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/