I know that it’s a old post… If your issue are the same that I come across, I could solve this issue following this answers from Reddit post Plasma browser extension - failed to connect to the native host.
Your Firefox must be what comes with KDE Neon. The issue is because the apparmor. Run the commands in your terminal:
sudo ln -s /etc/apparmor.d/usr.bin.firefox /etc/apparmor.d/disable/
sudo apparmor_parser -R /etc/apparmor.d/usr.bin.firefox
Disclaimer: I didn’t understand what it really did, but now the extension works.