Firemaking is the fastest skill to get to 99. The fastest way is to go to Skillz under the emotes tab and keep clicking the chest found there until you level up.
Alternatively, you can actually make a lot of money using this skill by going to shops under the emotes tab.
Here, you should buy magic logs and then burn them using a tinderbox, (Tinderbox is bought from general store at skills)
Doing this will give you money each time you burn the log, and is actually one of the best money-makers in the game.
However, this method gives very little experience, and is not recommended for training purposes.
It is entirely possible to train Firemaking without ever actually making a fire!