the askubuntu.com site has a rss/atom feed which is available via the rss icon in the awesomebar of firefox. if you click on that icon, the page firefox wants to open is feed://http//askubuntu.com/feeds.
as you can see this is formatted wrong as it should be https://askubuntu.com/feeds
if you click on the feed icons of opera and ie they are going to the right address, so it's firefox only.
<link rel="alternate" type="application/atom+xml" title="Feed of recent questions" href="/feeds">-- I'd rather assume this is a bug in Firefox. Also, it works as expected in my Firefox 3.6.12 -- What version are you using?