I'm surprised after all of the reviews I've done that I haven't reviewed Sage yet. I've recommended this reader to a lot of individuals and all of the users have said what a great Feed Reader it is.
The Sage extension supports the standard feed formats: 2.0, 1.0, 0.9x and Atom Feeds. Sage has included a Feed Discovery, meaning if there is an RSS feed on a web page, it'll detect it and add it for you to Sage.
One of the things I always look for in a reader is the ability to export or import OPML files. Sage handles this task easily and utilizes the Firefox's storage through their LiveBookmarks and bookmarking system.
One downside to Sage is that there isn't a way to work with enclosures. If you podcast, you understand what I'm saying. If you receive an RSS feed and an MP3 is attached to it, you want to listen or download it. Sage does not give you that ability.
If you don't like the way your styles look in the reader, the community has developed a lot of styles to use in Sage. Copy the styles into the Sage directory and you have your own newspaper-style reader. Adding an RSS feed is easy. Just pick a RSS feed, enter it into Sage, and read. Feed and Read!
Documentation is provided in the form of a wiki. So if you have any problems with Sage, help is only a click away.