Investigate setting MOZ_DISABLE_NONLOCAL_CONNECTIONS to prevent proxy bypasses
It turns out Mozilla has an environment variable, MOZ_DISABLE_NONLOCAL_CONNECTIONS, that might be helpful for preventing proxy bypasses or, if that's not the case right now, it might be patched so that it does indeed function as one would expect. See: https://lists.torproject.org/pipermail/tbb-dev/2018-March/000818.html for more context.