As reported in bug 1663536 comment 1, the increase in printing errors has dropped back down again. Dropped down suspicously low, in fact. It turns out that in bug 1653319 I failed to notice that switching to the new API required adding new telemetry reporting calls... :-(

Actually never mind, I'm wrong. Everything's fine. We still report the errors that we previously reported.

To expand on the details a little, we report errors using PrintUtils._displayPrintingError(). This function isn't called directly on the old PrintUtils based codepaths though. It is called in response to "Printing:Error" events occurring in the print document and being seen by the PrintingChild.jsm code. We still, for now, load PrintingChild.jsm into the print documents and therefore continue to report telemetry for the same errors.