On Firefox 52, the CSS
order property, which allows to sort flex items by an arbitrary order, is ignored in some cases if those items are separated by an absolutely-positioned element like this:
<div style="display:flex"> <div style="order:2">B</div> <div style="position:absolute"></div> <div style="order:1">A</div> </div>
Mozilla developers are aware of the regression and working on the solution.
Update: This issue has been fixed with Firefox 53 and Firefox ESR 52.1.