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.
Sage is one of the oldest RSS feed readers for Firefox, started in 2004.
Sage is build on top of Python, the Gecko SDK and Ant, and most of its code is written in Javascript.
If you are interested to have a look at the source code, you can go to our repository at Github.