Very simple + sane extension that I found.
Puts an RSS icon into your URL bar when the webpage you’re on has an RSS feed. The button is hidden, if it does not.
When you click the button, it copies the feed URL into your clipboard, so you can add it into your RSS reader. If there’s multiple RSS feeds on a webpage, it shows a little dropdown and then when you click one of the entries, it copies the feed URL.
When I say “RSS”, I do also mean Atom and possibly others.
I use RSSPreview for this. If a feed is available and you click its icon, it gives you a menu where you can select a feed to visit. RSSPreview basically changes how feeds are displayed inside Firefox.