He won't be fired unless your word literally is the perfect description for the season. It'd take a complete implosion. Like a ~4 win season with multiple massive blow-outs. Even something like 6-6 where they're remotely competitive and he's fine.
They just gave him an extension. They're still paying Willie. They have no money to buy him out, and buy out another HC from wherever he currently is. And the only way they'd start digging in the darkest of corners to find it is if he completely implodes, as you mentioned.