self-starter vs ball of fire

self-starter and ball of fire both are nouns.

Word	Noun	Adjective	Verb	Adverb
self-starter	Yes	No	No	No
ball of fire	Yes	No	No	No

As nouns, ball of fire is a hypernym of self-starter; that is, ball of fire is a word with a broader meaning than self-starter:

Other hypernyms of self-starter include fireball, human dynamo, powerhouse.

self-starter (noun)	ball of fire (noun)
an electric starting motor that automatically starts an internal-combustion engine	someone whose career progresses rapidly
an energetic person with unusual initiative	a highly energetic and indefatigable person