The word "not firm" indicates that something is unsteady, unstable or insecure. There are many synonyms that can be used to express this idea. Some common alternatives to "not firm" include wobbly, shaky, weak, feeble, flimsy, rickety, unstable, uncertain, unsettled, precarious, and volatile. Each of these synonyms paints a slightly different picture of the state of the thing in question, but all suggest that it lacks stability and could easily collapse or fail. When choosing the most appropriate synonym, it's important to consider the context of the sentence and the intended tone or message.