That was removed from Firefox in version 64

https://support.mozilla.org/en-US/kb/live-bookmarks

Honestly just returning the icon that appeared when the site had a feed would be a huge win. Browser integrated feed-readers and bookmark notifications were nice but dedicated RSS readers are probably going to be better. You can click the feed icon and the browser can redirect you to your preferred feed reader.

There are extensions that do this but having it built-in would make it widely available and easier to set up.

Vivaldi has had an in-built RSS client for years now, taking after Opera's original feature years back.

Edit: link since I notice they have a page about it. https://vivaldi.com/features/feed-reader/

Any RSS support I've seen in web browser sucked. Use a real RSS reader like QuiteRSS and you won't care to have it in your browser.

Without having dipped into the code before, how hard would it be run a fork of Chromium that did this?