Here are some eyewitness accounts, together with my caveats:
A little less than halfway down the webpage:
there begins an extended quotation from: History of the Great Conflagration, Sheahan & Upton, Chicago 1871, the first two paragraphs of which are reproduced below:
"The summer of 1871 had been excessively dry; the moisture seemed to be evaporated out of the air; and on the Sunday above named the atmospheric conditions all through the Northwest were of the most peculiar character. The writer was living at the time in Minnesota, hundreds of miles from the scene of the disasters, and he can never forget the condition of things. There was a parched, combustible, inflammable, furnace-like feeling in the air, that was really alarming. It felt as if there were needed but a match, a spark, to cause a world-wide explosion. It was weird and unnatural. I have never seen nor felt anything like it before or since. Those who experienced it will bear me out in these statements.
"At that hour, half past nine o'clock in the evening, at apparently the same moment, at points hundreds of miles apart, in three different States, Wisconsin, Michigan, and Illinois, fires of the most peculiar and devastating kind broke out, so far as we know, by spontaneous combustion."
And here are my caveats:
The people who died without having been singed were, most likely, asphyxiated to death by a phenomenon observed following the firestorms of the Second World War in which the fires suck all the oxygen from the air.
I don't find the account of Port Huron, MI mill manager Allison Weaver to be credible for two reasons (this is the lawyer part of me speaking), the first being that he claims to have knowledge of the speed with which the house and the mill were consumed by the flames even though his own account says that he was hiding in the well at the time, the second being that I don't see how he, himself, was not asphyxiated in the same manner as those residents of Hamburg who took refuge in solid blast-proof bomb shelters during that city's firestorm of 24 July, 1943. I think he spun a tale in order to impress the mill owner with an account of his heroic efforts to save the owner's property.