How to see the print media CSS in Firebug?

Actually, be aware that you might see @media print CSS when you don't expect it.

Like SO uses:

[..]@media print{#sidebar,#nav,[..],div.vote{display:none;}}[..]

...and hence one might expect the CSS panel in Firebug to somehow show:

@media print {
  #sidebar, #nav, [..], div.vote {
    display: none;
  }
}

But instead it shows the CSS as if the @media print is actually active, like:

#sidebar, #nav, [..], div.vote {
  display: none;
}

(See also the related issue report: CSS Panel does not have @media UI.)