I run firefox on Debian. I made a clean install with no cookies, or no information input from my side. I've also disabled all mumboo-jumboo healthy and crash report tracking. I've disabled 'shared additional data' too.
At this time, there is no TCP connection to any website, then I leave firefox open at the 'create new tab'. When I turn off firefox, there are no packets too. When I turn it on, I see random TCP packets to websites I've visited (days ago) like 'duckduckgo' and 'cloudflare'. (I suspect they are HTTPS, since they are sent over TLS to port 443 of the respected servers).
Why? (The issue is also visible on Chromium)