By definition, The order property for flexbox does not work with pixel values. But once it is edited through Firebug value spinner, it iterates through pixel values. Though the issue is not major and the incorrect usage of pixel value is highlighted by the red box shadow in spinner, it is to be noted that currently order can accept negative values just as how z-index works.
In below screenshot, I have used negative order values on the right hand side and it illustrates the corresponding output on the left.
In short, order spinner should works as z-index value spinner works.
Issue #7922: CSS flexbox order property accepting pixel values
I've fixed that. Thanks for the report!
Thanks for the fix, Sebastian! Cheers 👍