Why do we commonly refer to the last makkah, Makkas Bechoros, in the feminine form? Even according to a medrash I once heard (but did not see) that the female firstborn also died, the main point of the miracle was regarding the male firstborn (i.e., Pharoh).
If you want to answer that since the preceding word, makkas, is used in the feminine form, then I would respond that we should be consistent for all of the makkos, and use the feminine form for all of them (e.g., makkas kinnos). Furthermore, the fast on Erev Pesach is called Taanis Bechorim, not bechoros, even though it is preceded by taanis, which presumably is feminine.
Indeed, Rashi in Berachos 9a (last line) calls it the "makkas ha'bechorim!"
Yossie Levitin, Spring Valley, NY
In Tanach we find many feminine examples of Bechoros. Hevel brought "mi'Bechoros Tzono," in Devarim we find "Bechoros Bekarcha," and in some places it says "Bechorei," such as Nechemyah 10:37, where both "Bechoros" and "Bechorei" appear.
However, as the Gemara in Chulin 137b says, "The Torah's language is distinct, and the Sages language is distinct." In Chazal, the form is always "Bechoros" (as in "Maseches Bechoros," etc.). This may be due to the fact that in Aramaic, words like this always end with "os" in the plural, such as "Mo'ados" and not "Mo'adim."
It should be noted that there are masculine words which end in "os" in the plural even in the Torah, such as "Luchos," which are masculine, and "Shulchanos," which are masculine, so this is not a clear-cut feminine word.
It is also possible that if it were written "Bechorim," it could easily be confused with "Bikurim" which has the same spelling in Hebrew, and therefore Chazal preferred "Bechoros" to separate the two concepts.
Yoel Domb