If I edit [the attached testcase](https://bug1616361.bmoattachments.org/attachment.cgi?id=9127379) (or use devtools) to change `.header` to have `display:inline-block` instead of `display:inline`, Chrome maintains its same stacking order (no change), and Firefox changes to match Chrome. So our remaining disagreement on that testcase seems to be due to some subtlety around handling of `display:inline` vs `inline-block` (as a relatively-positioned thing that forms a stacking context due to having nondefault 'z-index')...
Bug 1616361 Comment 8 Edit History
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If I edit [the attached testcase](https://bug1616361.bmoattachments.org/attachment.cgi?id=9127379) (or use devtools) to change `.header` to have `display:inline-block` instead of `display:inline`, Chrome maintains its same stacking order (no change), whereas Firefox changes to match Chrome (we move the orange stuff to be furthest-back in the stacking order). So our remaining disagreement on that testcase seems to be due to some subtlety around handling of `display:inline` vs `inline-block` (as a relatively-positioned thing that forms a stacking context due to having nondefault 'z-index')...