eldest vs firstborn

eldest and firstborn both are nouns.

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

As adjectives, eldest and firstborn are synonyms defined as:

As nouns, eldest and firstborn are synonyms defined as:

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

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

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