How to show CSS in current @media query in Firefox?

Update 2019

To show current media queries open settings (F1 from dev tools) and check "Show Browser Styles" under the Inspector section.

The media query will be shown just to the right of the file name and line number.

Firebug doesn't have that feature implemented. It was requested in issue 5329, though Firebug is discontinued now, so it won't be implemented anymore.

Though the Rules side panel of the Firefox DevTools does show @media queries.

To get them displayed uncheck the option Show original sources within the options panel.

As [upcoming versions of Firebug will integrate into the DevTools][4], they will inherit this feature.