Taxis are available but are costly. They are the only comfortable and safe mode of transportation. Donot board local city bus as you may be thrown out of the bus due to over crowding or be prepared to lose your wallet, purses and other appareils.
Planes from France(Air France), Nigeria(Virgin Nigeria) have flights once and twice a week respectively.
Monrovia's airport was unlike any I've been to. Everything is hot and crowded and disorganized and tedious. Give yourself lots of time and be very patient if you find yourself here.