You are underselling Thor quite a bit in regards to those burns.
You do know Thor is essentially fire proof right? It took the heat of a neutron star to burn him and even then it took extended exposure to produce visible damage, he survived in Surtur's realm unharmed, took heat from a concentrated flamethrower from Pre-Eternal Flame Surtur (although he blocked the initial blast).
I doubt regular ol' flames from burning debris could leave burns on Thor, who do you think Thor is? Captain America?
Thanos and the BO had advanced weaponry on their person, not to mention on Thanos’s ship which it seemed had been attacking them for a while. I don’t think I am under selling Thor at all.
None of the weaponry or abilities displayed by the BO or Thanos himself inflict burning damage except for the Power Stone.
Did the power stone cause burning when it was held to Thor’s head though? Did it in Guardians?