The word "un-firmer" means "less firm or steady." Its antonyms would be words that mean "firmer or more steady." These antonyms could include words like "stable," "strong," "secure," "solid," "reliable," "consistent," "resolute," and "steadfast." These words all connote qualities of firmness and strength, both physical and mental. Whether we're talking about the foundation of a building or the resolve of a person, these antonyms are a reminder that strength and stability are important qualities to strive for in all aspects of life.