After sync between Chrome and FF left an incredible mess, I reorganized the few bookmarks that remained. I exported those, then tried to restore an older set. Xmarks.com claimed to be restoring them, but it refreshed with the newly reorganized set. This even when I chose the earliest possible restore set, some 5 years back. So, in essence, 'restore' does nothing at all.
Exporting an old set, by the way, leads only to a "server busy" error.