Firefox 2.0 has an new feature that allows it to hand-off RSS subscription requests to other RSS clients. It's very simple to create your own. Here is the code I wrote that installs Rmail as an RSS subscription handler.
<
script type="text/javascript">function SubWithRmail()
{
navigator.registerContentHandler('application/vnd.mozilla.maybe.feed',
'http://www.r-mail.org/bm.aspx?rss=%s', 'Rmail');
}
</script>
<input type="button" onclick="SubWithRmail()" value="Make Rmail your Firefox RSS handler" />
http://developer.mozilla.org/en/docs/DOM:window.navigator.registerContentHandler