FireFox and AjaxControlToolKit Combobox

I'm using the .NET framework 4.0 and the corresponding version of the Ajax control toolkit.

On my page I have a combobox defined like this:

<asp:ComboBox ID="cbUserName" AutoCompleteMode="SuggestAppend"
        CaseSensitive="false" runat="server" BackColor="#FFFFCC">
        </asp:ComboBox>

In IE9 the combobox allows me to either type in it or select from the list of users. In FireFox, however, it's not letting me type in the box. The dropdownlist also doesn't filter when I'm typing in FireFox either. I'm wondering if anyone else has seen this behavior, and if there's a work-around?

EDIT

For what it's worth, the demo on the ASP.NET Ajax Control Toolkit site actually works fine in FireFox.