Fireworms are bristleworms from the taxonomic family Amphinomidae; generally speaking (this is not a hard and fast rule by any means), these have clusters/tufts of white bristles rather than the typical little spikes with a single bristle sticking out (if that makes sense); they also frequently have red/orange, fluffy looking bunches of gills mixed in with the bristles. Some good examples to look at are Hermodice carunculata and Eurythoe complanata.
As mentioned, there are exemptions to these guidelines, and some of the other fireworms can look a lot more like regular bristleworms, but this should give you a decent, general ID guide.
Regular bristleworms should really only ever pose a threat to critters that are already on death's door, so if you see regular bristleworms eating something in your tank, that thing was probably already dead/nearly dead regardless. Fireworms are a slightly different story though.
Encrusting SPS, crabs, and serpent stars should (to my knowledge) be totally safe from fireworms.
Very small, soft-shelled crustaceans (like pods or very tiny shrimp), snails, and clams could all be at risk with fireworms, especially if they're not in great health/if they're dying.
I haven't heard of fireworms going after Zoanthids or Xenia, but it's possible that they might; when they go after corals, they tend to go after branching corals (Acros, Millies, Gorgs, etc.) and anemones rather than encrusting corals (the way their mouths work, encrusting species would likely be very, very difficult for them to eat). To my understanding, though, most fireworms prefer other foods over corals and nems, so they're generally harmless/beneficial, acting as CUC for your tank and basically eating whatever your fish don't unless you have too many of them in the tank/they're not getting enough food.
In other words, keep fireworms well fed and with their population under control, and they really shouldn't cause many issues for you.
