Mountainous as heck
Your forests fill relatively well separated alpine valleys, with lots of glacial lakes. The dragons roost in the mountain peaks. Fire breathing has evolved here for a couple of reasons, from your average, winged, non burny giant flying lizard.
The first is hunting - we see this behavior in firehawks, where they seem to use burning things to flush out prey. While the dragons are too big to maneuver among the trees, they simply set fire to one end of the valley, and the animals living there panic and run. They can then either pick them off with controlled flaming, or dive down and grab them as they flee into the water.
The second is thermals - being able to set fires makes flight easier for these big, ungainly flying things. They may not be able to get over the mountains without this help.
From this, I'd propose a proto-draconic ancestor - much smaller, with limited fire starting ability. These dragons would be closer in size to flying lizards, and would use a limited fire breath to ward of predators. Eventually, they start using little blazes to fly greater distances, and even starting fires to migrate to new valleys.
I'd also propose a dragon life cycle - dragons hibernate for long periods, perhaps into the multiple years, probably high up on the mountains. Every few years, they emerge, set fire to the valley, mate, gorge themselves on prey that's either been cooked or picked off from the lake, lay eggs in the embers, and then return with their young to the top of the mountains, where they hibernate in caves.
It solves for the ecological devastation issue of a giant burning lizard, and matches with other highly destructive lifecycles we find in nature (i.e, locusts), where a swarm emerges after a long period of hibernation.