Use: Permanently teaches you this glyph. Your Icy Veins causes your Frostbolt, Frostfire Bolt, Ice Lance, and your Water Elemental's Waterbolt spells to split into 3 smaller bolts that each do 160% damage, instead of increasing spell casting speed.
Elementalist talent builds had never quite achieved a position of PvE competitiveness in terms of damage dealing when compared to either the Frost or Fire trees. Previously, Elementalist Mages would use Fireball in raid situations due to its naturally higher damage, seeing half of all their talent investment (that in Frost) go down the drain and being unable to compete with fully specced Fire Mages. That changed with the introduction of Frostfire Bolt in Patch 3.0.2.
The Bolt, in essence, gained the benefits of both Frost and Fire damage talents (with the exception of restricted ones like Improved Fireball or Empowered Frostbolt). Elementalist Mages could benefit from Ice Shards and Ignite simultaneously, for instance, which was a massive increase to their DPS. Frostfire Bolt's stated goal from Blizzard was to make Elementalist builds viable in raids.
Patch 4.0.6 (8-Feb-2011): Frostfire Bolt mana cost has been reduced to 9% of base mana.
Patch 3.0.3 (04-Nov-2008): The periodic damage effect from Rank 2 of this spell now benefits from spell power gains.; Reduced the mana cost of Fireball, Frostbolt, Frostfire Bolt, Arcane Blast, Arcane Missiles, and Arcane Explosion.