Firefox itself never had a real rss-reader. It had rss-parser which was incorperated into the bookmark-ui, making it barely useful for the purpose if consuming

Firefox itself never had a real rss-reader. It had rss-parser which was incorperated into the bookmark-ui, making it barely useful for the purpose if consuming feeds. 3rd-party feedreader coming as addons were more useful than this and they were not really good either. So it's no surprise this feature had low usage and was thus removed.