That's because of an old version of the spec:
Absolutely positioned children of a flex container are not themselves
flex items, but they leave behind "placeholders" in their normal
position in the box tree. These placeholders are anonymous inline
boxes with a width, height, and line-height of ‘0’, and they interact
normally with the flexbox layout algorithm.
This was later modified:
The static position is intended to more-or-less match the position of
an anonymous 0×0 in-flow ‘flex-start’-aligned flex item that
participates in flex layout, the primary difference being that any
packing spaces due to ‘justify-content: space-around’ or
‘justify-content: space-between’ are suppressed around the
hypothetical item
But Firefox didn't implement that change.
