That's the certs, two old (still valid though) and two new ones (they have different DN's though)...
Logging into the page requiring mutual-auth I only see the two old ones in the list:
I checked and it is the same in Chrome though...
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.