They are stackable, and one firelighter is used for each log burnt. These logs then can be burned using a tinderbox, with each of them giving 10 xp more than a normal log. Gnomish firelighters obtained as a possible loot from Treasure Trails can be used to dye a normal log to the colour of the lighter used.
With the release of Barbarian Training, you can now use bows in place of tinderbox to light logs upon the completion of the Firemaking part of the miniquest. Your character will then drop the log on the ground, kneel, light the log and will move 1 square west of the log unless you have failed to light it, in which case your character will just stand on it. To light a fire, you "Use" the log with the tinderbox, or vice versa. This simple tool can be bought from general stores throughout RuneScape, or you can just ask the Woodsman Tutor in Lumbridge for one. The most common tool used in Firemaking would be a tinderbox, shown above. This is currently the most effective way to train cooking in F2P, as the ranges in that version of the game all require a bit of walking from the bank. The fires that you light can be used in the Cooking skill. As you level up, you'll be able to burn higher level logs. It is closely linked to Woodcutting, as you can burn the logs that you cut. Firemaking is used to light a few things including logs and light sources.