When does the game check whether you have Ferocious or not? After or before tapping the creatures? Is "tap creatures, they don't untap " a single effect or two separate effects?
As with Force Away, you follow Icy Blast's instructions in the order they're written; tapping the creatures and setting up the effect that stops them from untapping are two separate instructions, followed sequentially, so the creatures will be tapped before you check for Ferocious.