The special-use .onion domain name, defined in RFC7686, receives special treatment in the Tor browser and in Firefox: https://searchfox.org/mozilla-central/rev/6c8d325e61b0b445ed2e04899da38c3a4c266cba/netwerk/protocol/http/nsCORSListenerProxy.cpp#979-984

It seems like this behavior should be standardized since any browser could be setup to proxy traffic over the Tor SOCKS5 proxy.

I also filed an issue against the Referrer Policy spec to address the Referrer header.