If Firefox is not using the whole sheet of paper, this can be caused by having extracted the wrong paper size settings from Windows when reading the print driver data. Clearing it can involve finding some obscure settings, but here goes:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste print and pause while the list is filtered

(3) For each setting for the problem printer, right-click and Reset it. The fastest way is to right-click with the mouse and then press the r key on the keyboard with your other hand.

Note: In a couple other threads involving Brother printers, the preference printer_printer_name.print_paper_data was set to 256 and when the user edited it to 1 that fixed the paper size problem. If you see a 256 there, you can edit the value by doubling-clicking it or using right-click>Modify.

Any improvement?

Thank you so much! Resetting everything for that printer worked. :)