-enable the sideloaded Firefox Color add-on that carries the
             # Nord chrome theme, which Firefox otherwise leaves disabled.