Her firstborn. The meaning is, not that she had afterward any other child; but it is a way of speech among the Hebrews, to call them also the firstborn, who are the only children.
Till she brought forth her firstborn son (Matthew 1:25). From these words Helvidius and other heretics most impiously inferred that the blessed Virgin Mary had other children besides Christ: but Saint Jerome shews, by divers examples, that this expression of the Evangelist was a manner of speaking usual among the Hebrews, to denote by the word until, only what is done, without any regard to the future. Thus it is said, Genesis 8.6 and 7, that Noe sent forth a raven, which went forth, and did not return till the waters were dried up on the earth. That is, did not return any more. Also Isaias 46: 4, God says: I am till you grow old. Who dare infer that God should then cease to be. Also in the first book of Machabees 5.54, And they went up to mount Sion with joy and gladness, and offered h0Ɩ0cαųsts, because not one of them was slain till they had returned in peace. That is, not one was slain before or after they had returned. - God saith to his divine Son: Sit on My right hand till I make thy enemies thy footstool. Shall he sit no longer after his enemies are subdued? Yea and for all eternity. Saint Jerome also proves by Scripture examples, that an only begotten son, was also called firstborn, or first begotten: because according to the law, the firstborn males were to be consecrated to God: Sanctify unto Me, saith the Lord, every firstborn that openeth the womb among the children of Israel, & c. Ex. 13.2. (Douay Rheims commentary)
"First-born son": it is usual for Sacred Scripture to refer to the first male child as "the first-born" whether or not there were other brothers (cf., for example, Exodus 13:2; 13:13; Num 15:8; Hebrews 1:6). The same practice is to be found in ordinary speech; take, for example, this inscription dating from approximately the same time as Christ was born, which was found near Tell-el-Jedvieh (in Egypt) in 1922, which states that a woman named Arsinoe died when giving birth to "her first-born son". Otherwise, as Saint Jerome explains in his letter Adversus Helvidium, 10, "if only he were first-born who was followed by other brothers, he would not deserve the rights of the first-born, which the Law lays down, until the other had been born" - which would be absurd, since the Law ordains that those first-born should be "ransomed" within a month of their birth (cf. Numbers 18:16).
However, Jesus Christ is first-born in a much deeper sense independent of natural or biological considerations - which Saint Bede describes in these words, summarizing a long tradition of the Fathers of the Church: "Truly the Son of God, who was made manifest in the flesh, belongs to a more exalted order not only because he is the Only-begotten of the Father by virtue of the excellence of his divinity; he is also first-born of all creatures by virtue of his fraternity with men: concerning this (his primogeniture) it is said: 'For those whom He foreknew He also predestined to be conformed to the image of His Son, in order that He might be the first-born among many brethren' (Romans 8:29). And concerning the former (his being the Only-begotten) it is said 'we have beheld His glory, glory as of the only Son from the Father' (John 1:14). Thus, He is only-begotten by the substance of the Godhead, and first-born through His assumption of humanity; first-born by grace, only-begotten by nature. This is why He is called brother and Lord: brother, because He is the first-born; Lord, because He is the Only-begotten" (In Lucae Evangelium expositio, in loc.).
Christian Tradition teaches, as a truth of faith, that Mary remained a virgin after Christ's birth, which is perfectly in keeping with Christ's status as her first-born. See, for example, these words of the Lateran Council of 649:
"If anyone does not profess according to the holy Fathers that in the proper and true sense the holy, ever-Virgin, Immaculate Mary is the Mother of God, since in this last age not with human seed but of the Holy Spirit she properly and truly conceived the divine Word, who was born of God the Father before all ages and gave him birth without any detriment to her virginity, which remained inviolate even after his birth: let such a one be condemned" (canon 3). (Navarre)
Following are more verses regarding our Lady's Perpetual Virginity (also considering how "first-born" does not necessarily mean more are born after as the "firstborn" can be the only born - Mary did not have other children as the Protestants wish the Scriptures to assert);
Luke 1:34 How can this be, since I do not know man.
Luke 2:41-51 - age 12, Jesus evidently only son of Mary.
Mark 6:3 - "the son of Mary" not "a son of Mary".
Matthew 27:56 - Mary the mother of James & Joseph (called brothers of Jesus) is also
John 19:25 - Mary the wife of Clopas.
John 19:26 - entrusted Mary to John, not a younger sibling.
John 7:3-4 - brothers advise like elders: "go to Judea, manifest self" unthinkable for younger siblings (see next verse).
Mark 3:21 - set out to seize him, "he is out of his mind". More "untils" (for those who try to use Matthew 1:25 to "prove" that Mary is not ever-Virgin as the Church has always taught) that do not imply the reverse happened afterwards.
Matthew 28:20 - I am with you always, even till the consummation of the world. (and after)
1 Timothy 4:13 - until I arrive, attend to reading, teaching ... (but don't end all reading, teaching and so on after I arrive)
1 Corinthians 15:25 - He must reign until His enemies are underfoot. (and after I hope)
Luke 1:80 - John in desert until day of his manifestation (and after). Firstborn can be the only born.
Exodus 13:2; Numbers 3:12 - consecrate first-born that opens womb.
Exodus 34:20 - first-born among your sons you shall redeem.