Add the following javascript code in your aspx page.
Code:
<head id="Head1" runat="server">
<script type="text/javascript">
//disabling browser back button for mozila
function noBack(){window.history.forward()}
noBack();
window.onload=noBack;
window.onpageshow=function(evt){if(evt.persisted)noBack()}
window.onunload=function(){void(0)}
</script>
</head>