Background: The environment is a Windows 10 based peer to peer network with one PC acting as file and print server. The printer is a Xerox WorkCentre, shared via UNC path to the clients, connecting via fixed IP address (not WSD). OS: Microsoft Windows 10 Pro, 64-bit (build 19043) Mozilla: Mozilla Firefox ESR (x64 en-US) 78.12.0 * On this network the Xerox is defaulted to A4 paper and all PC's pull these settings from the print server. * The local PC settings also show default paper size as A4. * Most programs (Word, Outlook, Notepad, Print Test Page) respect these settings and so when printing, the print job prints as expected. * Firefox will print non-PDF pages to A4 as expected. '''''* PDF files, when opened in Firefox, do not print without causing a paper sizing related error on the printer.''''' * In turns out the when querying the advanced settings, the Firefox PDF engine is defaulting to Letter paper. So there are two issues as I see it: 1 - The Firefox PDF engine is not respecting the default paper size of the default printer. 2 - There is no way I can see to tell Firefox to use A4 paper. As a side note none of the printers have a locally installed printer, I have no idea if this makes a difference. My workaround has been to install Adobe Reader and set this as the default PDF reader both in Windows and Firefox. Does anyone know how to set Firefox to A4 for it's PDF engine?