Printer is OKI C531DN connected via network with A4 paper.
Works fine in Firefox 86 on Windows. Works fine in Firefox 76 on Linux (Mint, OpenSuse Leap).
Problem with Firefox 86 in Linux. The job goes to the printer which then complains about paper size - needs a4_210x297mm. I can press OnLine on printer and job completes OK. Chrome works OK and I can print to PDF then print that OK.
Values in about:config
print.printer_OKI_DATA_CORP_C531.print_paper_height 11.6945
print.printer_OKI_DATA_CORP_C531.print_paper_id iso_a4_210x297mm
print.printer_OKI_DATA_CORP_C531.print_paper_size_unit 0
print.printer_OKI_DATA_CORP_C531.print_paper_width 8.26378
On other instances the unit is 1 and measurements are in metric. Paper_id is iso_a4 or from memory I think there is a media_id set to 1. It looks like the paper_id is what is causing the problem. The printer knows a4 but not a4_210x297mm.
If I reset the values FireFox resets them to the “broken” values when I request printing.
Reporting here in addition to Mint and Mozilla because Tumbleweed has updated to FF86 and issue has now appeared there.