JavaScript-var在Firefox下未定义，但在Chrome中未定义

I want to draw an chart with Highchart. Before the chart is drawn, I collect the series, the user wants to see via jQuery Shopping Cart.

I save every dropped item in array choice.

The data is from php-file output.php . It prints out an JSON-Array for the chart-builder-method.

Everything works with Google Chrome browser. But not with Firefox

It generates the url-output output.php?string=undefined

//Global var
choice = new Array();

//Shopping Cart
$(function () {
    $("#catalog").accordion();
    $("#catalog li").draggable({
        appendTo: "body",
        helper: "clone"
    });
    $("#cart ol").droppable({
        activeClass: "ui-state-default",
        hoverClass: "ui-state-hover",
        accept: ":not(.ui-sortable-helper)",
        drop: function (event, ui) {
            choice.push((this));
       $(this).find(".placeholder").remove();
            $("<li></li>").text(ui.draggable.text()).appendTo(this);
        }
    }).sortable({
        items: "li:not(.placeholder)",
        sort: function () {            
            $(this).removeClass("ui-state-default");
        }
    });
});



// For individually tab - Draw chart after the container is put into the site
function drawChart() {  

    //output.php -> there the chart gets the data
    var url = 'output.php?string=' + choice[0].innerText;

   //later in the code ...
   $.getJSON(url, 
    function(data) {  

What could be the problem?