incendio

The false seam first, because it is a good one. Incendio ends in what looks like -end-, the passive participle of obligation: pagenda is payable, farenda is to-be-done, legenda is worth reading. Read incendio that way and you would need a verb inci, which does not exist, and you would get "a thing that must be inced", which is not a fire. Incendi- is one root, all eight letters of it, borrowed from Latin incendium and not built from anything.

Where the pieces really are. Latin incendere is in- plus candere, to glow white. The same candere gives kandelo the candle, kandida candid (originally shining white), and English incandescent. So there is a seam, it is a Latin one, and it lies in a completely different place from the one the Esperanto reading suggested.

The fence with fajro. A fajro is fire in general, and a good thing as often as a bad one: the fire in the hearth, the fire you cook on. An incendio is always a disaster - a building alight, a forest alight, something destroyed. You light a fajro and you fight an incendio, and the words are not swappable. Brulo and brulego, from bruli, cover the same ground more plainly, and bruligi is to set alight.

In use. Incendia is the adjective. Incendii is to set a serious fire deliberately, and incendiisto is the arsonist. The fire brigade is the fajrobrigado and a fireman is a fajrobrigadano, both built on fajro rather than on this word, which tells you something about which of the two Esperanto treats as basic.