firewood vs pine knot

firewood and pine knot both are nouns.

Word	Noun	Adjective	Verb	Adverb
firewood	Yes	No	No	No
pine knot	Yes	No	No	No

As nouns, pine knot is a hyponym of firewood; that is, pine knot is a word with a more specific, narrower meaning than firewood:

Other hyponyms of firewood include cordwood, backlog, brand, firebrand.

firewood (noun)	pine knot (noun)
wood used for fuel	a joint of pine wood used for fuel