Warning: this skill is completely useless, incredibly tedious to train, and is a waste of time.
This is my guide to 99 firemaking. Including expensive and cheap ways.
Expensive way:
Levels 1-15: 61 normal logs
Levels 15-30: 183 oak logs
Levels 30-35: 101 willow logs
Levels 35-45: 373 teak logs
Levels 45-50: 295 maple logs
Levels 50-58: 782 mahogany logs
Levels 58-60: 255 eucalyptus logs
Levels 60-75: 4,626 yew logs
Levels 75-99: 38,927 magic logs
Cheap way:
Levels 1-15: 61 normal logs
Levels 15-30: 183 oak logs
Levels 30-99: 144,679 willow logs