What's the problem in firefox with block height?

Question

css

Why is there some kind of gap between the blocks? (Chrome and other browsers do not have it)
I initially thought that this was due to the sub-pixel height, but the browser itself must calculate its norms if I set flex: 1 or height 100% or height calc ( 100% / 7) .
How to fix?
https://codepen.io/poylar/pen/popyVOV?editors=0100