Попробуй вот эту функцию (взято с руководства c
http://www.htmlguru.com/)
function Is() {
var agent = navigator.userAgent.toLowerCase();
this.major = parseInt(navigator.appVersion);
this.minor = parseFloat(navigator.appVersion);
this.ns = ((agent.indexOf(\'mozilla\')!=-1) && ((agent.indexOf(\'spoofer\')==-1) && (agent.indexOf(\'compatible\') == -1)));
this.ns2 = (this.ns && (this.major == 2));
this.ns3 = (this.ns && (this.major == 3));
this.ns4b = (this.ns && (this.minor < 4.04));
this.ns4 = (this.ns && (this.major >= 4));
this.ie = (agent.indexOf("msie") != -1);
this.ie3 = (this.ie && (this.major == 2));
this.ie4 = (this.ie && (this.major >= 4));
this.op3 = (agent.indexOf("opera") != -1);
this.win = (agent.indexOf("win")!=-1);
this.mac = (agent.indexOf("mac")!=-1);
this.unix = (agent.indexOf("x11")!=-1);
}