Printing from firefox is mostly straight forward with a few required margin and format settings as shown below.
Print dpi should match the dpi in printer settings.
From Menu > Print > Page Setup, choose settings as shown in images above.
Landscape mode. Empty out headers and footers. Print background colors and images. Enable shrink-to-fit. Zero margins.
You can choose Menu > Print or print from the QR code page to print the label.