No one really knows for sure. Dominic Lieven's new(ish) book Russia against Napoleon is a good book which addresses this.

Some say the fire was intentional, some say accidental, but I would say most modern serious scholars contend it was accidental, in much the same way Tolstoy describes it. Had it been intentional, it seems that the conflagration would have been much more total. As it happened, the fire spread quickly, and destroyed much, but it wasn't systematic. Systematically burning a massive city like Moscow wouldn't be too hard given a few hundred police or soldiers or firefighters. Station them at various parts of the city and just light something big on fire, so if any one fails, the others would largely continue to spread.

Instead, Moscow burned from probably one location and spread outwards from there, just like in the great Chicago fire. There's also no reliable documents which claim it was intentional.