if (/Firefox[\/\s](\d+\.\d+)/.test(navigator.userAgent)){ //test for Firefox/x.x or Firefox x.x (ignoring remaining digits);
 var ffversion=new Number(RegExp.$1) // capture x.x portion and store as a number
 if (ffversion<=5)
  document.write("<link rel='stylesheet' href='css/fonts_all.css' type='text/css' />")
 else 
 document.write("<link rel='stylesheet' href='css/woff.css' type='text/css' />")
}