mdns: I disabled MDNSResponder and now I can't use firefox, but I can connect through opera (chrome based)

I am trying to strengthen my macos, and I have read that mdnsresponder has some vulnerabilities, so I disabled it (after disabling sip, then restarted with sip enabled).
So now I can't connect to the internet. However, when I try to navigate with Firefox, I can only connect to My other applications cannot connect either.

But with the Opera browser I can connect to any site! As you might guess, it is related to google dns.

So, I currently have these questions, could you help me with any of them if you know anything?

1) Why does opera have a connection? Does it mean that Opera always uses google dns even if I don't put or similar in my network settings? Does it mean that he has some potential & # 39; spy & # 39 ;?

2) Is there a way to disable mdnsresponder and have the possibility of having a network?