Blizzard requires a button press for each spell. You can also put a reset timer on there (or have it reset when you hit another button or right click it). Otherwise, if your mob dies before you case fireblast, the next time you click the macro, it will cast fireblast. If you tell it to reset, then it will reset to pyroblast.
See the two stickies by Neuron and Cogwheel for more macro info.
http://forums.worldofwarcraft.com/bo...Id=11114&sid=1