Good question! Whether it’s actually infringement is a legal judgment I’m certainly not qualified to make. 🙂 But my understanding is that it hinges on whether a court thinks a “reasonable person” could be confused. For example, a clothing brand called “Firefoxy” would probably be in the clear since Mozilla isn’t in the clothing business. And maybe even a clothing brand named “Firefox” might be okay! For example, Apple Computer and Apple Records (founded by The Beatles) coexisted nicely for a long time until Apple Computer started getting into the music-selling business. I forget how it got resolved (maybe a licensing agreement?) but The Beatles’ music wasn’t available on the iTunes Music Store for a looooong time while that dispute was going on.
Firefish is an online service and software package, the very space Mozilla operates in, so there’s at least a case to be made that reasonable people might incorrectly assume it’s from Mozilla. It’s come up many times in this discussion already, and we as active Fediverse users are already pretty well informed about this!