Per https://drafts.csswg.org/css-align-3/#valdef-align-content-space-between , the fallback alignment for justify-content: space-between is safe flex-start.

Here, safe is defined as forcing start alignment when the aligned content causes overflow, per: https://drafts.csswg.org/css-align-3/#valdef-overflow-position-safe

In cases where the flex container's main axis is reversed (flex-direction: row-reverse or flex-direction: column-reverse) and the flex container itself establishes the scrollport by having overflow:scroll, the safe keyword here actually makes things unsafe -- it forces the overflow off of the end side, where content is unscrollable.

Testcases:
https://bugzilla.mozilla.org/attachment.cgi?id=9472153 (column-reverse)
https://bugzilla.mozilla.org/attachment.cgi?id=9472158 (row-reverse)

Current Firefox release 136.0.1 renders those^ testcases as orange squares without scrollbars due to following the spec on this point.

I'm planning to revert this behavior and remove safe as part of our fallback alignment here, to match other browsers; and I think we should update the spec as well.