In DragonVale for the iPhone and iPad, the Frostfire Dragon is a hybrid dragon with the cold and fire attributes, similar to the Blue Fire Dragon.
To get a Frostfire Dragon, you need to breed a Cold Dragon with a Fire hybrid. Some examples are Cold + Firefly, Cold + Lava, Cold + Blazing, etc.
Note that any of these combinations do not guarantee results, and the different combinations have different percentages for success. Keep trying though and you’ll eventually get your Frostfire Dragon.