Firefox removes core product support for RSS/Atom feeds | Use Tables! is a pretty good blog post for this as well.
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.