Both Cryo Ammo and Cryo Blast can act as the source for a Cryo Explosion Combo. When a target under the effects of one of these abilities is hit by an eligible 'detonator', such as Incinerate, Biotic Charge, or a Frag Grenade, for a killing blow, a large area of effect explosion will occur, which will freeze targets in the vicinity (making them eligible targets to be shattered), as well as doing a large amount of damage to armored targets.
In addition, there are specific upgrades to the Incinerate ability that give it a substantial damage bonus against frozen targets. In addition, the various 'freeze' powers (Cryo Ammo and Cryo Blast) both offer similar damage bonuses against frozen targets through upgrades, though they are unable to detonate a combo.
There is no special benefit to freezing a target that's on fire. None of the frost abilities are capable of acting as a detonator.