Their docs[0] make it seem like little would be gained from a privacy/security perspective as it seems pretty secure/private (at least to me).
[0] https://support.mozilla.org/en-US/kb/how-firefox-sync-keeps-...
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?
Since cookies and sensitive site data aren't a part of syncing, I guess I could see this being pertinent to your point of data being used to identify you.
That being said, if I were a malicious actor, this sounds like a really low reward if you manage to compromise/crack encrypted Firefox Sync data.
Ultimately, I agree with your point of reducing information exposure by self-hosting. My line of thought was more wondering what arguments there were on why this may or may not be worth setting up.