var msg = "This website works best with IE 7+, Firefox 2+, and Safari 3+. Please use one of these browsers for the best possible experience.";
var showAlert = false;
var userAgent = window.navigator.userAgent;
if (userAgent.indexOf("Firefox") > -1) {
  var version = parseInt(userAgent.substring(userAgent.indexOf("Firefox/") + 8));
  if (version < 2) 
    showAlert = true;
}
else 
  if (userAgent.indexOf("MSIE") > -1) {
    var version = parseInt(userAgent.substring(userAgent.indexOf("MSIE") + 5));
    if (version < 7) 
      showAlert = true;
  }
  else 
    if (userAgent.indexOf("Safari") > -1) {
      var version = parseInt(userAgent.substring(userAgent.indexOf("Safari/") + 7));
      if (version < 522) 
        showAlert = true;
    }
    else {
      showAlert = true;
    }
if (showAlert) {
  alert(msg);
}