This is a sample that can detect if you are working on IE, Netscape, Firefox or Opera, where all the the browsers it also checks which version except Firefox:

    // Internet Explorer

  var ie = document.all != null; //ie4 and above
  var ie5 = document.getElementById && document.all;
  var ie6 = document.getElementById && document.all&&(navigator.appVersion.indexOf("MSIE 6.")>=0);

  // IPhone
  var iphone = browser.indexOf('iPhone') != -1;  

  var ns4 = document.layers != null;
  var ns6 = document.getElementById && !document.all;
  var ns = ns4 || ns6;
 
  // Firefox
  var ff = !document.layers && !document.all;
 
  // Opera
  var op = navigator.userAgent.indexOf("opera")>0;
  var op7 = op && operaVersion() <= 7;
  var op8 = op && operaVersion() >= 8;
 
  // Detects the Opera version
  function operaVersion() {
    agent = navigator.userAgent;
    idx = agent.indexOf("opera");
    if (idx>-1) {
      return parseInt(agent.subString(idx+6,idx+7));
    }
  }