AMO: https://addons.mozilla.org/en-US/firefox/addon/brief/
GitHub: https://github.com/brief-rss/brief
It also includes a `Page action` so that an RSS icon is shown in the address bar on sites that support RSS/Atom. Great for discovery.
Edit: Really strange, that `Brief` is missing in the curated list of readers that mozilla is linking to from https://support.mozilla.org/en-US/kb/feed-reader-replacement... as alternative: https://addons.mozilla.org/en-US/firefox/collections/mozilla... (This looks like a really poor list) I've seen `Brief` recommended all the time by users. I don't really mind that Firefox removes its RSS support: It was missing many features anyway and had a really poor UX.