A fire mountain is probably what you think it is: A volcano.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The 山 reading is ざん instead of さん because of rendaku (sequential voicing).
Mt. Fuji is a volcano.
There is some fire coming out of the top of the volcano.
There are many volcanoes in Hawaii.