On Firefox, FoxyProxy Classic supports configuring SOCKS proxy over Unix domain sockets. This is done by configuring a host of the form file:///path/to/socket. The port is ignored.

This does not seems to be working anymore with the new version of FoxyProxy. Using a Unix domain socket SOCKS proxy actually works with proxy.onRequest():

browser.proxy.onRequest.addListener(handleProxyRequest, {urls: ["<all_urls>"]});

function handleProxyRequest(requestInfo) {
  return {
    type: "socks",
    host: "file:///run/user/1000/proxy.socks",
    port: 9999,
    proxyDNS: true,
  };
}