SOCKS socket does not support spaces and other special characters
While working on #20111 (moved), Kathy and I found a bug in Firefox's SOCKS socket support: special characters, including spaces, are not decoded before the configured path is placed inside a Unix domain sockaddr. This causes the SOCKS connection to not work at all when a Unix domain socket is used with a path that contains spaces (or other URL-special characters).
We are working on a patch which we will also want to uplift to Firefox. I will file a Firefox bug later.