The original manuscript is extant here and definitely reads “in the favor of the freemen”—that is, with the definite article the in the expression “in the favor of X”. Joseph Smith removed the the in his editing for the 1837 edition, in agreement with general usage in English. Elsewhere in the text, there are four other occurrences of the phrase “in favor of X”:
But there are no other instances of “in the favor of X”. Except for the first occurrence (in Alma 10:26), these examples of “in favor of X” are extant in the original manuscript. In particular, the nearby occurrence of “in favor of kings” (in Alma 51:8) forms a contrastive pair with the earlier “in the favor of the freemen” (in Alma 51:7), so one wonders whether the extra the before favor in verse 7 might not be an error. It is quite possible that the extra the is due to the predominance of the in the surrounding text in verse 7 (“the voice of the people came in the favor of the freemen”). In the Oxford English Dictionary (under definition 6 for the noun favour), the phrase is nearly always “in favor of X”, but there is one 1556 example (from Aurelio and Isabell ) cited with original accidentals as “Hoo well have you spoken in the favoure of the wemen” (that is, “how well have you spoken in the favor of the women”). Another Early Modern English example is found in the OED under definition 4 for the noun undertaker (also given here with original accidentals):
Examples of “in the favor of ” can be found in current English, as in the following example from (dating from no earlier than 2004): “and hence Benjamin Franklin was in the favor of choosing the turkey as USA’s national symbol”. Although there is only one occurrence of the phrase “in the favor of X” in the earliest Book of Mormon text (here in Alma 51:7), the critical text will restore the the in that instance since this usage is possible, although infrequent.
Summary: Restore in Alma 51:7 the definite article the before favor in the phrase “in the favor of the freemen” (the reading of the original manuscript); although the expression “in the favor of X” is unusual, there is occasional evidence for it in current English as well as in Early Modern English.