Firefish is much more customisable, both for users and from an Admin point of view. For example, I like that as an Admin I can upload a bunch of emojis that have particular meaning to the instance theme. I can also create a site wide theme (but users can change that if they want to). I can follow people and interact with people on any microblogging service that runs activitypub (so Mastodon etc). I found installing Firefish very easy, using their ubuntu installer .sh file.

There's absolutely nothing wrong with Mastodon, its great software. I just find firefish more versatile.