var agt=navigator.userAgent.toLowerCase();

if (agt.indexOf("firefox")  != -1) {
  document.write("<link rel='stylesheet' href='http://dodoskido.com/styles-site-fx.css' type='text/css'/>")
} else {
  document.write("<link rel='stylesheet' href='http://dodoskido.com/styles-site.css' type='text/css' />")
}