Sometimes it is inconvenient or inappropriate to have the autocomplete feature of IE or FireFox.  I found it blocked the custom autocomplete feature I was building for my app.  Just add autocomplete="off" to the form to turn it off: 
<form id="formSignOn" autocomplete="off" method="post" runat="server"></form>
In ASP.NET I added it by using the following line:
Me. Form.Attributes.Add("autocomplete", "off")