The first time you print, Firefox will save the printer that was used to '''print_printer''', even if you have '''print.always_print_silent''' set. After that point, Firefox will use the printer saved to '''print_printer''', not the system default printer. The only way you can prevent Firefox saving the printer name to '''print_printer''' is to set '''print.save_print_settings''' to false, but that will prevent Firefox saving any print settings, not just '''print_printer'''. It is possible that you can get Firefox to do what you want (it's not clear exactly what you want, though) by printing once without '''print.always_print_silent''' set (to get Firefox to store most print settings), then remove '''print_printer''', set '''print.always_print_silent = true''' and '''print.save_print_settings = false'''. It '''may''' then use the system default printer but still pick up useful print defaults from the rest of the saved prefs. However, I have not tested whether this works. It also has the risk that some of the print settings that have been saved to prefs are not actually valid with the new system default printer.