The word "un-firmest" does not exist in the English language. However, the antonyms of the word "firm" can be used to describe the opposite of "firmest." The antonyms of "firm" are "soft," "weak," "flabby," "loose," "unsteady," "insecure," and "wavering." These words describe something that lacks strength, stability, or solidity. For example, a soft mattress is the opposite of a firm mattress, and a weak argument is the opposite of a firm argument. It is important to choose the right antonym to convey the correct meaning in a sentence.