Firefish and Mastodon are both microblogging platforms (like Twitter) on the fediverse. Kbin and Lemmy are both forum style platforms (like Reddit).
Where Lemmy and Mastodon are for people who prefer a more basic just the necessities experience, kbin and Firefish provide a more stylish experience with more options and customization.
https://github.com/plenaryapp/awesome-rss-feeds