Enable Silent Printing
To enable silent printing:
Make sure the printer Firefox uses by default is the one you want to silently print to.
1. Type about:config in Firefox's location bar and hit Enter
2. Right click anywhere on the list that appears and select New > Boolean
3. Enter the name as print.always_print_silent, set the value to true and hit OK
4. Find print.show_print_progress, set the value to false and hit OK
From now on when something prints it will automatically be sent to the default printer.