If you are closing the browser, onunload="javascript:window.close()" has no use (actually, it will display a message box asking you whether you want to close the window).
But if you are just refreshing your page, onunload="javascript:window.close()" will close the window, that may not be what you want.
if you want to handle the onunload event on the frameset, try
<frameset onunload="doWhateverHere()">
.....
</frameset>