When using a AutoCompleteExtender from the AjaxToolkit, Firefox may show it’s own AutoComplete List over the one you want. To disable that, you need to disable autocomplete on the Field or the entire Form.

To do that, just add an autocomplete="off" to the Textbox in the Markup Code or add the Attribute on Runtime like this:

Textbox1.Attributes.Add("autocomplete", "off");