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.
Nice, I thought something like this existed natively on some other chromium browsers too. Convenient it can be added to ff now.