For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/M17N:fonts/openSUSE_Tumbleweed/M17N:fonts.repo
zypper refresh
zypper install mozilla-zilla-slab-fonts
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/M17N:fonts/15.6/M17N:fonts.repo
zypper refresh
zypper install mozilla-zilla-slab-fonts
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/M17N:fonts/15.5/M17N:fonts.repo
zypper refresh
zypper install mozilla-zilla-slab-fonts