Steps to reproduce: (1) Find a page you want printed to PDF. (2) File -> Print... (3) In the GTK dialog, select Print to File, then PDF, and specify the path you want to save the file to. (4) Hit OK. In nsDeviceContextSpecGTK::EndDocument(), mSpoolFile has the generated PDF ready to deliver. It calls gtk_print_job_send(mPrintJob, print_callback, mSpoolFile, ns_release_macro);. In print_callback, aError says "Operation not supported", and our code calls mSpoolFile->Remove(true). So just when we're on the cusp of success... we die and report nothing to the user. The Print to PDF operation has silently failed.
In a more modern build we are not seeing this.