Company fires their employee at 8:30AM on the spot: Employee is now without income and likely health insurance and his ability to pay rent, buy food and merely exist might now be in jeopardy.

Employee quits at 8:30AM on the spot: Company is at worst set back a bit until they backfill that job. At best they don't even notice. Unless that employee was some keystone that held the whole business together, there is no existential threat to the company.