This is a question of whether MSS that do not have the word must be considered superior to MSS that DO have the word in question. Some MSS do, and some do not.
Please bear in mind that printed editions of the Greek New Testament (NT) from Erasmus' Novum Instrumentum omne (1516) to the Elzevier edition (1516) comprise the Textus Receptus (TR). No scribes who copied from Greek MSS available to them in the first few centuries A.D. were involved. People like Erasmus and the Elzevier brothers worked from existing material and MSS to produce a Greek text of the NT to get round the problem of there only being an old Latin text of the NT. They knew the desperate need to produce translations from the original language the NT had first been written in, and people were put to death for daring to produce a Greek text of the NT. Their work came to be know as "The Received Text" - received from the apostles through a chain of scribes. But once those MSS had been translated into old Latin and church law was then passed forbidding anyone to go back to the Greek, on pain of death, a dreadful situation arose.
Now it also needs to be borne in mind that the best, most reliable Greek MSS of the first few centuries were worn to a frazzle because of constant use, and copies of them had to be made before they became too worn. However, if a copy of the Greek text had been made that contained several errors, it would be shelved, or even binned. This is where the claim of those using particular MSS that did not have 'firstborn' in Mat. 1:25 needs to have the questions raised, "Why did such older MSS not have that word? Had that been a scribal mistake - an omission on the part of one scribe, then magnified by other scribes working from that flawed copy? Is that why such older MSS were found to be in really good condition, on a monastery shelf (or was it even in a bucket?)"
This is a simplified way of pointing out that older is not necessarily more accurate (when it comes to Bible MSS). But because the Novum Testamentum Graece (NU Text) included some new MSS not in the TR, differences arose, as in this example.
So, did the NU-Text make a scribal error? No. The collection of texts was compiled by a committee based on critical analysis of many Greek MSS. That committee did no scribal copying of Greek MSS. They were a group of scholars in the late 1800s who decided to add previously unfound MSS to the selection of MSS they thought modern translations should work from. It is the selection of particular MSS in the late 1800s that form the reason for the NU not having 'firstborn' before 'Son' in Mat. 1:25.
Here is an explanation provided in the notes of The Companion Bible re. vs.25, which it renders as "And knew her not till she had brought forth her firstborn Son...":
"These words [her firstborn son] are quoted by Tatian (A.D. 172) and
twelve of the Fathers before cent. 4; and are contained in nearly all
MSS except the Vatican and Sinaitic (cent. 4). All the Texts omit 'her
firstborn' on this weak and suspicious evidence. But there is no
question about it in Luke 2:7."
This implies that the Vatican and Sinaitic 4th century MSS that were chosen to go into the NU collection of texts may have had a scribal omission in that verse. That omission was compounded by the committee deciding to include those MSS, which had never been included in the TR text. That's one way of looking at it. But however one looks at it, any scribal error is far more likely to be missing an existing word out, than adding one that never was in the MSS being copied.
