Yes you can actually self-host both Firefox sync server [1] and use Firefox accounts (which also can be self-hosted [2] and someone put something simpler in docker image [3]). And those can be used even with Firefox itself not only the forks.

[1] https://github.com/mozilla-services/syncstorage-rs

[2] https://mozilla.github.io/ecosystem-platform/tutorials/devel...

[3] https://github.com/jackyzy823/fxa-selfhosting

I don't see how your 2 questions are related to each other.

> Do any of these forks have the ability to sync, either with Firefox or something self hosted?

The Firefox Sync web service is provided by Mozilla but can be self hosted: https://github.com/mozilla-services/syncstorage-rs. That could also be used in forks. See e.g. https://librewolf.net/docs/faq/#can-i-use-firefox-sync-with-... . I don't understand what you mean by sync with Firefox.

> Or are they all just basically reskins with a single toggle added or such?

Hard to generalize, but definitely not all of them. see e.g. https://lwn.net/Articles/1012453/

LibreWolf has a Firefox Sync option, though it's disabled by default

Good point. I'd actually be happy to pay a couple of bucks a month for a good syncing solution based on an open source protocol, to make it easier for me to use the same history and preferences across browsers, IDEs and other such tools. It's actually a similar need and setup to that of a password manager, so I wonder if this is something Bitwarden could take on.