FF does not use the default printer settings, like doublesided print. Why not?
The printing preferences for my printer (Epson XP-630) are set to print on both sides. When I print something with FF, it prints on one side only. So I need to go to printing properties each time to set to print on both sides.
Why does FF not use the default printer settings?
I don't know "why" - it is the way it is. Many preferences for print drivers are only available via Properties for each printer. I addition to 2-sided, there is no setting in Firefox for Color / Greyscale as much more common 'toggle' preference.
Firefox uses "last used" as the default for the few printer preferences that are in Firefox; not the system defaults. I can only guess that the "age" of the printer code (circa 1995 when Netscape wrote the original Gecko printer code) and the cross- operating system compatibility for Firefox is part of the reason.