-text_field_with_auto_complete- browser problems

Hello,
i'm using the following code in my rails app. works like a charm on IE.
but on FF no autocomplete popup is displayed. can somebody help?

rails code:
<p>
    <%= text_field_with_auto_complete :person, :nachname, {},
    {
     :select => 'name',
     :after_update_element =>
       "function(element,value) " +
       "{ " +
       remote_function(:update=>'Kundendaten',
         :url=>{:action=>:update_kundendaten},
         :with=>"'adr_id='+value.id") + ";" +
       "}"
    } %>
</p>

html result:
<p>
   <style> div.auto_complete {
             width: 350px;
             background: #fff;
           }
      div.auto_complete ul {
             border:1px solid #888;
             margin:0;
             padding:0;
             width:100%;
             list-style-type:none;
           }
     div.auto_complete ul li {
             margin:0;
             padding:3px;
           }
     div.auto_complete ul li.selected {
             background-color: #ffb;
           }
     div.auto_complete ul strong.highlight {
             color: #800;
             margin:0;
             padding:0;
           }
</style>

<input id="person_nachname" name="person[nachname]" size="30" type="text" />
<div class="auto_complete" id="person_nachname_auto_complete"></div>

<script type="text/javascript">
//<![CDATA[
var person_nachname_auto_completer = new Ajax.Autocompleter('person_nachname', 'person_nachname_auto_complete', '/vermietung/auto_complete_for_person_nachname', {afterUpdateElement:function(element,value) { new Ajax.Updater('Kundendaten', '/vermietung/update_kundendaten', {asynchronous:true, evalScripts:true, parameters:'adr_id='+value.id});}, select:'name'})
//]]>
</script>
</p>