javascript firefox中onclick()函数不能用的问题

<a >  var downloadLink = document.getElementById('downloadFileLink');
        downloadLink.onclick = Function("location.href='DownloadHandler.ashx?fileName=" + fileName + "&filePath=" + filePath + "'");
        downloadLink.style.display = 'block';
        downloadLink.style.display.visibility = 'hidden';
        //downloadLink.onclick();  //this sentence can't run in firefox
        if (document.all && typeof (document.all) == "object")   //IE  
        {
            downloadLink.fireEvent("onclick");
        }
        else {
            var e = document.createEvent('MouseEvent');
            e.initEvent('click', false, false);
            downloadLink.dispatchEvent(e);
        }
        return false;