Firefox returns many 'Gtk-WARNING **: Unknown paper size Custom Size 1' messages whenever I do anything that seems to make Firefox think I'm going to print; actually trying to print isn't necessary. I have no printer; I use printing to capture a page as a PDF. Because Firefox cuts the pages in the middle of images I have a custom size, 200 lines long (the longest acrobat handles) to minimize (usually eliminate) page breaks. It works: I get pages that long. This warning message merely annoys me. Page length doesn't matter: I get the same complaint when I make Custom Size 1 8.5x11 I suspect that the real problem is some service Firefox calls doesn't recognize non-standard page sizes and that Firefox doesn't use that service for my purposes so it doesn't matter.