Firefox 57 / Quantum: emulate print styles

After the GCLI (Developer Toolbar) has been removed, the feature is now once again available in the latest nightly version of Firefox. Print style emulation can now be toggled from the styles panel in the Inspector of the Developer Tools. It's the little page icon in the upper right, next to the buttons to change the hover state, toggle classes and add CSS rules (toggled active in the screenshot):

As of May 2019, this has not yet landed in the current stable release (66), but it is available in the current Developer Edition and the Nightly Channel (68.0a1 and above).

Update August of 2019 (v68)

Version 68 has landed in the stable release channel, so the feature is now available in all Firefox distributions.

[UPD] this console has gone starting from v62. But after some time toggle button for switching into Print mode is available in DevTools again. Please check another answer below(unable to delete accepted answer)

I thought it's impossible. But suddenly have found answer in another topic:

1. press Shift + F2 to get special browser console
2. type "media emulate print"
3. hit "Enter"

That's it.