In the following set of words, which is different than the others?

Question

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Someone or something that is firm is strong and unlikely to change. This is different from the rest of the words, which mean "wavering; likely to change."
 Fickle is an adjective used to describe something that changes frequently, especially when it comes to loyalty and affections. It is similar to the other words in the set since they are used to describe things that change often or are not constant.
Inconsistent is an adjective used to describe something that does not stay the same throughout time.
Unsure is an adjective that is used to describe something that is frequently changing, or when something is able to be changed.
Variable is an adjective that is used to describe something that is not consistent or having a fixed pattern.

In the Following Set of Words, Which Is Different Than