So, I've recently, for the sake of experiment, done something that isn't recommended: I've configured Firefox to use the TOR Socks listener as a proxy.
As I expected, the regular sites, such as Bing (my homepage) opened. After opening check.torproject.org, I tried something which I was nearly certain would fail (but I wanted to see what error message I'd get): I opened the URL of a ONION service, namely, facebookcorewwwi.onion.
I expected that Firefox, being an ordinary browser, would make the DNS request (over the TOR SOCKS proxy) attempting to resolve the IP of facebookcorewwwi.onion, and, since .onion services don't have an actual IP address, fail with some error message.
I got very surprised when it actually opened the URL. Furthermore, it claimed that the IP of facebookcorewwwi.onion is 0.0.0.0:443. Can you explain to me what's going on here?