There are several words that can be considered antonyms for the phrase "firms up." Some of the potential antonyms include softens, loosens, weakens, slackens, and relaxes. These words are all in direct contrast to the idea of something becoming more solid, stable, or established. For example, when bread dough is kneaded, it firms up, but when left out too long, it softens and loses its structure. Similarly, when a person is stressed, their muscles tense up, but when they relax, their muscles loosen. Understanding the antonyms of "firms up" can help individuals better articulate the specific changes they wish to make.