I was going to join calckey.social/firefish.social but I'm a little hesitant now because mastodon.art defederated with it, and I follow multiple accounts from that instance.

I'm on calckey.world, and it seems to be federated with mastodon.art. at least post from there appear on my global feed

Hi from lemm.ee👋

Firefish user here. While I do agree with the statement, I'd like to add that Firefish integrated really well in "twitter-verse". You can see post from Mastodon, Misskey, Akkoma/Pleroma and other platforms. So the small userbase isn't much of a problem.

I, personally, find Misskey/Firefish a much better alternative to Twitter than mastodon. It has everything you'd expect: Nice and clean UI, "quoted retweets" ability, working search functionality, post reactions and much more.

However you still need to go to an accounts instance to see its subscriptions/subscribes. But I guess it will be fixed in future

Both Lemmy and Kbin users can see each other's content. This is the beauty of Fediverse