Firm pillows work well for people who need extra support to keep their bodies aligned. People who sleep on their side are usually happy with firm pillows as there’s no risk of their heads curving and leading to neck pain. Plus-sized people also generally prefer firm pillows as they place more pressure on them.
Stomach sleepers and petite people usually need softer pillows to prevent pressure build-up or pain. Some back sleepers prefer firm pillows, but others may do better on soft or medium ones.