jqueryajax is not working in FireFox

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Ajax

jQuery

I am working on asp.net website in which i am getting data by using jquery ajax.
here is my code.

     function PostSubChapter(qbt_id) {
    debugger;
    var v1 = 'qbt_id:' + qbt_id;
    $.ajax(
    {
        type: "POST",
        url: '<%= ResolveUrl("~/QuestionBankSubChapters.aspx/GetChapters") %>',
        data: '{' + v1 + '}',
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (result) {
            if (result.status === "OK") {

                alert('Comment posted');
            }
            else {

                fnDisplaySubChapter(result, qbt_id);
            }
        },
        error: function (req, status, error) {

            alert("Sorry! Post failed due to error");
        }
    });
}

and my web method is

[WebMethod]
public static List<bo.questionbanksubchapters> GetChapters(int qbt_id)
{

   BAL.QuestionBankSubChapters oQBTSC = new BAL.QuestionBankSubChapters();
   List<bo.questionbanksubchapters> lstQBTSC =      oQBTSC.getQuestionBankSubChapters(qbt_id);
   return lstQBTSC;
}

for your understanding here i am mentioning the link of the website
Link is : "http://skillgun.com/Home.aspx"

Open these in Fire fox and chrome both then Click on Arithmetic then it will display 2nd screen.
Just see the o/p in both the browser you'll understand. In chrome its working properly but in Firefox its not.I am not getting understand what is the reason behind these.Sample code already i mentioned above.
I am using jquery-1.8.3.min.js for getting the result.The callback function is working fine in chrome and IE but not working in FF
I checked a lot and find my data is coming but a the time of displaying it in FF its showing nothing.

Please help me....