English fire is the ordinary name for combustion: flame, heat, smoke, and the event of something burning. It is also the hearth or campfire you tend, the blaze that destroys a building, and, by extension, the discharge of guns, the heat of feeling, and a verb for lighting, shooting, or dismissing someone from a job. Those later uses sit on a very old noun.
Native, not borrowed
The origin of the word fire is not a loan from French or Latin. It is a native Germanic inheritance. Old English already had fȳr, often written fyr, for the physical substance and for a fire as an instance. First English use belongs to the Old English period, before 1150. The rest of West Germanic tells the same story: Old Saxon fiur, Old Frisian fiūr, Old High German fiur (and an older fuir), Dutch vuur, German Feuer. North Germanic is mixed. Old Norse had rare poetic forms of this family, but the everyday Norse word for fire was a different one, related to modern Scandinavian eld. Swedish and Danish fyr in the sense of a lighthouse is a later borrowing from Low German, not the direct continuation of the old Norse inheritance.
The current spelling fire is recorded from about 1200. For centuries it competed with Middle English fier. The older spelling did not vanish overnight: fiery still carries that ie, a fossil of the form that lost the spelling contest around 1600.
An r-stem that English kept
Behind Old English sits Proto-West Germanic fuir, itself a regularized shape of Proto-Germanic fōr. The deeper reconstruction is Proto-Indo-European péh₂wr̥; older handbooks write the same root as paewr. Indo-European had two distinct names for fire. One, this péh₂wr̥, was neuter and treated fire as an inanimate substance. The other, h₁n̥gʷnis (older egni), was masculine and treated fire as an active, almost living force. Latin ignis and English ignite and igneous belong to the second family. English fire belongs to the first. That split is the same kind of contrast linguists note for water: one word for the stuff, another for it as a power.
The PIE noun was one of a small class of neuter r/n stems. The nominative and accusative ended in r; other cases used n. Hittite still shows both sides of that pattern in pahhur and related forms. Most daughter languages leveled one consonant or the other. West Germanic generalized the r, which is why English, German, and Dutch have r in the modern word. Gothic went the other way and generalized n, yielding fon and related fun-forms. Old Norse funi, a word for flame, is another n-form from the same stock. The phonological path from the PIE noun through Germanic into Old English fȳr is acknowledged as complex. Specialists disagree on some of the intermediate vowels, and it is honest to leave that as unsettled rather than force a single schoolroom story.
Whether the PIE noun contains a substantivizing suffix on a shorter root is also discussed and remains uncertain; clearly related verbs that would settle the analysis are lacking. What is not in doubt is the family of r-forms outside Germanic.
Cognates that still look like fire
Once the r-form is in view, the cousins are easy to name. Ancient Greek pyr is the source of English pyre and of the pyro- in pyromania. Armenian hur, Umbrian pir, and Tocharian por and puwar belong here as well. Latin did not keep this noun as its common word for fire, but related material survives in words such as purgo, to cleanse, which English later borrowed as purge. None of this is folk etymology: the sound correspondences are regular. The everyday Latin word ignis is not a cognate of English fire at all; it is the other Indo-European name for the same phenomenon.
From hearth to gunpowder to the office
The noun’s core sense stayed close to combustion. What multiplied were figurative and technical extensions. By the mid-fourteenth century English was using fire for burning passions. On fire is attested around 1500; in fire a little earlier. Play with fire in the sense of courting disaster is nineteenth-century, from the ordinary warning to children. The pairing of fire and sword as a formula for devastation is older still, reaching English by the later Middle Ages and later echoing a Latin phrasing with flame and iron.
Gunpowder added another cluster. The verb meaning to discharge a firearm is from the 1520s, originally literal because a match or other fire set the charge off. The noun for a volley of shots follows in the later sixteenth century. To fire bricks or pottery, exposing them to kiln heat, is recorded from the 1660s. Fire away as a cue to go ahead is eighteenth-century.
The employment sense is much newer. American English of the 1870s uses fire out and then fire alone for dismissing a worker. The usual explanation is a pun on discharge, which already meant both send a shot and release from a post, helped by an earlier slang use of fire for throwing someone out of a place. That office meaning is not in Old English. The Old English verb fyrian meant roughly to supply with fire; that verb did not survive into Middle English. The later verb fire is a conversion from the noun, first in figurative inflame or arouse around 1200 and only later in the literal set alight.
If one fact should stick, it is the split inside the family. Gothic kept the n of the old r/n paradigm; English kept the r. Greek pyr and German Feuer are the same noun as fire, while ignite is a cousin from the other Indo-European name for flame. The spelling fiery is a reminder that Middle English had not yet chosen the form we now write.