The firemaking skill affects indirectly the time you need to let something burn. It actually influences the temperature rise an object gets when you hit it with any "Firemaking" object, such as the Firemaking Bow. Every level adds 0.5 degrees to the temperature rise.

Example: You have firemaking level 10 and your firemaking object has a minimum tempearture rise of 10 degrees. If you hit any object, this means you get 10 + (10/2) = 15 degrees temperature rise. Note: Numbers are always rounded down, so 14.5 degrees will be 14 degrees.

For every hit you do with a Firemaking object you get 0.5 XP.