I cleaned the burner of the gas stove.
The fire department took time to identify the origin of the fire.
The burner opening of the gas stove is dirty, so it's hard to light.
The fire department investigated the scene to determine the point where the fire started.
ノズル is the general loanword for any nozzle, while 火口 refers specifically to the nozzle or opening of a burner.
Written identically as 火口, かこう means a volcanic crater, while ひぐち (this entry) means a burner nozzle or the point where a fire started, so context and reading must distinguish them.
mouth · opening; hole; mouth (of a container) · entrance; exit; gate · speech; talk; gossip · taste; palate · mouth to feed · job opening; vacancy · invitation; summons · kind; sort; type · beginning; start · counter for mouthfuls, shares, stove burners · counter for swords, blades
Compound of 火 (hi, 'fire') and 口 (kuchi, 'mouth, opening'), literally 'fire mouth'. The reading ひぐち is a standard rendaku form.