Unable to download files from application in Mozilla firefox

We have a link in one of our web pages. On click of this link we open a pop-up window in which we display all documents that a user has uploaded to our application. We are opening the pop-up window as a model dialog using the function 'window.showModalDialog()' in javascript.

We have a download link for each attachment uploaded. The download link triggers a post-back event which downloads a file from the server. During the download, the 'save-as' dialog box opens.

Now the problem is, during the execution of above scenario in Mozilla Firefox browser, once the 'save as' dialog box opens (as shown in the attachment), the whole dialog box freezes. The 'save-as' or 'Cancel' buttons does not work. The 'save as' dialog box can be closed only using the window close (X) button. As a result, we are not able to download the file which we intend to. This problem does not occur with the Internet Explorer browser and so we are able to download the file. We are facing this problem only with Mozilla Firefox.

This 'save as' dialog box also works perfectly if the window with all attachments is opened as an ordinary window (using the function 'window.open()') instead of the modal dialog.

I have attached code and word document with code.

Thanks.

if (!IsPostBack)
            {
                if (Request.QueryString.ToString() == "instructions")
                {
                    pnlInstructions.Visible = true;
                }
                else
                {
                    root = System.Configuration.ConfigurationSettings.AppSettings["attachement_root"].ToString();
                    filepath = Session["filepath"].ToString();
                    filename = Session["filename"].ToString();
                    string[] s_arr = filepath.Split(new char[] { '\\' });
                    filename = s_arr[s_arr.Length - 1];//filepath.Substring(filepath.LastIndexOf("\\"), filepath.Length-filepath.LastIndexOf("\\"));
                    if (filepath != null)
                    {
                        if (File.Exists(filepath))
                        {
                            Response.ClearHeaders();
                            Response.Buffer = true;
                            Response.ClearContent();
                            Response.Clear();
 
 
                            FileInfo DownloadFileInfo = new FileInfo(filepath);
                            string Ext = DownloadFileInfo.Extension;
 
                            if (Ext != null)
                            {
                                Ext = Ext.ToLower();
                            }
 
                            switch (Ext)
                            {
                                case ".exe":
                                    //Exe file
                                    Response.ContentType = "application/octet-stream";
                                    break;
                                case ".zip":
                                    //Zip file
                                    Response.ContentType = "application/x-zip-compressed";
                                    break;
                                case ".pdf":
                                    //Pdf file
                                    Response.ContentType = "application/pdf";
                                    break;
                                case ".doc":
                                    //MS Word
                                    Response.ContentType = "Application/msword";
                                    break;
                                case ".docx":
                                    //MS Word
                                    Response.ContentType = "Application/msword";
                                    break;
                                case ".dll":
                                    //Dll file
                                    Response.ContentType = "application/x-msdownload";
                                    break;
                                case ".html":
                                    //Html file
                                    Response.ContentType = "text/HTML";
                                    break;
                                case ".txt":
                                    //Txt file
                                    Response.ContentType = "text/plain";
                                    //Response.ContentType = "application/notepad";
                                    break;
                                case ".jpeg":
                                    //Jpg picture
                                    Response.ContentType = "image/JPEG";
                                    break;
                                case ".jpg":
                                    //Jpg picture
                                    Response.ContentType = "image/jpg";
                                    break;
                                case ".bmp":
                                    //bmp picture
                                    Response.ContentType = "image/bmp";
                                    break;
                                default:
                                    Response.ContentType = "application/octet-stream";
                                    break;
                            }
 
			
                            Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);
                            Response.TransmitFile(filepath );
                            Response.End();
 
                        }
                    }

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sample-code.doc
sample-code-to-open-window-as-mo.doc