The original heteroclitic alternation of r/n is preserved.
Simms posits several stages of development of this word. Since no single paradigm can be asserted for Proto-Germanic at any given point in time, the developments are detailed below.
Paradigm 1: The original paradigm as inherited from PIE.
Paradigm 2: An alternative paradigm was later formed based on the stem *fu- and regular n-stem endings. The two formations coexisted side by side for a period.
Paradigm 3: At a certain point in time, the two paradigms above were blended, attaching n-stem endings to the stem *fun-. Paradigm 1 was lost after that, paradigms 2 and 3 continued to exist side by side.
After the above stages, paradigms 2 and 3 finally split. In paradigm 2 the -r- was generalised to all forms, creating *fōr, *fuïr-, which is found in North and West Germanic. In paradigm 3, the -n- was generalised to the nominative, creating *fōn, *funin- as found in Gothic and in the Old Norse form funi.
- Old English: fȳr
- Old Frisian: fiūr, fiōr
- Old Saxon: fiur
- Low German: Füür
- Old Dutch: fuïr
- Old High German: fuïr (early), fiur
- Old Norse: fúrr, fýr, fýrir, funi
- Gothic: 𐍆𐍉𐌽 (fon)
- Portuguese: fona
- Don Ringe, From Proto-Indo-European to Proto-Germanic, Oxford University Press, 2006
- Douglas P.A. Simms - The Words for "Fire" in Germanic, 2009