The firebrick seems very flakey. Is this normal?
It’s not out of the question. Firebrick differs from common brick in that it has an alumina content that makes it dense and allows it to capture and refract heat. As a consequence it can be brittle and small chips out of the edges are not uncommon.
I had one brick lose a large corner, and had to pull it out and replace it. I was pounding it with the rubber dead blow mallet and one tap it the corner at just the right spot.
The ones in your photo don’t look like they’re in that class, and I wouldn’t worry about them. They will not catch the edge of a peel, and they will not affect your cooking in any way.
This does point to the need to take care with your mallet while you’re putting the hearth together. I’m looking at the joints and overall it looks like you solved the leveling issue quite nicely.