VerifyEval で以下を入力し、期待値をtrueにすればOK!
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;