TL;DR: from Firefox 64 onwards, RSS/Atom feed support will be handled via add-ons, rather than in-product.

What is happening?

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.

Why are you doing this?

By virtue of being baked into the core of Firefox, these features have long had outsized maintenance and security costs relative to their usage. Making sure these features are as well-tested, modern and secure as the rest of Firefox would take a surprising amount of engineering work, and unfortunately the usage of these features does not justify such an investment: feed previews and live bookmarks are both used in around 0.01% of sessions.

As one example of those costs, “live bookmarks” use a very old, very slow way to access the bookmarks database, and it would take a lot of time and effort to bring it up to the performance standards we expect from Quantum. Likewise, the feed viewer has its own “special” XML parser, distinct from the main Firefox one, and has not had a significant update in styling or functionality in the last seven years. The engineering work we’d need to bring these features, in their current states, up to modern standards is complicated by how few automated tests there are for anything in this corner of the codebase.

These parts of Firefox are also missing features RSS users typically want. Live bookmarks don’t work correctly with podcasts, don’t work well with sync, and don’t work at all on any of Mozilla’s mobile browsers. They don’t even understand if an article has been read or not, arguably the most basic feature a feed reader should have. In short, the in-core RSS features would need both a major technical overhaul and significant design and maintenance investments to make them useful to a meaningful portion of users.

Looking forward, Firefox offers other features to help users discover and read content, and the move to WebExtensions will make it much easier for the Mozilla community to bring their own ideas for new features to life as well.

What will happen to my existing live bookmarks?

When we remove live bookmarks, we will:

1. Export the details of your existing live bookmarks to an OPML file on your desktop, which other feed readers (including ones that are webextensions) support importing from.
2. Replace the live bookmarks with “normal” bookmarks pointing to the URL associated with the live bookmark.
3. Open a page on support.mozilla.org that explains what has happened and offers you options for how you could continue consuming those feeds.

This will happen as part of Firefox 64, scheduled for release in December 2018. We will not change anything on Firefox 60 ESR, but the next major ESR branch (currently expected to be Firefox 68 ESR) will include the same changes.

https://www.gijsk.com/blog/2018/10/firefox-removes-core-product-support-for-rss-atom-feeds/