It does everything Mastodon does but more, honestly. I use both, and definitely prefer Firefish. But I’m a developer so a lot of things about Mastodon really bothered me. The core difference is Firefish fka Calckey is being developed much faster and with a more modern stack. The click to play MFM feature was developed in a few days when the community was concerned about potential seizures due to unasked for auto playing or animated text.
A few key features: QT & Full text search (search I don’t use except for specific posts so can’t speak to that) MFM & cat mode (these are just fun, Misskey flavored markdown has things like tada and sparkle and rainbow. People make art with it)