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 just tried it on Android Firefox, and well, it does work, but you can definitely tell that it wasn't built for Android Firefox.
It doesn't show up in the URL bar and rather just in the extension menu. In there, it still gets hidden when no RSS feed is available. It copies to the clipboard as expected. And when there's multiple feeds, it opens a separate page to show the dropdown in a very small font. Definitely usable enough, since you're not going to interact with it all the time, but I wouldn't give it the same glowing review...