Well, what can I say about this word? I bet a big deal there.

A lot of Chinese guys like to sleep on a firm mattress. If we are asked to talk about this matter with English-speaking people, most people will use the word "hard". Unfortunately, it is improper to use the word "hard" in this context. We should say "I like firm mattres", or "I like to have a firm one." Absolutely, there is no such things as a hard mattress. If we want to foister a good habit of saying "the firm mattres", we may need some exercises.

Personally, I prefer firm mattress to those soft ones.