Firmus was, according to the HA, a usurper. The historical record is less clear.
Historia Augusta 29.IV
“Firmus was nevertheless of huge stature with prominent eyes, curly hair, a scarred forehead, a darker complexion on his face while most of his body was white, although it was tough and hairy, so that many used to call him a Cyclops. He used to eat a lot of meat and allegedly ate an ostrich in a day. He drank some wine and a lot of water. He was extremely strong in mind, most robust in nerves to the extent that he overcame Tritannus, whom Varro mentions. For he endured an anvil placed on his chest and struck constantly while he seemed to be rising up rather than lying down because he was face up supporting himself on his hands.
Yet, [Firmus] had a drinking competition with Aurelian’s generals when they wanted to test him. For, when a certain Burburus, one of the standard-bearers and a notable drinker, challenged him to a drinking context, he sucked down two pails of wine but was still sober after the banquet. When Burburus asked him, “Why didn’t you drink the dregs?” he responded “Fool, the earth is not drunk.” We are pursuing lighter notes here, we must speak of more important ones.”
Fuit tamen Firmus statura ingenti, oculis foris eminentibus, capillo crispo, fronte vulnerata, vultu nigriore, reliqua parte corporis candidus sed pilosus atque hispidus, ita ut eum plerique Cyclopem vocarent. 2carne multa vescebatur, struthionem ad diem comedisse fertur. vini non multum bibit, aquae plurimum. mente firmissimus, nervis robustissimus, ita ut Tritannum vinceret, cuius Varro meminit. 3nam et incudem superpositam pectori constanter aliis tundentibus pertulit, cum ipse reclinis ac resupinus et curvatus in manus penderet potius quam iaceret. fuit tamen ei contentio cum Aureliani ducibus ad bibendum, si quando eum temptare voluissent. nam quidam Burburus nomine de numero vexillariorum, notissimus potator, cum ad bibendum eundem provocasset, situlas duas plenas mero duxit et toto postea convivio sobrius fuit; et cum ei Burburus diceret, “Quare non faeces bibisti?” respondit ille, “Stulte, terra non bibitur.” levia persequimur, cum maiora dicenda sint.