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.
If you'd just like to start using Sage, head to the website to get started.
$ git clone git@github.com:petea/sage.git
$ cd sage
$ curl [Gecko SDK download URL] | tar -x
$ ant
If all goes well, you'll find the XPI file under the sage/build folder.
With the XPI built, you can install it by opening it from Firefox or copying it to your Firefox profile directory. Either way, you'll need to restart Firefox.