The reporter in bug 1661751 comment 17 was actually using Linux, i.e. our CUPS backend, I think. `nsPrinterListCUPS::SystemDefaultPrinterName` doesn't seem to have any obvious places where it could return anything other than NS_OK, so I'm not sure how it's failing in their case. But it does definitely look like it can fail on Windows as noted above, at least...
Bug 1699343 Comment 1 Edit History
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The reporter in bug 1661751 comment 17 was actually using Linux, i.e. our CUPS backend, I think. `nsPrinterListCUPS::SystemDefaultPrinterName` [doesn't seem to have any obvious places where it could return anything other than NS_OK](https://searchfox.org/mozilla-central/rev/b7b156e53643f0237f3e98a76e5fc7fa9e3b4e71/widget/nsPrinterListCUPS.cpp#179-201), so I'm not sure how it's failing in their case. But it does definitely look like it can fail on Windows as noted above, at least...