It was never unusable beyond the stability issues large instances (from 1k to howevermany people ff.social had) had. For smaller instances it worked fine and continues to do so. The issues with large servers were the result of it being based on an ancient codebase (Misskey v12) with extremely questionable changes thrown on top (muting enough words could cause the entire instance to slow down), and the issues with ff.social were specifically caused by throwing everything at the wall to try to duct-tape that ancient codebase to function (ScyllaDB was the nail in the coffin i believe…?)
Firefish itself is still going (see firefish.dev), there are forks like Iceshrimp which reigned in the issues enough for larger servers to not fall over every few seconds (iirc both the infosec.exchange hosted Firefish instances migrated over which caused the main issues to be found and fixed). I wouldn’t be surprised if “Modern” Firefish took the most important changes over from Iceshrimp (the devs are friendly, and the Mastodon API implementation and some security fixes were shared between both)
If you want something a bit lighter, Misskey itself is still ongoing, and there are forks like Sharkey that do some of the modifications Firefish and similar forks did to tailor it towards a non-Japanese audience.
(And Iceshrimp.NET is a project worth keeping an eye on, which aims to get rid of the technical debt of the Misskey codebase by completely rewriting it, but is not ready for much more than a single user instance just yet considering it’s been a thing for just about a year)