ajaxToolkit ComboBox don’t work with firefox

I use the ajaxToolkit ComboBox with following code

<ajaxToolkit:ComboBox ID="ComboBoxPoli" runat="server"  DataSourceID="SqlDataSourcePoli" AutoCompleteMode="SuggestAppend"  DataTextField="Poli" DataValueField="Poli" DropDownStyle="Simple" MaxLength="0"  ></ajaxToolkit:ComboBox>

With IE work fine but with firefox when I try to write something (anything) the control remain blank. If the control has something (same words) and try to hit a key from my keyboard the control return to blank.

 I talk about the textbox of control and not for the list.

The list items displayed fine.