Hi,
I have almost the same issue discussed in Navigation bug going back to forms tab: mouse navigation freezes
The issue is: Besides the fact the item is navigable. Sometimes, not always, but frequently, the form looses the
ability to correctly focus and change the record with the mouse.
With keyboard. Works. With mouse does not.
If i close the forms and start it again. It works.
Differences from the post I mentioned: We use Firefox ESR. Doesn´t matter if a popup opens or not. Just the fact to change windows
(example: the user change to Excel to copy a text and paste into oracle text item) triggers the bug.
Any tips? (below, versions info)

When the bug happens and user press any key in the item where the cursor is (but is not the current record),
the forms triggers: FRM-41008: Undefined function key
It´s very hard to reproduce the error. I couldn´t in my PC. It happens with many users. All PC´s has the same program and versions installed.
None of this helped me:
https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=201322916457108&id=239820.1&displayIndex=4&_afrWindowMod…
https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=201964346697345&id=760250.1&_afrWindowMode=0&_adf.ctrl-s…
https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=201697538484298&id=1228363.1&displayIndex=7&_afrWindowMo…
Forms [64 Bits] Versão 11.1.2.2.0 (Produção)
Oracle Database 11g Release 11.2.0.4.0 - 64bit Production
Java Plug-in 10.67.2.01
JRE 1.7.0_67-b01 Java HotSpot(TM) Client VM
WebUtil Version: 11.1.2.2
Forms Applet: 11.1.2.2
Mozilla Firefox ESR: 45.4.0.