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.

Reads RSS (2.0, 1.0, 0.9x) and Atom feeds Newspaper feed rendering customizable via style sheets Feed Discovery Integrates with Firefox’s bookmark system Imports and exports OPML feed lists Support for a number of locales: Argentine Spanish, Catalan, Chinese, Czech, Danish, Dutch, Finnish, French, German, Greek, Hungarian, Italian, Japanese, Korean, Polish, Portuguese, Russian, Serbian, Slovenian, Spanish, and Swedish Simple, one step install / uninstall