I posted a "Help fish gone" once too.
Of course if you have no Net Top, then first place to look is on the Floor. Firefish will certainly jump if spooked (by other fish, or you).
When I lost a fish in past (jumping out), you'll be surprised how much distance they can flop, or flop themselves under something.
However one time a fish didn't jump out since (net was tight), but disappeared. It was similar size to a FireFish.
I tore apart my sump. Nothing.
Then after a few days, I decided to look into my overflow box (which I looked into before). Nothing.
But I noticed a bit of color, so stopped the flow. He was in there in corner fighting the current to not go down drain.
Also I too had a firefish. He was around in tank, but one day just dissapeared.
I forgot about him after doing a good search.
1 year later I was upgrading the tank. Removed all fish, corals, then rocks.
Tank was cloudy and I started draining.
When there was barely any water left, there was the Firefish.
He must have just not wanted to come out. Must have been eating catching food drifting by in whatever cave he was (for a whole year, without being seen once).
All the best, I know how you feel. Hope you find him Alive.