var ns = false;                                                
var browser = navigator.userAgent.substring(0,9);      
if (browser == "Mozilla/3" || browser == "Mozilla/4") ns = true;

var dot_normal , dot_hot;
if (ns){
   dot_normal = new Image (42,10);
   dot_normal.src = "image/dot_off.gif";
  
   dot_hot = new Image (42,10);
   dot_hot.src = "image/dot_on.gif";
}

function switchimage (imagename, target)
{
   if (ns) document.images[imagename].src = target.src;
}