<html>

<head>
<title>Port Scanner for Mozilla Firefox</title>
</head>

<body>

<form name="my_form" id="my_form" method="post" action="#" onsubmit="return displayMessage();">
<p align="center">URL/Domain Name/IP Address: <input type="text" size="20" name="uri" value="" placeholder="http://www.google.com" required/></p>
<table align="center" border="1" width=100% bgcolor="#f4fff0" bordercolor="green" cellspacing="2" cellpadding="2">
<tr>
<td align="center">
Start port: <input type="text" size="5" name="start_port" maxlength="5">
End port: <input type="text" size="5" name="end_port" maxlength="5"><br>
<input type="submit" value="Scan all ports" name="scan_all"><br>
<input type="submit" value="Scan only open ports" name="scan_open">
</td>

</tr>
<tr>
<td align="center">
<input type="text" size="20" name="scan_tcp"><br>
<font color="#b1b1b1">Enter port numbers separated by space. (e.g. 21 25 80)</font><br>
<input type="submit" value="Scan TCP ports" name="scan_tcp_ports">
</td>
</tr>
<tr>
<td align="center">
<input type="submit" value="Scan common service ports" name="scan_common_tcp"><br>
<input type="submit" value="Scan common Trojan ports" name="scan_common_trojan_tcp">
</td>
</tr>
</table>
</form>
<script type="text/javascript">
function displayMessage()
{
if(document.my_form.uri.value==0)
{
document.write("<font color='red'><i>Please enter URL/Domain Name/IP Address.</i></font>");
return false;
}
document.write("<DIV id='loading'><BR><BR><font color='#FF6600'><i>Please wait... The ports are being scanned...</i></font></DIV>");
document.getElementById("my_form").submit();
}
window.onload=function()
{
document.getElementById("loading").style.display="none";
}
</script>
</body>
</html>

I want to submit above form. But it shows me JavaScript error in Console that
"TypeError: document.getelementbyid(...) is null"
This error is for code :
document.getElementById("my_form").submit();

I am using Firefox 24.0 and Windows 7. I have installed only 1 add-on. That is 'Firebug 1.12.3'.
This code works fine for other browsers Except Firefox.
Please help me to solve this problem.