EndNote X9.3.3 includes a fix to the Kopernio integration on Firefox.
Thank you for bearing with us.
We apologise that the Firefox version of Kopernio cannot export the PDF with the reference to Endnote from the PDF Viewer. A fix will be available within 2 months for Windows and 3-4 months for MacOS.
Let us know if you need any help or if you have any questions. Please email us at firstname.lastname@example.org.