For those (like me) who actually liked the old system of a folder with "bookmarks" in it there is Livemarks [0].
Thank you for the two add-on links. I would add Feed Preview [1] to the list.
[1] https://addons.mozilla.org/en-US/firefox/addon/feed-preview/
Thank you for sharing this! I had not encountered an RSS addon with 'subscribe' functionality before. This addon basically restores everything I wish had not been removed from Firefox.
(It's a shame I couldn't have found this on my own. addons.mozilla.org is unbelievably bad for discovering useful addons. It takes a thread like this to turn you on to the good stuff.)
[1] https://addons.mozilla.org/en-US/firefox/addon/awesome-rss/
[2] https://addons.mozilla.org/en-US/firefox/addon/rsspreview/