Exactly what I was looking for! Two most important features of this addon is that is uses a sidebar and it automatically detects feeds on webpages.

It would be nice if dark theme is added.

Dead simple, functional, and seamless integration into older FF versions. Exactly what I was looking for, and I suspect I'll be using this for my RSS needs from now on. The only problem I have is that the add-on does not save my feeds when exiting the browser and opening it again, so I have to always import the feed. That aside, I look forward to what the developer does with it.

I like it a lot. Found via Hacker News.