You go in "Edit/Preferences/Feeds/Subscribe to the feed using" here you can choose an application used to subscribe to feeds.
Now when you go to a webpage with RSS content and click on the RSS icon, a page shows up asking you what to do and you can choose to open it with your application but nothing happens ...
I tried with the corresponding version of Firefox and its working as expected (your selected application is launched).
Yesod -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]