var ievs = (/MSIE (\d+\.\d+);/.test(navigator.userAgent));

var ok = 1;

if (ievs){
   var iev=new Number(RegExp.$1);
   if (iev==7) {
       //document.write('<b>Stai usando il browser: Explorer 7</b>');
       ok = 0;
       document.write('<link rel="stylesheet" href="css/stile.css" type="text/css">');
   }
}

if((ind = navigator.userAgent.indexOf("Firefox")) > -1 ) {

	if(ok == 1 ) {
       //document.write('<b>Stai usando il browser: Firefox</b>');
       document.write('<link rel="stylesheet" href="css/firefox.css" type="text/css">');
       ok = 0;
	} 
} 


if(ok == 1 ) {
     //document.write('<b>Stai usando il browser: Firefox</b>');
     document.write('<link rel="stylesheet" href="css/stile.css" type="text/css">');
     ok = 0;
}