 What if companies fire employees that try to unionize?

If you're literally penniless and need immediate funds, see the section on welfare. Only join companies that allow employees to unionize. Or find funding and form a cooperative. If enough people join the union that there aren't enough ununionized empoyees left, the employers will have no choice. If that is not the case, either the conditions must not be so bad, or there are more people than needed in the industry, which means some people need to leave and libertarianism is just preventing wasted human effort.