"He is the image of the invisible God, the firstborn of all creation; because by means of him all [other] things were created in the heavens and upon the earth, the things visible and the things invisible, no matter whether they are thrones or lordships or governments or authorities. All [other] things have been created through him and for him. Also, he is before all [other] things and by means of him all [other] things were made to exists," (Col. 1:15-17, for context. The New World Translation--Emphasis added. Note the NWT's addition of 'other' into the text four times. This is discussed here).
The Jehovah's Witnesses interpret the word "firstborn" here to mean "first created" because it is consistent with their theological presupposition that Jesus is a created thing. Of course, Jesus, the Word become flesh (John 1:1, 14) is not a created thing but that hasn't stopped the Watchtower organization from claiming He is. Nevertheless, there is a Greek word for "first created," and it was in use at the time of Paul's writing to the Colossians. He did not use it here. The Greek for "firstborn" is proto with tikto which would give us "firstborn" and that is what we find here in Colossians 1:15. The Greek for "first created" would be proto with ktizo, and it is not used here.
Second, the Biblical use of the word "firstborn" is most interesting. It can mean the first born child in a family (Luke 2:7), but it can also mean "pre-eminence." In Psalm 89:20, 27 it says, "I have found David My servant; with My holy oil I have anointed him . . . I also shall make him My first-born," (NASB). As you can see, David, who was the last one born in his family, was called the firstborn by God. This is a title of preeminence.
Third, firstborn is also a title that is transferable: Gen. 41:51-52, "And Joseph called the name of the first-born Manasseh: For, said he, God hath made me forget all my toil, and all my father's house. And the name of the second called he Ephraim: For God hath made me fruitful in the land of my affliction."
Jer. 31:9, " . . . for I am a father to Israel, and Ephraim is My firstborn."
Scripture best interprets Scripture. Firstborn does not require a meaning of first created as the Jehovah's Witnesses say it means here. "Firstborn" can mean the first born person in a family, and it can also be a title of preeminence which is transferable. That is obvious since Jesus is God in flesh (John 1:1, 14) and is also the first born son of Mary. In addition, He is the pre-eminent one in all things. The Jehovah's Witnesses should consider this when they examine Col. 1:15. They should also abandon the Watchtower which guides them in their thinking and believing.