Sorry, Firefox doesn't have a preference for that setting.

Maybe is possible to change this value in code? Where Firefox keeps printer settings information ?

The '''Configuration Editor''', more commonly referred to as '''about:config''' is where those setting are stored; and available for editing for experienced users. https://support.mozilla.org/en-US/kb/about-config-editor-firefox If you use '''print.print''' in the Search Bar at the top of the about:config page you will see the available related preferences. The only thing close might be '''print.print_via_parent''', but my search for that pref in MDN came up empty. https://developer.mozilla.org/en-US/ ''With the change from XML to HTML within Firefox, and changes made to the MDN website my search skills have bottomed out when it comes to accessing information from MDN.'' Here's a Google search for that "print.print_via_parent" preference, which is turned on by default. https://www.google.com/search?q=print.print_via_parent&sourceid=mozilla-search&start=0&start=0&ie=utf-8&oe=utf-8num=100 Perhaps you'll find an answer from one of those search returns.