I print a number of label sizes on a Brother Laser as well, and have zero issues. Most of the media size and handling is done by the printer itself, and the software has little to do with it…for the most part.
- What specific printer is this?
- Can you upload an image of the misprint to illustrate what’s happening?
- Have you tried forcing the system print dialog when printing versus just using whatever Firefox gives you?
- Can you print a proper label with any other programs, or do they all behave similarly?

























When you go to print, just look for the link that sends out the print job to your system settings. That bypasses the browser print stack. Probably solves your issue right there.