My (admittedly limited) understanding of it is that forest fires do affect the weather, and do so in very complex ways. On one hand, you get big updrafts which suck in air and make the fire spread and get more intense. Sometimes these updrafts are so strong that it basically creates fire tornados.
On the other hand, one of the products of combustion is water vapor, and adding a lot of water to the air combined with ash that forms nucleation sites for water can lead to rain. Sometimes you get these sudden downbursts that may douse a small area with water, but push air out and cause the fire to expand outward away from it.
You can end up with a lot of chaos, with some affects trying to make the fire bigger and some trying to make it smaller, which makes the spread of the fire very difficult to predict.