I think fire here is referring to zeal in spiritual life.
Never be lacking in zeal, but keep your spiritual fervor, serving the Lord – Romans 12:11
Gotta keep the focus (to get occupied with meaningful and purposeful activities) and zeal/fire no matter how tired. But then, gotta also have knowledge too, as it is written.
It is not good to have zeal without knowledge, nor to be hasty and miss the way – Proverbs 19: 2
For I can testify about them that they are zealous for God, but their zeal is not based on knowledge – Romans 10: 2
I like how Neil Phelan Jr. described it:
“Zeal without knowledge is like a horse without a bridle or a jet without a pilot. It is a great force without direction. Zeal is a force and knowledge is for the direction of that force.”