Sage was one of the extensions that we missed a lot, since we moved to Firefox 3. Sage was last updated on January 2007 and since then the lead developer Peter Andrews and his team seems to have lost interest in the project.
Sage is an excellent RSS reader addon for Firefox. This is how the sage developers describe it.
Sage is a lightweight RSS and Atom feed reader extension for Mozilla Firefox. It's got a lot of what you need and not much of what you don't.
And this sentence is true to core. You get only what you want in Sage. No extra fat, no unwanted bells and whistles.
Sage-Too have retained all existing features of Sage. So if you are interested to know the features of Sage-Too, the Sage features page ought to be your destination.