The name is way too similar to the Firefox trademark and could create the impression that Firefish is associated with Mozilla. I suspect some lawyers are currently in a huddle trying to figure out how to send a Cease and Desist letter that won’t completely piss off the community.
(Trademark law, at least in the US where Mozilla is headquartered, requires organizations to actively defend their trademarks. So just ignoring Firefish would be risky, even if they don’t actually mind the similarity.)