@Gwen-Dragon
I installed like described the nss-tools from openSUSE tumbleweed repository via:
sudo zypper in mozilla-nss-tools
Afterwords I installed the certificate, downloaded from FritzBox router:
certutil -d sql:$HOME/.pki/nssdb -A -t "P,," -n fritz.box -i ~/Downloads/boxcert.crt
And the verification shows me, that it was well installed:
certutil -d sql:$HOME/.pki/nssdb -L
I think now someone else can do it easier 😉
Update - after an restart, vivaldi shows a green locker symbol.