I'm trying to find a paper that states otherwise -- that we can't have all neurons fire simultaneously, but all I can find is Quora questions (like this) which mentions that if all neurons will fire together we will have a seizure.
Note: this answer is imprecise; read its comments below.
"...based on the Hodgkin–Huxley model calculates by the method of ion counting and power integration that an action potential consumes 2.468 × 10−7 J of biological energy produced by the hydrolysis of Adenosine triphosphate (ATP)..." (ref. https://link.springer.com/article/10.1007/s11571-018-9503-3)
Supposing we have 100 billion neurons in the brain, the total energy consumed would be:
1E11 * 2.468E-7 J = 2.468E4 J
So 24KJ to generate a single action potential for each neuron of the brain.
To lift an apple 1 meter requires roughly 1 J. So the energy needed to make each single neuron fire once would be enough to lift 24 thousand apples 1 meter, all at the same time.
So that is why it is impossible: the brain would catch fire. An we would starve.