firstborn vs eldest

firstborn and eldest both are nouns.

Word	Noun	Adjective	Verb	Adverb
firstborn	Yes	Yes	No	No
eldest	Yes	Yes	No	No

As adjectives, firstborn and eldest are synonyms defined as:

As nouns, firstborn and eldest are synonyms defined as:

firstborn and eldest: the offspring who came first in the order of birth

firstborn (noun)	eldest (noun)
the offspring who came first in the order of birth	the offspring who came first in the order of birth

firstborn (adjective)	eldest (adjective)
first in order of birth	first in order of birth