This indeed appears to be in the MozillaFirefox-branding-openSUSE package: The
'susefox-20120626.tar.bz2' tarball contains this code snippet in
content/pluginInstallerService.js:338:
executable.initWithPath("/sbin/OneClickInstallUI");
But AFAICS building this whole tarball is disabled in the .spec file:
https://build.opensuse.org/projects/openSUSE:Factory/packages/MozillaFirefox-branding-openSUSE/files/MozillaFirefox-branding-openSUSE.spec?expand=1
%global susefox 0
...
...
%prep
cp -a %{SOURCE6} COPYING
%if %susefox
%setup -n susefox -q
mv COPYING COPYING.susefox
%endif
%build
%if %susefox
./build.sh
%endif
...which means that it was an intentional decision not to support this directly
anymore. Security considerations?