This is a bug in the MozillaFirefox-branding-openSUSE package. It was not adjusted to account for Leap 16:
%if %suse_version >= 1550
# Tumbleweed
%global suseversion Tumbleweed
%endif
Open bug report.
It is the same source for Tumbleweed and Leap. Build happens in different environments.
Mod edit: Unnecessary comment removed.