You are un-fireable for the usual reasons for which people outside academia worry about being fired.
Layoffs aren't a thing in academia. Poor performance in the classroom isn't punishable. Failure to bring in grants isn't punishable. You can't be fired for disagreeing with your boss. You can (in most cases) publicly criticize the administration you work for, and advocate for many (yes, not all) controversial ideas.
That's an American thing. By default, you can fire anyone at any time for no reason. Universities then overcompensate and give extensive protections for tenured faculty.
In Europe, it's more common that a professor has roughly the same job security as a teenager in their first real job. There are some exceptions due to academic freedom, but they are mostly about the substance of the work rather than the performance in it. And other independent professionals, such as doctors, lawyers, and civil engineers, often have similar exceptions.
They are, but extremely infrequently and when they are it is minor numbers, at least at the largest universities. And when they are laid off it is for financial reasons not performance.
Layoffs aren't a thing in academia. Poor performance in the classroom isn't punishable. Failure to bring in grants isn't punishable. You can't be fired for disagreeing with your boss. You can (in most cases) publicly criticize the administration you work for, and advocate for many (yes, not all) controversial ideas.