SUMMARY
When plasma-nm is first started after power-on, Firefox or any other browser does not connect to IPV6 sites. After 'Disconnect' and 'Connect' all is OK!

STEPS TO REPRODUCE
1. Power on machine - login as user
2. Start FireFox or other browser
3. IPV6 - "Server Not Found" - IPV4 are OK

OBSERVED RESULT
IPV4 sites function as usual

EXPECTED RESULT
IPV6 sites function as usual

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 20.04.1
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 
Qt Version: 5.12.8

ADDITIONAL INFORMATION
To ascertain when this problem started, I used several cloned back copies of my OS. When these were saved no problem existed. Each one exhibited the problem when loaded---may mean the problem exists in my /home directory.

Tried renaming plasma-nm config files. I purged plasma-nm and reloaded. Problem still exists.

Compared a very old copy of config files with current. Can see no difference. I must admit that I am not entirely sure that I got to all the right config files.

Searched previous reports but nothing hitting the mark.

This problem happened long ago but the situation resolved itself when I upgraded the OS.

Earlier in my issue the solution was resetting my modem. It worked.