How can I print straight to a thermal printer without dithering?

In a hardware store, we use a USB thermal printer (CITIZEN CT-S4000) at the desktop POS to print receipts on 104mm thermal roll. The billing software runs online, and generates a pdf receipt, which we print from the browser at the cashier's desktop box, running a Linux flavor based on Debian 10 (BunsenLabs). I chose to use Firefox because it is the default browser and because it automatically adjusts the length of the page so the printer cuts the receipt off correctly. However, Firefox also scales the receipt down incorrectly : The text is dithered, and the printer does not use the whole page,

Question by Conrado on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.