Player's Handbook[]
A character with fire-building proficiency does not normally need a tinderbox to start a fire. Given some dry wood and small pieces of tinder, he can start a fire in 2d20 minutes. Flint and steel are not required. Wet wood, high winds, or other adverse conditions increase the time to 3d20, and a successful proficiency check must be rolled to start a fire.
Player's Option: Skills & Powers[]
A character with this proficiency can build a fire in 1d20 minutes, as long as there is dry wood and some small bits of tinder. Add another d20 minutes for each of these factors: the wood (or tinder) is wet, it's raining or foggy, or the winds are strong. A proficiency check is required if conditions are bad and the character is forced to work without shelter.