Try Google Reader. No, it's not an extension but RSS feeds actually work much better with an online reader for several reasons.
1) Most offline readers only pull the latest feed entries when your computer is on or when you choose to pull them down. As RSS feeds only show the latest 10-20 entries if any more have been made since then you won't get them. This to me defeats the point of RSS as if you don't get the complete feed you have to visit the site and manually check what stories you've missed. Online readers like Google Reader get round this by constantly checking feeds all the time and updating their server cache of them, so whenever you visit Google Reader you'll have every entry posted since your last visit, not just the last ten.
2) The second major reason for an online RSS reader is like web based email you can check your feeds from any computer and automatically have anything you read there marked as read when you check it elsewhere.