[Selenium]Change User Agent with IDE

var pref = Components.classes["@mozilla.org/preferences;1"]
           .getService(Components.interfaces.nsIPrefBranch);

var nsISupportsString = Components.interfaces.nsISupportsWString
    ||Components.interfaces.nsISupportsString;

var ua = (Components.classes["@mozilla.org/supports-wstring;1"])
    ? Components.classes['@mozilla.org/supports-wstring;1'].createInstance(nsISupportsString)
    : Components.classes['@mozilla.org/supports-string;1'].createInstance(nsISupportsString);

ua.data ="Mozilla/5.0 (Linux; U; Android 2.3.6; en-us; Nexus S Build/GRK39F) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1";

pref.setComplexValue("general.useragent.override", nsISupportsString, ua);
true;