Firefox posses its own Print Options dialog (File > Print), but if you want to make use of default OS settings:
- In the address bar type "about:config" and press Enter.
- In the Filter field search for "print.extend_native_print_dialog".
- Double click on this entry to change its value to False
- Close the about:config window.
As shown below the difference between the 2 print dialogs:
Firefox print options:
Windows print options: