I said it should not exist. Meaning there is no reason for it’s existence because the Arch community
kvantum package supports Qt 5 & 6.
Anyway, just use Mullvad VPN. There is really no benefit to using Mozilla VPN seeing how they use the same servers. Also Mullvad is updated much more frequently and actually builds.
I have the same problem and I correct it (partially) with the unset of the QT_STYLE_OVERRIDE variable. It’s seem that QT don’t support the kvantum value.
I corrected it in the ugliest way :
mv /usr/bin/mozillavpn mozillavpn-exe
cat << EOF > /usr/bin/mozillavpn
chmod +x /usr/bin/mozillavpn
And tadaaaa ! It works.
I hope you have a better idea to correct this.