Firefox popup on form change password

Hi,i get this popup on form change password but IE,Chrome don’t have it
I don’t understand how to remove this popup

this is code

<html>
<head>
</head>
<body>

<center>

<form action="" method="post" name="Form_ChangePass" onsubmit="javascritp:return checkForm()">

   <input name="Old_Pass" type="password" size="50" maxlength="100" id="TextForm2" value=""/>

    <input name="User_Pass" type="password" size="50" maxlength="100" id="TextForm2" value=""/>

    <input name="User_Confirm_Pass" type="password" size="50" maxlength="100" id="TextForm3" value=""/>
  

<INPUT TYPE="image" SRC="../img/ok.png" BORDER="0" ALT=""><a href=""><img src="../img/cancel.png" border="0" onclick="document.getElementById('form').reset();"/></a>

</form>
</center>
</body>
</html>

on form i have 3 password
when i key : 1=a,2=b,3=b i get popup
1 = a,2=a,3=b i get popup
1=a,2=b,3=a i get popup
1=a,2=b,3=c i don’t get popup

What’s it?My Firefox version is 3.6.8