If you have a fiery breath weapon, do you need flint and steel?

[sirpercival]

Or can you start a cooking fire with your exhalation?

Asking for a friend.

[Garryl]

What is it with saying "asking for a friend" that makes people know that you're asking for yourself but are trying to hide it for some reason? I'm asking for some friends, so they can ask questions for me or something the details are not important.

Strictly speaking, a typical fire damage breath weapon doesn't set things on fire. It just deals fire damage, and unlike fireball, it lacks the little bits about setting flammable material alight. There's nothing in the rules, therefore, that explicitly allows this. Nothing that denies it either, though. Specific breath weapons may have their own rules on the matter, but they usually just follow the standard wording if they don't just refer you to the general breath weapon rules.

It's the kind of thing that makes sense, though, from a simulationist perspective, so ask your DM's opinion on the matter. Personally, I'd be inclined to rule in favor if I were DMing... except your you, Sirpercival. If I were DMing for you, I'd rule against it, hard, and probably dock you a few thousand xp for even asking such a silly question. Good thing you're asking for a friend, right?

[altpersona]

I think of it like a sword...

'I' can swing my sword X times per period to inflict damage. 'I' can use my X ability X times per period to inflict damage.

I can use my sword to spread mayo on a sandwich, if the jar is large enough or if I have another implement transfer the mayo to the sword from the jar in an extra step that defeats action economy.

'I' can use my X ability to do X menial task outside the paradigm of 'causing damage'.

this is merely like, my opinion man...

In our OSR game our Cantrips are based on spells you have memorized or if known if a spont caster. having a force effect memorized might be used to keep the rain off you... or a light spell to make your staff glow in a cool way or provide minor illumination, or if you have a fire spell... cooking is easier.   

[Nifft]

I would definitely allow it.

If your DM is a strict stickler for the rules, you might need to do something like...
- Take a nice dry stick and put it on rocks, perhaps in front of a river or a cliff, and arrange yourself such that only the stick, the rocks, and open water (or sky) are in the area of your fiery breath.
- Exhale, dealing fire damage to the stick, which hopefully also sets the stick on fire.
- Carry the burning stick to wherever you want the fire to be.

Alternately, if you have multiple breath weapons (e.g. Dragonfire Adept level 2):
- Exhale fire, dealing fire damage and starting a bunch of fires.
- Take one stick from this burning area.
- Exhale cold, putting out fires.
- Carry stick to camp.

[Braininthejar]

[Maelphaxerazz]

I would say yes, you can light campfires with a breath weapon. While normal use of breath weapon does not cause things to catch on fire, the rules for forest fires and wildfires (DMG 87 and RC 107) say that sufficiently dry vegetation can be lit on fire by lightning strikes or careless use of fire. If lightning strikes can light dry vegetation, then I think flaming breath weapons should be able to do the same. After all, though the breath weapon does not have "lights things on fire" in its description, neither does Call Lightning or Control Weather.

You won't be able to light wet wood on fire, but it will light the same things a flint and steel can light, and probably more.