Just checked LibreWolf a hardened Firefox as I thought they disable it. Turns out its just off default and "There aren't significant downsides as Firefox Sync encrypts your data locally before transmitting it to the server." I thought there might be another reason to have it off (giving Mozilla your email?) but seems OK. https://librewolf.net/docs/faq/#can-i-use-firefox-sync-with-librewolf-is-it-safe-to-do-so