function getContents(aURL){
  var ioService=Components.classes["@mozilla.org/network/io-service;1"]
    .getService(Components.interfaces.nsIIOService);
  var scriptableStream=Components
    .classes["@mozilla.org/scriptableinputstream;1"]
    .getService(Components.interfaces.nsIScriptableInputStream);

  var channel=ioService.newChannel(aURL,null,null);
  var input=channel.open();
  scriptableStream.init(input);
  var str=scriptableStream.read(input.available());
  scriptableStream.close();
  input.close();
  return str;
}

try{
  alert(getContents("chrome://browser/content/browser.css"));
}catch(e){alert(e)}