Let's see, what WTS publications say about that matter, what does the term "firstborn" means according to the Aid To Bible Understanding (1971):
„BIRTHRIGHT. The right that naturally belonged to the father's firstborn son. Under the patriarchal system the oldest son became the head of the family upon the death of the father, with authority over the others as long as they were in the household. He was responsible to care for the members of his father's household. He also succeeded to the father's position in representing the family before Jehovah. The firstborn generally received the father's special blessing. (Gen. 27 :4, 36 ; 48 :9, 17, 18) Moreover, he was entitled to two parts of the father's estate ; that is, he received twice as much as each of his brothers. Under the Mosaic law a man with more than one wife could not take the birthright from the oldest son and give it to the son of a specially loved wife .-Deut. 21 :15-17 .
In patriarchal times the birthright could be transferred by the father to another son for a cause, as in the case of Reuben, who lost his right as firstborn due to fornication with his father's concubine.(1 Chron. 5 :1, 2) The firstborn could sell his birthright to one of his brothers, as did Esau, who despised his birthright and sold it to his brother Jacob in exchange for something to eat. (Gen. 25 :30-34 ; 27 :36 ; Heb. 12 :16) There is no record that Jacob asserted his purchased birthright in getting a double share of Isaac's property (which was movable or personal property, for Isaac owned no land, except the field of Macpelah, in which was a cave for a burial place) . Jacob was interested in the passing on of spiritual things to his family, that is, the promise given to Abraham concerning the seed .-Gen. 28 :3, 4, 12-15 . With respect to the kings of Israel, the birthright seems to have carried with it the right of succession to the throne. (2 Chron . 21 :1-3) However, Jehovah, as Israel's real King and their God, set aside such right when it suited his purposes, as in the example of Solomon .-1 Chron. 28 :5.
Jesus Christ, as the "first-born of all creation," always faithful to his Father Jehovah God, has the birthright through which he has been appointed "heir of all things." -Col. 1 :15 ; Heb . 1 :2" (p. 237-238)
Gotcha! So according to the WTS "first-born of all creation" means, that Jesus Christ "has the birthright through which he has been appointed "heir of all things."" What can I say? I can just agree with the "faithful and descreet slave" :-) It's pretty much the very same interpretation I've suggested on this verse myself:
"Colossians 1:15 simply means that He owns, enjoys the position of the Heir, the ruler in relation to the whole of creation. The direct continuation clearly explains this when he adds: because in him all things were created..."
So, according to your boss I'm not wrong about that... :-)
Let's check what the same book writes about "Firstborn":
“From earliest times the firstborn son held an honored position in the family and was the one who succeeded to the headship of the household He inherited a double portion of the father's property. (Deut . 21 :17) Reuben was seated by Joseph at a meal according to his right as firstborn. (Gen . 43 :33) But the Bible does not always honor the firstborn by listing sons according to birth . The first place is often given to the most prominent or faithful of the sons rather than to the firstborn .-Gen. 6 :10 ; 1 Chron. 1 :28 ; compare Genesis 11 :26, 32 ; 12 :4. The firstborn came into considerable prominence at the time that Jehovah delivered his people from slavery in Egypt. [..]According to another WTS publication God’s Eternal Purpose Now Triumphing for Man’s Good, page 28 we read,
David, who was the youngest son of Jesse, was called by Jehovah the "first-born," due to Jehovah's elevation of David to the preeminent position in God's chosen nation and his making a covenant with David for a dynasty of kings. (Ps. 89 :27) In this position David prophetically represented the Messiah .-Compare Psalm 2 :2, 7 with 1 Samuel 10 :1 ; Hebrews 1:5. Jesus Christ is shown to be "the first-born of all creation" as well as "the first-born from the dead ." (Col. 1:15, 18 ; Rev. 1:5 ; 3:14) On earth he was the firstborn child of Mary and was presented at the temple in accordance with Jehovah's law. (Luke 2 :7, 22, 23) The apostle Paul speaks of the followers of Jesus Christ who have been enrolled in the heavens as "the congregation of the first-born ."-Heb. 12 :23 . At Job 18 :13 the expression "first-born of death" is used to denote the most deadly of diseases.” (p. 584)
"Our thinking about this here reminds us of what is said in the eighth chapter of the book of Proverbs, where divine wisdom is pictured as a person who talks about himself. Of course, in the original Hebrew text of Proverbs, the word “wisdom” (hhakh·mahʹ) is in the feminine and speaks of itself as a female person. (Proverbs 8:1-4) Of course, divine wisdom does not have any separate existence apart from God. Wisdom always existed in Him and so was not created. For this reason it is interesting to hear how wisdom speaks of herself as a feminine person..."I can just agree on that, well done, "faithful and discreet slave" ! :-)