First-born and Firstborn

Firstborn and first-born are semantically related In some cases you can use "Firstborn" instead the phrase "First-born" as a noun or an adjective.

Firstborn adjective - First in order of birth.

First-born and firstborn are semantically related in older person topic. You can use "First-born" instead the word "Firstborn" as a noun or an adjective, if it concerns topics such as senior, born earlier, most senior.