Set an empty function to be called on window.onunload:
window.onunload = function(){};
e.g.
<html><body>
<script type="text/javascript">
window.onload = function() { alert('window.onload alert'); };
window.onunload = function(){};
alert('inline alert');
</script>
<a href="https://stackoverflow.com/questions/2638292/1.html">Click Me!</a>
</body></html>
Source:
http://www.firefoxanswer.com/firefox/672-firefoxanswer.html (Archived Version)