PDF files won't print after they have been edited

I sell on vinted, which generates a PDF label to print. Some of the courier sites add a lot of nonsense text to their labels, which I cover up with a white box to save ink. These edited PDFs used to then print just the label without the nonsense text, but since a recent Firefox update they have started printing an empty page instead. Saving the edited PDF and then opening it again in Firefox does not work. The edited labels still print okay in Microsoft Edge.