>Aufhebungsvertrag

That is something both sides have to agree on. So it can not be considered "firing".

>Freistellung

You are still employed, just have no tasks assigned to you. Completely different scenario for the employee, who now can look for a new job, while still being paid as if he were employed. Arguably it is even better than being let go, but having to continue working. Definitely anything but a "firing on the spot".