Sync works fine, at least for bookmarks and history, but I cannot choose what to sync, the check boxes are greyed out: https://i.imgur.com/atDU75z.png

You are not currently able to Sync to a custom server in Firefox Preview. Doing that requires access to about:config which is not set up in the release version.

You can probably use Firefox Lockwise [https://lockwise.firefox.com/] to sync passwords.

Firefox sync is amazing and one of the reasons I never got hooked on chrome (I tried using it for a full year). I often do research on iOS for coding related topics. Send tab to device - boom, it’s there when I need it and I’ve got my text editor open.