Finally achieved empty tcpdump starting Firefox. Had to find and clear location.services.mozilla.com and push.services.mozilla.com from show-all in about:config. Then there were the following that are hard-coded, not appearing in about:config, for which /etc/hosts needed to be invoked:
firefox.settings.services.mozilla.com content-signature-2.cdn.mozilla.net prod.remote-settings.prod.webservices.mozgcp.net content-signature-chains.prod.autograph.services.mozaws.net
FFS do better.