Directly related to fire:
* Ignite: To set on fire.
* Ignition: The act of setting something on fire.
* Igniferous: Containing fire.
* Igneous: Formed by fire or volcanic heat, as in igneous rock.
Figuratively related to fire:
* Ignis fatuus: A misleading light, often seen over marshland; a will-o'-the-wisp.
* Ignominious: Characterized by disgrace or shame; shameful.
* Incendiary: Designed to start fires; tending to incite anger or violence.
* Incense: Aromatic gum or resin burned as an offering or to produce a fragrant smoke.
* Incandescent: Emitting light as a result of being heated to incandescence.
Less direct but still related:
* Ignorance: Lack of knowledge or information; a state of being uninformed. (This comes from the idea of "not knowing" or "not being enlightened.")
* Ignoramus: A person who is ignorant or uninformed.
Other words:
* Pyre: A funeral pyre, a pile of wood for burning a corpse.
* Pyromania: An obsessive desire to set fires.
* Pyrotechnics: The art of making and using fireworks.
Remember, these are just some examples. There are many more words with the root "ignis" that are used in different contexts.