[jQuery] Firefox problem on ajax loaded javascript

[nomen]

Hi all:

   I have a simple page.
   Clicking in a button it loads a content by Ajax to the page, let's
call to it "subpage".
   In "subpage" I have some HTML(some select lists) and a jquery code
in "$(document).ready(function(){...". The javascript code launches
another js ( http://www.yxscripts.com/cs/chainedselects.html ) and
this one affects to the loaded select list using the ID of the select
lists.
   In IE8, IE7, Chrome and Opera 10 it works perfectly, but, in
Firefox, the ready function is fired but Firebug says that there is an
error because it can´t find the select lists ID.
     What I´m doing bad?

Thanks in advance.