כִּ֣י לִי֮ כׇּל־בְּכוֹר֒ בְּיוֹם֩ הַכֹּתִ֨י כׇל־בְּכ֜וֹר בְּאֶ֣רֶץ מִצְרַ֗יִם הִקְדַּ֨שְׁתִּי לִ֤י כׇל־בְּכוֹר֙ בְּיִשְׂרָאֵ֔ל מֵאָדָ֖ם עַד־בְּהֵמָ֑ה לִ֥י יִהְי֖וּ אֲנִ֥י יְהֹוָֽה׃
For every male first-born is Mine: at the time that I smote every [male] first-born in the land of Egypt, I consecrated every male first-born in Israel, human and animal, to Myself, to be Mine, GOD’s.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation.)
As explained in my comment to the previous verse, in this passage the noun בְּכוֹר applies only to males; females are not in view.
Ramban’s comment at Exod 12:30 presumes that the scope of the tenth plague can be inferred from the subsequent ritual regulations; this is also the view of the Mishnah B’rurah at Orach Chayim § 470.1. I.e., the related metaphors and legal sequelae—i.e., that to which the plague narrative has given meaning—clearly involve only males. Given a presumed correspondence between narrative and ritual, I infer that the tenth plague killed only males. That is, the ancient audience would have understood the narrative in terms of the male-only rites with which they were already familiar, and to which the Torah account gives meaning.
In other words, the Torah is using a gender-inclusive term to refer to a male-only group, while the gendered nature of that group is understood from the context. This is a common way of using language, because it is communicately efficient. (Similarly in English, when discussing the National Basketball Association—a male-only league—we customarily refer to the team members simply as “players,” not as “male players.” Their gender goes without saying.)
As for rendering into English, the NJPS “first-born” is unduly vague. Contemporary readers come to the text with gender assumptions that differ from those of the text’s ancient audience. Because nowadays the English term “first-born” is usually used gender-inclusively, the unqualified NJPS rendering misleadingly implies that all genders are in view. Rather, when only males are intended, English idiom calls for more specific wording.
The Torah seems more vague as to whom the tenth plague killed than it is as to whom the later lore applies. Therefore the references to victims of the plague are rendered in terms of “[male] first-born” (with brackets that highlight the textual ambiguity), whereas the consequent references to Israelite life are to “male first-born” (without brackets).