The precursor and the predecessor. What's the difference? [closed]

They are synonyms, with a few caveats. Predecessor is usually a person, though not always. Also in many sciences, especially chemistry and biology, precursor has a specific meaning of "required to make."

For some examples, I think you're more likely to hear "He outshone his predecessor" than "He outshone his precursor." You're more likely to hear "Acetone is a common precursor of many compounds" than "Acetone is a common predecessor of many compounds."