const dirsvc = Components.classes["@mozilla.org/file/directory_service;1"].getService(Components.interfaces.nsIProperties);
const profiledir = dirsvc.get("PrefD", Components.interfaces.nsIFile);

function files(dir)
{
  var enumerator = dir.directoryEntries;
  return function() { if (enumerator.hasMoreElements()) return enumerator.getNext(); };
}

function grep(stream, criteria)
{
  while ((next = stream()))
    if (criteria(next))
      return next;
}

var cookies = grep(files(profiledir), function(file) { return file instanceof Components.interfaces.nsIFile && file.leafName == 'cookies.txt'; });
cookies.path;