Things that can naturally start a fire are the sun, lightning strikes, volcanic eruptions, spontaneous combustion of organic matter, meteor impacts, sun hitting glass or even sun hitting a magnifying glass, rockfalls, coal seam fires, strong winds, high temperatures and drought and dry vegetation.
Lightning is the leading natural fire starters, especially hot lightning, which is long duration lightning strikes during dry thunderstorms which can set vegetation on fire, especially in remote areas.
Molten lava, hot ash and gases from eruptions of volcanos can ignite surrounding forests.
Piles of decomposing organic material, like leaves, compost, manure etc can also generate enough internal heat to ignite by itself.
Sparks from falling rocks can ignite dry brush and start fires.
Underground coal deposits can burn for long periods of time and ignite surface fuels.
And although rare, meteors can also start fires naturally.
Dry vegetation and drought creates abundant fuel for fires.
High temperatures increase fuel dryness and heat and can lead to natural fires.
And strong winds can fan the flames and spread embers, which creates larger firestorms, like California's Santa Ana Winds.