Given my background, and area of study, I feel as if it's my duty to respond to this.
In regards to Colossians 1.15 ("He is the image of the invisible God, firstborn of all creation"), we could discuss partitive genitives (“i.e., one of the students of the class”), or genitives of subordination (“i.e., King over Israel”) and its correlative analogous genitive (“i.e., King of Israel”); however, I would instead like to direct attention to the preposition found in v. 16, ὅτι (“He is the image of the invisible God, the Firstborn of all creation, because…”), because our understanding of the subsequent things that Paul has to say is key to understanding what he meant by referring to Christ as “Firstborn of all creation.” The reason for Christ being called the “Firstborn of all creation” is “because all things were made in Him… through Him… and for Him.” This statement of course makes little or no sense if the phrase “Firstborn of all creation” is meant to mean that Christ is the first thing that God ever created. How is it that Christ is the first-created of all creation, because all things were created in, through, and for Him?
Christ’s creation of “all things” serves as the sole basis of Him being “Firstborn of all creation.” If I were to say, “He is president of the United States, because he received the most votes,” the ground clause “because he received the most votes” modifies the predicate nominative “president” (giving the grounds for it) but does not contribute anything to the subordinate prepositional phrase “of the United States.” In fact, you could remove the subordinate prepositional phrase without issue, which again seems to indicate that there isn’t a link between it and the ground clause.
Whatever we take “Firstborn of all creation” to mean (and there are divergent views within Orthodoxy), it must be on the basis of Christ existing before, and creating all things as indicated in the ground clause, “because in Him all things in heaven and on earth were created.” This is elsewhere expressed throughout the NT, where the authority of the Creator over creation derives from the very act of creation itself (Rev. 4.11, Hebrews 2.8).
With this in mind, consider the lexical field of πρωτότοκος ("Firstborn"). It is commonly recognized that “Firstborn,” as it is used throughout the OT is often used in reference to the preeminence of an individual. In the highly Messianic Psalm, iDavid (who here is a typification of the coming Messiah, Jesus Christ), the youngest amongst the sons of Jesse (1 Sam 16.11-13), is described and appointed as God’s “firstborn, the highest of the kings of the earth” (Psalm 89.27). It is Christ, as the Son of God, who is the “Firstborn” in the sense that He is the “heir” of all things, for everything that belongs to the Father also belongs to the Son (John 16.15, 17.10). As the “firstborn” is the heir to all of his father’s estate, so too is Christ the heir of all the Heavenly Father’s estate, namely, all of creation (Col 1.15, Heb 1.2, Psalm 2.7-8).
This understanding of πρωτότοκος ("Firstborn") sheds light on the preposition in v. 16. Christ is the "Firstborn" of all creation, because all things were created in, through, and for Him.