Following up from bug 1666964.

On macOS, http://localhost:631/printers/ shows that the default media size for my Brother MFC-L8690CDW series printer is iso_a4_210x297mm. However, if I clear the print prefs for this printer and then try to print to it, Firefox selects na_index-3x5_3x5in (i.e. 3 x 5″) as the default selected paper size. That's a bit annoying, especially given that the new UI initially hides the paper size field.

Checking nsIPrinter.paperList for this printer, the paper list contains the following sizes, in this order:

na_index-3x5_3x5in: "3 x 5″"
na_monarch_3.875x7.5in: "Envelope Monarch"
na_number-10_4.125x9.5in: "Envelope #10"
iso_a6_105x148mm: "A6"
iso_dl_110x220mm: "Envelope DL"
iso_a5_148x210mm: "A5"
iso_c5_162x229mm: "CEnvelope 5"
iso_b5_176x250mm: "Envelope B5"
na_executive_7.25x10.5in: "Executive"
iso_a4_210x297mm: "A4"
na_letter_8.5x11in: "US Letter"
na_foolscap_8.5x13in: "Foolscap"
na_oficio_8.5x13.4in: "8.5 x 13.4″"
na_legal_8.5x14in: "US Legal"
iso_a4_210x297mm_borderless: "A4 (Borderless)"
na_letter_8.5x11in_borderless: "US Letter (Borderless)"
na_legal_8.5x14in_borderless: "US Legal (Borderless)"
custom_12.02x12.02mm_12.02x12.02mm: "media.custom_12.02x12.02mm_12.02x12.02mm"
custom_12.02x99.99mm_12.02x99.99mm: "media.custom_12.02x99.99mm_12.02x99.99mm"
custom_17.02x53.85mm_17.02x53.85mm: "media.custom_17.02x53.85mm_17.02x53.85mm"
custom_17.02x86.87mm_17.02x86.87mm: "media.custom_17.02x86.87mm_17.02x86.87mm"
custom_21x99.99mm_21x99.99mm: "media.custom_21x99.99mm_21x99.99mm"
custom_23.03x23.03mm_23.03x23.03mm: "media.custom_23.03x23.03mm_23.03x23.03mm"
custom_24.05x24.05mm_24.05x24.05mm: "media.custom_24.05x24.05mm_24.05x24.05mm"
custom_28.96x41.91mm_28.96x41.91mm: "media.custom_28.96x41.91mm_28.96x41.91mm"
custom_28.96x89.83mm_28.96x89.83mm: "media.custom_28.96x89.83mm_28.96x89.83mm"
custom_28.96x99.99mm_28.96x99.99mm: "media.custom_28.96x99.99mm_28.96x99.99mm"
custom_28.96x208.96mm_28.96x208.96mm: "media.custom_28.96x208.96mm_28.96x208.96mm"
custom_29.97x99.99mm_29.97x99.99mm: "media.custom_29.97x99.99mm_29.97x99.99mm"
custom_38.02x89.83mm_38.02x89.83mm: "media.custom_38.02x89.83mm_38.02x89.83mm"
custom_38.02x99.99mm_38.02x99.99mm: "media.custom_38.02x99.99mm_38.02x99.99mm"
custom_38.95x99.99mm_38.95x99.99mm: "media.custom_38.95x99.99mm_38.95x99.99mm"
custom_39.03x47.84mm_39.03x47.84mm: "media.custom_39.03x47.84mm_39.03x47.84mm"
custom_49.95x99.99mm_49.95x99.99mm: "media.custom_49.95x99.99mm_49.95x99.99mm"
custom_51.99x28.87mm_51.99x28.87mm: "media.custom_51.99x28.87mm_51.99x28.87mm"
custom_54.02x99.99mm_54.02x99.99mm: "media.custom_54.02x99.99mm_54.02x99.99mm"
custom_54.86x99.99mm_54.86x99.99mm: "media.custom_54.86x99.99mm_54.86x99.99mm"
custom_58.25x58.25mm_58.25x58.25mm: "media.custom_58.25x58.25mm_58.25x58.25mm"
custom_60x87mm_60x87mm: "media.custom_60x87mm_60x87mm"
custom_61.98x28.87mm_61.98x28.87mm: "media.custom_61.98x28.87mm_61.98x28.87mm"
custom_61.98x99.82mm_61.98x99.82mm: "media.custom_61.98x99.82mm_61.98x99.82mm"
custom_61.98x99.99mm_61.98x99.99mm: "media.custom_61.98x99.99mm_61.98x99.99mm"
custom_61.98x183.9mm_61.98x183.9mm: "media.custom_61.98x183.9mm_61.98x183.9mm"
custom_61.98x208.96mm_61.98x208.96mm: "media.custom_61.98x208.96mm_61.98x208.96mm"
custom_61.98x209.3mm_61.98x209.3mm: "media.custom_61.98x209.3mm_61.98x209.3mm"
custom_72.98x99.99mm_72.98x99.99mm: "media.custom_72.98x99.99mm_72.98x99.99mm"
custom_80.77x99.99mm_80.77x99.99mm: "media.custom_80.77x99.99mm_80.77x99.99mm"
custom_96.86x99.99mm_96.86x99.99mm: "media.custom_96.86x99.99mm_96.86x99.99mm"
custom_103.97x99.99mm_103.97x99.99mm: "media.custom_103.97x99.99mm_103.97x99.99mm"
custom_106.68x99.99mm_106.68x99.99mm: "media.custom_106.68x99.99mm_106.68x99.99mm"
custom_107.95x99.99mm_107.95x99.99mm: "media.custom_107.95x99.99mm_107.95x99.99mm"
custom_120.9x99.99mm_120.9x99.99mm: "media.custom_120.9x99.99mm_120.9x99.99mm"
custom_142.92x99.99mm_142.92x99.99mm: "media.custom_142.92x99.99mm_142.92x99.99mm"
custom_143.76x99.99mm_143.76x99.99mm: "media.custom_143.76x99.99mm_143.76x99.99mm"
custom_153.92x99.99mm_153.92x99.99mm: "media.custom_153.92x99.99mm_153.92x99.99mm"
custom_179.83x99.99mm_179.83x99.99mm: "media.custom_179.83x99.99mm_179.83x99.99mm"
custom_190.67x99.99mm_190.67x99.99mm: "media.custom_190.67x99.99mm_190.67x99.99mm"
custom_203.88x99.99mm_203.88x99.99mm: "media.custom_203.88x99.99mm_203.88x99.99mm"
custom_238.76x99.99mm_238.76x99.99mm: "media.custom_238.76x99.99mm_238.76x99.99mm"

So it looks like cupsGetDestMediaDefault call in nsPrinterCUPS::DefaultSettings is really just returning the first paper size in the list?

If we can't figure out how to get better information out of CUPS, maybe just having nsPrinterCUPS::DefaultSettings() get the full list of printers and switch the default to Letter to A4 if one of those is supported and depending on locale would be better in the common case?

This really hasn't been the case for me on my macOS 10.15.6 machine.

I have three printers connected and I've verified that changing my Settings > Printers & Scanners > Default Printer correctly updates the printer returned from cupsGetDestMediaDefault().

Each time I verify the output from:

layout/base/tests/chrome/test_printer_default_settings.html

But when I tried to use the IPP method on macOS, I switched my default from US Letter to A4 and only two of my printers correctly updated.
I believe that Hiro had a similar experience, which is why we decided to keep cupsGetDestMediaDefault() on macOS, over using the IPP.

I'm really curious to know what's causing these inconsistencies.

So looks like the default page sizes are inconsistent in different cases. I mark this as S3 for now.