Molten Fury is a mage talent that increases damage done to targets with less than 35% health by 4% per rank, up to 12% at max rank 3. This talent provides a straight 12% DPS increase for the last one third of a mob's health, for an overall DPS increase of 4% on average. The talent is popular among Fire-specced PvE raiding mages. The last 35% of a raid boss fight can be the most critical, with low mana and dead players. By the 35% mark, aggro is firmly established and DPS classes are free to unload, making the effective DPS increase much more valuable. Also, because this is a multiplicative damage increase, it increases more effectively as other vulnerabilities are applied to the target (such as [Curse of the Elements]) as well was gaining increases from talents such as [Fire Power].
Although this talent is situated in the Fire tree it applies to all spell damage caused by the mage.
Besides boss fights also consider using this talent in a [Blizzard] build, where the gradual decrease in mob health increases your effective kill speed by 4% in the optimal case (12% * (35/100)).