We can't build Firefox 60 ESR for s390 and ppc arches because the devtoolset and rust-toolset is not prepared for these platforms.
Despite builds for these arches are not imported to ET, excluding them brought some problems last time .
Upstream rust does not have a 31-bit s390 target at all. They do have 32-bit powerpc, but we haven't been building it. We agreed early on that Firefox would only need Rust for the 64-bit targets and i686.
Okay, closing is wontfix.