I have 2 asp.net textboxes in an update panel. Both textbox controls have some javascript attached to autotab to the next field and to allow only numeric input. When I enter some data into the first field and press enter, focus shifts to the next field and the requiredfieldvalidator of the second field displays its "* required" error message, even though I've just entered the field. This is only happening in Firefox. How can I prevent the validator from firing when I first enter the textbox?

edit

Here's the code:

<asp:TextBox ID="add_ISBN" runat="server" Columns="14" MaxLength="17" CssClass="focus" />
<asp:TextBox ID="add_Qty" runat="server" Columns="4" MaxLength="4" />
<asp:RequiredFieldValidator ID="rfvQty" ControlToValidate="add_Qty" ErrorMessage="* required" ForeColor="Red" Display="Dynamic" EnableClientScript="true" ValidationGroup="Add" runat="server" />

In the codebehind:

 add_ISBN.Attributes.Add("onkeydown", "return isbnCheck(event, '" & add_Qty.ClientID & "')") 

And the javascript:

function isbnCheck(e, id) {
    e = e || window.event;
    var key = e.which || e.keyCode

    if (validIsbnChars.indexOf(parseInt(key, 10)) >= 0) {
        return true;
    } else {
        if (key == 13) {
            var nextfield = document.getElementById(id);
            if (nextfield) nextfield.focus();
            return false;
        }
        if (e.preventDefault) e.preventDefault();
        e.returnValue = false;
        return false;
    }
}

The javascript allows only a valid subset of characters, and if the user presses enter, sets focus to the next field.