refactor(nixos): scope agent guidance

configs/nixos/modules/desktop/apps/firefox.nix

@@ -67,6 +67,8 @@ in
 
           # Stylix uses a near-white selection background.
           # Use the Nord selection color because its text stays readable.
+          # Home Manager writes extension settings through Firefox's legacy storage backend.
+          # Any extension setting disables ExtensionStorageIDB for the whole profile.
           extensions.settings."FirefoxColor@mozilla.com".settings.theme.colors.popup_highlight =
             let
               c = hm.config.lib.stylix.colors;