The word ᴹQ. úr “fire” appeared in The Etymologies of the 1930s under the root ᴹ√UR “be hot”, but this word was deleted when Tolkien revised the meaning of the root to “wide, large, great” (Ety/UR). However, the root √UR “heat” reappeared in later writings (PE22/160), and úr “fire” appeared in The Feanorian Alphabet of the 1930s as the name of tengwa #36 [.] (PE22/23). It reappeared again in the version of that document from the 1940s, but with the gloss “fire, heat” (PE22/51). In the 1st edition of The Lord of the Rings, the name of tengwa #36 was úr “heat” (RC/736), revised in the 2nd edition to Q. úrë “heat” (LotR/1123).
Neo-Quenya: Based on the development of the names for tengwa #36, it seems 1930s úr “fire” >> 1940s úr “fire, heat” >> 1950s úr “heat” >> 1960s úre “heat”. However, the introduction of Q. úrë “heat” might mean that úr could once again be used for “fire”, and this has long been a popular word in Neo-Quenya. I would retain úr for that purpose, as it also allows us to salvage several fire-related related adjectives. It is possible, though, that the root √UR was restricted to “heat” and can no longer be used for “fire”.