You could put your self-hosted container in a self-hosted VPN to add a layer of protection.

But honestly, while encryption theoretically can make any data "safe" to hand to a 3rd party, it's just good "data hygiene" to keep it under your own control. If someone steals the Firefox sync database now, and some part of the encryption scheme is later broken by e.g. quantum computers, the thieves could retroactively decrypt your sync account data.

And there's also the question of metadata - when does your Firefox send data to sync, and how much? Are you sure there's nothing to glean from that?