From the Sage site:
“Sage is a lightweight RSS and ATOM feed aggregator extension for Mozilla Firefox. It’s got a lot of what you need and not much of what you don’t.”
Sage reads both RSS and Atom feeds. It integrates with Mozilla’s bookmark system, supports feed discovery, supports OPML files, has feed newspapers, is integrated with Technorait, and supports multiple languages. There is also a Sage Wiki.
With so many people choosing to run Firefox as their primary browser, it looks like a Firefox / Sage combination will be able to give Safari good competition for in-browser RSS reading.