If intelligent life emerged on a methane-based atmosphere, how would they make fire without toasting the planet/moon's atmosphere?
In a methane based atmosphere, oxygen cannot exist in large quantities precisely because it would oxidize (burn) the methane. If you were to have a local concentration of oxygen, it would probably cause a local fire. On earth, such fires (in reverse) can be seen in swamps as decomposing organic matter produces methane, and that methane burns at the surface of the water.
Adapted from Will-o'-the-wisp - Wikipedia
In modern science, it is generally accepted that most burning swamp gas is caused by the oxidation of phosphine, diphosphane, and methane. These compounds, produced by organic decay, can cause photon emissions. Since phosphine and diphosphane mixtures spontaneously ignite on contact with the oxygen in air, only small quantities of it would be needed to ignite the much more abundant methane to create ephemeral fires.
The same phenomenon would be likely to happen where oxygen was produced in a methane atmosphere.
So while we make fire by gathering fuel, aliens in a methane based environment would make fire by gathering oxidizers, such as oxygen, chlorine, etc.
Methane cannot burn without an oxidizer.
Presumably, if life emerged, it would be based on reversing the roles that carbon and oxygen play in the metabolism: lots of carbon around, little oxygen. The living things would sequester reactive oxygen, and little would be available in the atmosphere.
I have no idea if such chemistry is likely, or even possible. But "intelligent beings making fire" is the least worry of a species living in an environment containing both oxygen and reactive carbon. They'd have already burned to death a billion times over.
Whatever chemistry allows the organism to be alive would be adapted to make fast-moving reactions equivalent to fire. It wouldn't be "fire", per se; they might burn lumps of ammonium nitrate in the methane atmosphere. Since that's the only oxidizer available it's self-limiting and won't set off the atmosphere any more than burning wood ignites all of the oxygen in ours.