Prior to Firefox Quantum, Firefox had a plugin to allow websites to streamline printing without extra dialog boxes and additional pop-ups. Firefox Quantum still has the seamless printing functionality, but it no longer allows changing the settings for it to be accessed by a web page. The user must manually make the change to enable it. This is how to do it.
Open Firefox Quantum and type 'about:config' in the address bar and press enter. Click to confirm you know what you’re doing.
At the top of the page, enter 'print.always_print_silent' into the search bar (no quotes).
If you do not see anything appear, you will need to create a new entry. Right click anywhere on the page and selecting New > Boolean from the menu. In the popup enter print.always_print_silent as the preference name and then select True.
If after you searched for print.always_print_silent you do find the setting, right click on the value, and chose "Toggle" to set it to True.
This is what the completed setting should look like: