Works in Firefox and IE7, IE8
in code-behind put add this attribute to yout textbox
yourTextBox.Attributes.Add("onkeypress", "var key; if(window.event){ key = event.keyCode;}else if(event.which){ key = event.which;} return (key == 45 || key == 13 || key == 8 || key == 9 || key == 189 || (key >= 48 && key <= 58) )");

Firefox recognize event.wich