Did you uninstall and reinstall the printer driver/software? Printing is a printer software/driver issue. What version did you have before FF60.

I don't think it is the printer driver software. I just did some more tests. The Chrome Browser and Safari print duplex just fine. Preview on the Mac prints duplex just fine. Every other application that I use to print print duplex just fine, as far as I know, except Firefox. So it seems the only common denominator is Firefox. My experience in replacing drivers to try to fix problems usually break more things than fix them. So I am reluctant to go that path right now. What I have been doing is using Print Friendly & PDF to print the page to pdf, and then print it with Preview as duplex. And I just did another test, I disabled this add-on and tested duplex printing again... it failed. So this add-on is not the cause either. Any more ideas? Sure seems to me to be Firefox. It has been this way for many weeks, I just hadn't got around to reporting it, hoping it would go away. I try to keep all software up to date, from MacOS, to Firefox, etc. on the assumption that newer is better.

I forgot to mention, Thunderbird duplex printing also works fine. Another reason I don't think it is the printer driver.