From the article:
Mastodon is a great microblogging platform, but maybe you want to try something new. Firefish made a splash recently, and a lot of people are talking about it. If you’ve played with it for a while and want to officially take the plunge, here’s some tips on moving from your Mastodon Instance to a Firefish one.
I poked at it a bit and it does indeed let you log in with a Firefish account, but it seems like it doesn’t support threaded replies like Firefish and also lacks the Antenna feature?
yea, it’s still a pretty basic implementation at the moment. Hopefully it’ll be added in the future.