I have 2 Fully Trained Firesteeds, both with good Stats/Resists, one has 105.4 Resist and another has 107.7 if i remember right. Resist is the only skill that can be above 100, and that's if they had over GM Resist before tame.
Solo, a Fire Steed is pretty weak/frail, and a Nightmare outclasses them in everything. However, Fire Steeds have Daemon/Equine Pack Insticts, so if there's more than one Firesteed, then the damage they do is SIGNIFICANTLY boosted. 2 Firesteeds together can outdamage a Great Wyrm or a Mare+Runebeetle Combo, yet they're still easily killed. The fastest way i've found to roll over the Cu Sidhe spawn in the Twisted Weald is to get a Tamer to use his Trained Cu Sidhe to tank, then use Firesteeds to dish out the damage, drops the wild Cu Sidhes like a rock. Once got another Tamer to bring 2 Firesteeds as well, we were dropping Paragon Cu Sidhes in under 10-15 secs with 4 Firesteeds and a Cu Sidhe. Firesteeds are only good if there's more than one.