For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:mozilla/openSUSE_Tumbleweed/home:manfred-h:mozilla.repo
zypper refresh
zypper install MozillaFirefox
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:mozilla/openSUSE_Slowroll/home:manfred-h:mozilla.repo
zypper refresh
zypper install MozillaFirefox
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:mozilla/15.6/home:manfred-h:mozilla.repo
zypper refresh
zypper install MozillaFirefox
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:mozilla/15.5/home:manfred-h:mozilla.repo
zypper refresh
zypper install MozillaFirefox