I use a similar set up. Librewolf on Linux and IronFox on Android. You could still use a Firefox account to sync, but I wouldn’t. I’ve heard there’s a way to host an older version of Firefox sync locally, but I haven’t looked into it.
Yes IronFox is a fork of Mull, and though it does have a couple of differences in opinion on the balance of privacy and usability, it's very similar. I've been using it since shortly after we lost Mull.
And this is the latest sync server that doesn't rely on discontinued versions of Python: https://github.com/mozilla-services/syncstorage-rs/. It's not a full, plug and play solution, and it doesn't support PostgreSql so I haven't set it up in my self hosted environment yet, but plan to eventually.