Earlier and Firstborn

Earlier adjective - (comparative and superlative of `early') more early than; most early.

Firstborn and earlier are semantically related In some cases you can replace term "Firstborn" with "Earlier", this adjectives are similar.

Firstborn adjective - First in order of birth.

Earlier and firstborn are semantically related in born earlier topic. You can use "Earlier" instead an adjective "Firstborn", if it concerns topics such as most senior.