Control Effects - Interaction between Molten Primordial and Willbreaker
The creature will come under your control by Willbreaker's effect, Molten Primordial's ability will not be applied.
Let's break down the interaction step by step:

Aaron (Active player) controls a Willbreaker. Nick (Non-Active Player) controls a Bear Cub.
Aaron casts Molten Primordial, there are no responses, and it enters the battlefield. Stack is Empty
Molten Primordial's ability goes onto the stack, then you choose targets for it. Aaron chooses to target Nick's Bear cub. Bear cub is a legal target, as it is a creature that Aaron's opponent controls.
The Bear cub has been targeted, so this triggers Willbreaker's ability, which goes onto the stack.

The Stack now looks like this:
(Top) Willbreaker Control ability
Molten Primordial's Control ability
and begins to resolve.

Aaron gains control of Nick's Bear Cub (through Willbreaker).
Bear cub is no longer a legal target for Molten Primordial's ability, as it is not a &quot;creature that player (Nick) controls&quot;. As a result it is countered as per the comprehensive rules(emphasis mine):

608.2b: If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word “target,” are now illegal. If the spell or ability is not countered, it will resolve normally. Illegal targets, if any, won’t be affected by parts of a resolving spell’s effect for which they’re illegal. Other parts of the effect for which those targets are not illegal may still affect them. If the spell or ability creates any continuous effects that affect game rules (see rule 613.10), those effects don’t apply to illegal targets. If part of the effect requires information about an illegal target, it fails to determine any such information. Any part of the effect that requires that information won’t happen.



Final Result:
Aaron control's Willbreaker, Molten Primordial, and Bear cub. If at any point Willbreaker leaves the battlefield, Nick will gain control of the bear cub again, even if it is this same turn.