The adjective firm describes something that's strong and unwavering. If your great Aunt Martha had a firm belief that children should be seen and not heard, you and your siblings might have spent your childhood driving her crazy.
Something that is solid can also be described as firm. When you take a stick of butter out of the fridge, it's firm, and it needs to soften before you cream it with sugar. Another definition for the adjective firm is unwavering or loyal. You might be a firm supporter of your favorite baseball team, even though they haven't made it to a World Series in more than twenty years.