Fire Power is a mage talent talent that increases damage done by fire spells by 1% per rank, up to 3% at max rank 3. This is a multiplicative talent that is applied after the addition of spell damage item bonuses, and is multiplied together with similar talents, buffs and vulnerabilities.
This damage multipler stacks with other effects that increase damage by a percentage, such as debuffs and other mage talents like [Molten Fury]. These percentages are multiplied together when stacking.
The damage multiplier applies to total damage, including the base damage, bonus from spell damage gear, and any other damage multipliers. The following formula gives the total damage of a Fire spell, E, where B is the spell's base damage, c is the spell damage coefficient, d is the amount of fire spell damage the mage has, and X is the number of points in Fire Power. Critical strikes are not considered.
Patch 4.0.1 (2010-10-12): Fire Power is now a Tier 2 and 3 Ranks talent, down from Tier 6 and 5 Ranks. Now increases Fire spells damage by 1/2/3%, down from 2/4/6/8/10%. Also gives your Flame Orb a 33/66/100% chance to explode at the end of its duration.
[Flame Orb]
[Molten Fury]. These percentages are multiplied together when stacking.
Patch 5.0.4 (2012-08-28): Removed. 
