Sorry for your loss!

So...do you think this 'fuzz ball' was on him when you got him and you just didn't notice or something that developed visually while in your tank? Usually a fungus or something similar would show signs well before being fatal. You have the issue of bacteria, especially vibrio, that can cause sores or flesh eating like issues. I have seen vibrio take stuff out fast if you aren't quick to the signs of it.

As too whether the rest of your tank is now vunerable....well...yes. But doesn't mean they will get it. Since we don't know what it really is....you really can't do anything but watch them and ensure everyone is eating and acting normally.

Make sure you water params are in check, you have good food and good environment.

You have a lid on your tank, right? Firefish are known for jumping out of tanks.