Same issues here with Firefox 56, and 55 on Windows 7 and Firefox 56 on macOS 10.12. Switching between current and previous versions of the Xmark plugin occasionally allowed a synchronization but that was rare so I turned off Xmarks in Firefox and started using Firefox's syncing.
Then, a couple of days ago Xmarks online lost all my bookmarks and then synchronized the loss to Chrome and Safari. Trying to restore from a previous backup on the Xmarks doesn't work since previous backups don't download properly and restoring from a previous backup just hangs up. All of this, what I've read in the forums, and LastPass' slowness and lack of communication on resolving this issue makes me wonder if there is not also a server component to all of this.
Fortunately, since I had previously switched off Xmarks in Firefox and started using Firefox sync I was able to restore bookmarks for Chrome and Safari (after turning off Xmarks for them) by importing Firefox bookmarks.This is a little more cumbersome that using Xmarks but at least it works.