I think that a super firestorm would create such updrafts that burning branches would be hurled 1000s of feet ahead. Firebreaks would have to be enormous!
Fires jump roads, main highways, sometimes. That would be at least 200 feet. Say if firebreaks were every mile and wide enough to stop most fires, and the forests were cleaned out of deadwood, low branches, and other ground fuels, just for consideration. That would cut forested area by 10%, but would probably have other benefits besides making fire control a lot easier, such as improved wildlife habitat. Maybe an environmentalist could do something actually useful and design a national firebreak system on his computer . . . optimum spacing, optimum width, that kind of thing.
Then, of course there are tundra fires, which Alaska gets every summer, huge fires, lots of smoke that travels hundreds of miles but burns only a few illegal cabins and no loss of human life usually.