Firefox bookmark sync for v52 working? AddOn for Chrome?

[pstover]

Is built-in Firefox-Sync to sync bookmarks with remote Mozilla Server working with older versions like FF v52.9.0 ESR as well?

if Yes; is it client side encrypted as well?

As far as I heard bookmarks are always synced automatically. Can I switch to sync-on-demand mode?
I want to let them sync only if I manually trigger it.

Further question: Can I somehow (with additional WebExtension) access to Firefox-Sync server from Chrome(-based) browsers as well?

Peter