1. Launch Firefox.
2. Type “about:config” in the address bar at the top of the browser screen, then press “Enter.”
3. Click the “I’ll be careful, I promise!” button to confirm that you want to access Firefox’s advanced settings.
4. Scroll through the alphabetically-ordered list of advanced setting preferences in the main screen.
5. Right-click the “print_printer” entry in the print section. Select “Reset” from the context menu. If more than one “print_printer” entry shows in the list, reset each entry in the same way.
sumber : Support Mozilla