I'm no expert with wireshark but here's what I did:
- Closed down thunderbird and my periodic network check.
- Made sure neither firefox nor librewolf were set to check for updates (/etc/firefox/policies/policies.json and /etc/librewolf/policies/policies.json) and set both of their home pages to "about:blank"
- Started wireshark and set it to capture eth0 traffic - no filters of any kind, just everything
- Ran firefox for one minute, then closed it
- Restarted wireshark
- Ran librewolf for one minute, then closed it
Firefox produced a few hundred lines of network traffic log. I haven't analyzed that log at length (and I may or may not ever do so).
Librewolf produced... NOTHING. I checked it again to make sure I hadn't made a mistake. Nothing.
I didn't have the patience just now to wait fifteen minutes for each of them, but the quick results look like a win for LibreWolf.