Zen supports FF sync out of the box. Just started trying it out, and it synced at least all the extensions without issue https://docs.zen-browser.app/faq#how-can-i-sync-my-data-acro...
I’m using Sync on LibreWolf just fine. I am interested in self hosting sync, but not there yet. I think it would be cool if someone could set up a sync server and charge a small fee for accounts. It’s all OSS and setting it up doesn’t look impossible. Just tedious.
I use two forks on a daily basis - Floorp and Waterfox. Both sync FF accounts just fine.
There's an extension called floccus [1] that can sync bookmarks between numerous different browsers (including both Firefox/Gecko and Chromium/Blink based browsers, mobile and desktop) and plug into a user-controlled NextCloud [2] or Linkwarden [3] instance running on a local homelab server for backups.
However, I try to minimize the number of installed extensions to reduce my own browser fingerprint. As others have replied, Firefox Sync works across most of today's popular downstream shadow-forks of Firefox (as in, forks that shadow the Firefox releases rather than truly forking the code). If you don't want to give Mozilla's enshittified corpse your email (I don't), just use an email service that supports aliases to sign up for Firefox Sync... Which is in any case a good idea generally: no two of your important online accounts should point to the same email, even if behind the scenes the aliases are all routing emails back to you.
[1]: https://floccus.org/
I make heavy use of bookmarks and switch between devices frequently. Manually keeping those synced would be a bit of a nightmare.
Assuming that Firefox forks can use Firefox extensions, I imagine I could get around this problem by using some kind of bookmark extension. I don't know what options are available for that, though.