There is a lot of confusion on bristleworms. When most people say fireworm, they are referring to the bearded fireworm
However, it is worth noting that fireworm is also a colloquial term for all polychaete worms with irritating bristles along their sides. Essentially, all bristleworms.
The bearded fireworm is not a huge risk because it is fearless. You won't ever have to wonder if you have one because it will never hide.
As for all the other bristleworms, they are harmless. UNLESS they get big. And some of the common species we see do get big of allowed to gorge on uneaten fish food and dead animals. But until you see a 8 inch long 1 inch thich bristleworm in your tank... RELAX. It's fine. It's just there to clean up! And for every one you see, there are probably 10 others munching away on fish poop and uneaten food in the background, so really, don't freak out if you see a worm.