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Firefox 80 comes with caching stylesheets per document group. Mozilla thinks that needs partitioning for their top-level site partitioning, which is roughly equivalent to first-party isolation. The relevant check implemented is

nsIPrincipal* Loader::PartitionedPrincipal() const {
  if (mDocument && StaticPrefs::privacy_partition_network_state()) {
    return mDocument->PartitionedPrincipal();
  }
  return LoaderPrincipal();
}

which is not checking the FPI pref. So, I guess we need to investigate what the FPI story is.