There is no extension like Xmarks (when it worked well). It had a user-friendly web UI. I could expand the different backups and see their content. I could choose a backup to make it the current syncing bookmark set. I could remove backups. I could view the contents of the current set. Many other options.
I would say that in terms of features, ES sync/backup is somewhere between the built-in Firefox sync and Xmarks.
Right now I actually prefer the built-in Firefox sync because it's MUCH faster than ES sync. For example, after the initial sync, then pressing the sync icon makes it spin for maybe 4-5 seconds tops).
The one thing I use in ES is the auto-backup feature. Too bad there is no setting for the time period between backups (I think it's 15 min and it cannot be changed), but it's better than the built-in Firefox backup, which only backs up bookmarks once a day. So that's the only reason I'm using ES right now.
This is an opportunity for extensions like ES to fill the gap that Xmarks left, but it would require a lot of work by the team. Hopefully this thread will provide more ideas for improvement.
Customer support service by UserEcho