The basic Sindarin word for “fire”, derived from the root √NAR of the same meaning (LotR/942; PE17/38) and very well attested. It is derived from primitive ✱nār- since primitive long ā became au in Sindarin. It appeared as N. naur “flame” in The Etymologies of the 1930s with the same derivation (Ety/NAR). As a suffix it usually reduces to -nor, since au usually becomes o in polysyllables. As a prefix, though, it is often Nar- before consonant clusters, no doubt because the ancient long ā was shortened before it could become au.
Conceptual Development: In the Gnomish Lexicon of the 1910s, the word for “fire” was G. sâ with archaic form †sai (GL/66) clearly based on the early root ᴱ√SAH(Y)A “be hot” as suggested by Christopher Tolkien (LT1A/Sári; QL/81). In Early Noldorin Word-lists of the 1920s, the word for “fire” was ᴱN. byr or buir from primitive ᴱ✶ [mburyē] (PE13/139). Tolkien introduced naur in The Etymologies of the 1930s and stuck with it thereafter.
A root for “fire” first appearing as ᴹ√NAR “flame, fire” in The Etymologies of the 1930s along with derivatives like ᴹQ. nár(e)/N. naur “flame” (Ety/NAR¹). There was also an augmented variant ᴹ√ANÁR that served as the basis for “Sun” words: ᴹQ. Anar and N. Anor (Ety/ANÁR). These roots and the various derivatives continued to appear in Tolkien’s later writings in the 1950s and 60s (PE17/38; Let/425), and in one place Tolkien specified that nār- was “fire as an element” as opposed to √RUYU for an actual blaze.