Previously I had managed to force Firefox not to make unwanted automatic connections using gHacks user.js [0] and this official guide [1], but this new version makes a connection to firefox.settings.services.mozilla.com after startup no matter what. Not a big deal, but leaves a bad aftertaste from the upgrade.

[0] https://github.com/ghacksuserjs/ghacks-user.js

[1] https://support.mozilla.org/en-US/kb/how-stop-firefox-making...