A verb for “to fade” implied by the noun firith “fading”, a period of the year in late autumn (LotR/1107). It also appears to mean “die” given related words fíreb “mortal” (WJ/387) and firin or firen “mortal, dying” (PE17/101). Its ancient root √PHIR had nothing to do with death, so these meanings were probably borrowed from Quenya; see Q. fir- “to die” and Q. fírië “death” for discussion. As such, the Sindarin verb fir- probably applied only to the natural death of mortals, also used metaphorically when applied to inanimate things to mean “fade”. The pre-Quenya verb for “to die” seems to be [N.] gwanna- “to die, (lit.) depart”; see that entry for discussion.