Try this code it will disable the back button not only in firefox but also in MSIE, Safari and Opera.
<html>
<head>
<script type="text/javascript">
window.history.forward();
function noBack(){ window.history.forward(); }
</script>
</head>
<body onload="noBack();" onpageshow="if (event.persisted) noBack();" onunload="">
...
</body>
</html>
Or else force the cache to expire for this to work. Place the following code on your page code behind.
Page.Response.Cache.SetCacheability(HttpCacheability.NoCache)
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