Firefox removes core product support for RSS/Atom feeds

After considering the maintenance, performance and security costs of the feed preview and subscription features in Firefox, we've concluded that it is no longer sustainable to keep feed support in the core of the product. While we still believe in RSS and support the goals of open, interoperable formats on the Web, we strongly believe that the best way to meet the needs of RSS and its users is via WebExtensions.With that in mind, we have decided to remove the built-in feed preview feature, subscription UI, and the "live bookmarks" support from the core of Firefox, now that improved replacements for those features are available via add-ons.

I would assume most RSS users already use more capable RSS readers and/or browser extensions, so it makes perfect sense for Firefox developers to remove this functionality from the browser so they no longer have to maintain it.