Firefox has removed support for RSS in versions 64+.
This includes the feed preview feature that was quite useful for some people.
This extension attempts to recreate it.
Download at: https://addons.mozilla.org/en-US/firefox/addon/rsspreview/
From version 3.0 on, feed detection on web pages is available, but disabled by default. It can be enable in the addon options and it will add an rss icon in the address bar on pages that declares an rss or atom feed.