I have a faint memory of a previous issue where I think Firefox was found following the operating system's setting. Are you set to metric in Windows?

PS. Just to make sure, you did try the print.printer_YOURPRINTER.print_paper_size_type setting suggested in https://support.mozilla.org/en-US/questions/1120313, correct?

Yes I used print.printer_XP-320_Series(Network)_(Copy_1).print_paper_size_unit;1, changed the 1 to 0 and it would work one time and then be reset to 1 for the next print job. I assume that Windows is set to English units, don't know where to look for those settings.

P.S. Just checked Windows regional setting and it is set to U.S. so the system is not metric.

Alright, so it is not Windows' regional settings in this case, and you know which setting will yield the desired result.

If you are feeling confident, you could lock the setting/preference manually: http://kb.mozillazine.org/Locking_preferences. This is an exercise in basic coding, but should do the job. You will have to remember the manual lock later on though, should you wish to make the opposite change.

Before resorting to force, here is another Question and Solution (note the KB article links and see last post): https://support.mozilla.org/en-US/questions/919606.

Try resetting all your printer settings as described here: https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox#w_reset-all-firefox-printer-settings

Settings in Windows shouldn't be affecting this type of issue. Firefox is built to be cross-platform with regards to the "print functionality"; to work pretty much the same on Windows and Linux, and to a certain extent, Mac OSX also.

The user selected Paper Size in Firefox is what Firefox uses to 'set' metric vs. US Inch measurements for printable area (length and width of the paper being used) and for Margin settings.

There has been an issue with either print.printer_printer-description_Printer.print_paper_size_unit or print.printer_printer-description_Printer.print_paper_size_type becoming set to a 'max' number like 999, far out of the normal range for the value of that "Value". Seems to be related to some models of Brother and Samsung printer drivers. Doing the full printer reset of all preferences for that particular printer usually fixes it.

Thanks for the clarification regarding the internals, edmeister.

I've tried all suggested solutions except for locking the paper setting. Tried (1) modifying about:config printer settings and (2) resetting all printer settings in said file. Result is always the same, first print job display margins in inches and afterwards changes it back to metric units.

Finally came up with a solution, In the about:config file changed print.save_print_settings to false and it stopped firefox from resetting margins to metric after executing a print job.

Good work!