Recap
Reason
Solution
1. Wait for Firefox 66
The upcoming release of Firefox 66 which will ship on March 19, 2019 most likely mitigates this issue as we have been able to confirm using the current Firefox Nightly.
2. Install Firefox 66 Beta
Firefox 66 Beta, Revision 12 should already be sufficiently mature.
Workarounds
1. Explicitly request PDF from OPS
By going to “External sources » PDF and Images » OPS” through the context menu on the right side of the document header bar
you can explicitly request the PDF document from OPS, see also Problem when accessing PDF document of EP2706864A2 - #2 by Andreas.Motl.
2. Download PDF before printing
If you are not able to install a recent Firefox Beta release on your machine, please work around the problem by downloading the PDF documents before printing, e.g. by using the bulk download option as outlined on Auswahl an Druckschriften-PDFs herunterladen.
Outlook
Firefox 66 is around the corner and the problem will most likely go away by simply upgrading.
Please enjoy your research and sorry for the inconveniences this might cause to you and your colleagues.