Make a change to Firefox config:

1 – Type about:config in the address bar

2 – Right-click anywhere in the white space, click on "New –> Boolean" and type

print.always_print_silent

and set the value to True.

This will force Firefox to print with out any prompts. The drawback with using this is that you will not be able to make any changes to your print options (make sure everything is set first). To disable or remove the entry, right click and set it to false or reset and restart Firefox.