witness vs testifier

witness and testifier both are nouns.

Word	Noun	Adjective	Verb	Adverb
witness	Yes	No	Yes	No
testifier	Yes	No	No	No

As nouns, testifier is a hyponym of witness; that is, testifier is a word with a more specific, narrower meaning than witness:

Other hyponyms of witness include attestant, attester, deponent, deposer.

witness (noun)	testifier (noun)
testimony by word or deed to your religious faith	a person who testifies or gives a deposition
someone who sees an event and reports what happened
a close observer; someone who looks at something (such as an exhibition of some kind)
(law) a person who attests to the genuineness of a document or signature by adding their own signature
(law) a person who testifies under oath in a court of law

witness (verb)	testifier (verb)
be a witness to
perceive or be contemporaneous with