Thanks for checking, that points for me to a problem at the KDE side.
Found using the Internet that there is a way to open files from the command line using “kde-open5 <pdf-file>” and did give that a try.
That gave me an error related to FoxitReader (that I did installed for large pdf documents where Okular is slow) and checking things I saw the install was gone, not sure why.
Then installed the FoxitReader again tried kde-open5 again and saw Foxit was opened with the .pdf
Next I checked again the KDE settings and saw Foxit was the first preference for .pdf. Changed it back to Okular and then kde-open5 was opening Okular.
With that also “Open in System Viewer” works again in Firefox, problem solved!