fire watcher vs watchman

fire watcher and watchman both are nouns.

Word	Noun	Adjective	Verb	Adverb
fire watcher	Yes	No	No	No
watchman	Yes	No	No	No

As nouns, watchman is a hypernym of fire watcher; that is, watchman is a word with a broader meaning than fire watcher:

fire watcher: (during World War II in Britain) someone whose duty was to watch for fires caused by bombs dropped from the air
watchman: a guard who keeps watch

Other hypernyms of fire watcher include security guard, watcher.

fire watcher (noun)	watchman (noun)
(during World War II in Britain) someone whose duty was to watch for fires caused by bombs dropped from the air	a guard who keeps watch