The simplest words have the thorniest etymologies—water, dirt, head, air, food, etc. Those are the fundamental ones, the ancient ones, the ones that first needed articulation. It's no coincidence that our most necessary words are mono- or duo-syllabic.

fire (v.)
c.1200, furen, figurative, "arouse, excite;" literal sense of "set fire to" is from late 14c., from fire (n.). The O.E. verb fyrian "to supply with fire" apparently did not survive into M.E. The sense of "sack, dismiss" is first recorded 1885 in Amer.Eng. (earlier "throw (someone) out" of some place, 1871), probably from a play on the two meanings of discharge: "to dismiss from a position," and "to fire a gun," fire in the second sense being from "set fire to gunpowder," attested from 1520s. Of bricks, pottery, etc., from 1660s.

fire (n.)
O.E. fyr, from P.Gmc. *fuir (cf. O.S., O.Fris. fiur, O.N. fürr, M.Du., Du. vuur, O.H.G. fiur, Ger. Feuer), from PIE *perjos, from root *paewr- (cf. Armenian hur "fire, torch," Czech pyr "hot ashes," Gk. pyr, Umbrian pir, Skt. pu, Hittite pahhur "fire"). Current spelling is attested as early as 1200, but did not fully displace M.E. fier (preserved in fiery) until c.1600.

PIE apparently had two roots for fire: *paewr- and *egni- (cf. L. ignis). The former was "inanimate," referring to fire as a substance, and the latter was "animate," referring to it as a living force (see water).

Fire applied in English to passions, feelings, from mid-14c. Meaning "action of guns, etc." is from 1580s. Firecracker is Amer.Eng. coinage for what is in England just cracker, but the U.S. word distinguishes it from the word meaning "biscuit." Fire-engine attested from 1680s. The figurative expression play with fire "risk disaster" is from 1887; phrase where's the fire? "what's the hurry?" first recorded 1924.

Look at all the homonyms in that etymology: power, water, pyre, ignite... Fire is fundamental.