Becomes firm can have several antonyms, including softens, loosens, weakens, and becomes pliable. When something becomes firm, it usually means that it is solid, steady, and secure. However, the opposite happens when something softens or loosens, meaning it is unstable, weak or malleable. For instance, a cake becomes firm after it is baked, and it cannot be easily altered. If the same cake is left unbaked, it will be soft and pliable. Thus, the antonyms for becomes firm vary in meaning but generally convey the opposite qualities of solidity, stability, and security.