Fresh black coffee from just about any shop will cause 3rd-degree burns if you spill it in your lap. Mcdonalds coffee was not somehow unique in that respect, despite opinion to the contrary.
In this thread, meaning to condemn Mcdonalds by holding the "reasonable Starbucks" up as an example, Starbucks coffee has been pegged at "145-165F".
145F coffee can cause 3rd degree burns in less than five seconds. 165F coffee can cause 3rd degree burns in less than one second.
Industry standard coffee causes third degree burns.