Interesting tidbit about this. The reason we have so much fossil fuel in the first place is because of what happened during the Carboniferous era. Trees evolved the capability to produce lignin, the 'woody' material in trees. However, it took millions of more years for microorganisms to evolve the ability to decompose lignin.

So instead of what happens today when a tree dies and rots, the tree died and just piled up in the forest. I always think it's amazing to imagine what a forest must have looked like during the Carboniferous with all these dead trees just piled high. Trees were essentially like today's plastic: non-biodegradable refuse that stuck around for millennia. Eventually all those trees got buried and turned into fossil fuels (mostly coal).

And, interestingly, this process let to huge climate change back then, too - basically the reverse of what's happening now (and at a much slower pace). Trees sucked all this carbon out of the air and reduced global temperatures by about 8C.

Edit: One other note - there were of course still forest fires during the Carboniferous. It's believed they would have been incredible raging infernos with so much undecomposed combustible material.