A term used for Men meaning “Mortal”, appearing in the Quendi and Eldar essay of 1959-60, cognate to Q. Firya of the same meaning, both derived from √PHIR which was the basis of words for natural death (WJ/387). According to Tolkien this word was borrowed from Quenya, since the Noldor had pre-knowledge of the nature of Men having learned of them from the Valar. The plural form of Feir was Fîr and its class plural Firiath, the latter also appearing in contemporaneous Silmarillion drafts (WJ/219 footnote). It is unclear why this word did not become ✱Fair, since ei became ai in Sindarin monosyllables. Perhaps it remained Feir because it was an adaptation from Quenya, or it could be a conceptual remnant of its Noldorin form (see below).
Conceptual Development: Probably the first precursor to this word was ᴱN. fion “man, human being” from Index of Names for The Lay of the Children of Húrin compiled in the early 1920s (PE15/62), also appearing with the gloss “mortal man” in Early Noldorin Word-lists of the same period (PE13/143). In The Etymologies of the 1930s Tolkien gaven N. {fîr “man, mortal” >>} feir pl. fîr “mortals” under the root ᴹ√PHIR (Ety/PHIR; EtyAC/PHIR), hence with basically the same form, meaning and etymology as it had in later Sindarin.
An adjective meaning “mortal”, more literally “those apt to die”, a Sindarin adaptation of Q. fírima of the same meaning, both based on the root √PHIR having to do with natural death (WJ/387). It was also used as Fíreb to refer to Mortal Men, a variant of Feir of similar meaning. Tolkien said “Fíreb as compared with Fírima shows the use of a different suffix, since the S equivalent of Q -ima (✱-ef) was not current” (WJ/387).