'position().top' is null or not an object?-Collection of common programming errors

Your .css() arguments are backwards; it should be .css(, )

$('#pnlAdd .modalDialog').css("top", $("tr [style*=Blue]").position().top + "px");

Also, .position() will return null if $("tr [style*=Blue]") doesn’t return anything, causing an error. Try checking its length first.

var elements = $("tr [style*=Blue]");
if (elements.length) {
    $('#pnlAdd .modalDialog').css("top", elements.position().top + "px");
}

EDIT NOTE Firefox use initial caps on the colors (Blue) but IE use all lowercase (blue).