Very good, but you need the version from SageRSS.com Rated 5 out of 5 stars

A very good extension: Simple and clean, with no needless bells and whistles. The one downside is that Firefox updates break it every so often, such as Firefox 22, and it takes a while to update the version on addons.mozilla.org. However, the sageRSS.com works fine.

This review is for a previous version of the add-on (1.4.15.1-signed).