let
  mkPreferences = prefs: builtins.concatStringsSep
    "\n"
    (map
      (p: ''user_pref("${p}", ${builtins.toJSON prefs.${p}});'')
      (builtins.attrNames prefs));
in 
  mkPreferences {
    prefs = {
      "browser.urlbar.suggest.topsites" = false;
    };
  }