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'd like to note that RSSPreview restores features that were removed in Firefox 64.
And by "changing how feeds are displayed", it applies an XSL Transformation to display the feed as HTML (instead of XML) in the browser. XSL Transformation is pretty neat, you can use it on any XML document such as your own RSS feed, like I done on mine.