Karbon (a vector drawing application, not PDF viewer) is being picked up by KDE as the default PDF handling application instead of Okular. While Karbon can edit a PDF this is not ideal.
This change has only happened since the substitution of LibreOffice with Calligra. Evidence is the Firefox PDF handling options list.
To work around this what I did was open Firefox, print a page to PDF. Then right click the printed document and choose open with. there's a tick box to always open with and associate the application with that file type. I ticked that and now Okular opens PDFs when called upon by other applications like Firefox itself.
Okular was already in the associated PDF handler list above Karbon but was being ignored before I did the above procedure.