If Sync was Connected when you tried to Restore a bookmark backup from Firefox, chances are Sync won

If Sync was Connected when you tried to Restore a bookmark backup from Firefox, chances are Sync won't have your bookmarks on the Sync server.

If Sync was Disconnected try logging in and see if they will Sync back to that device.

What about other devices connected to that Sync account? Your bookmarks should still be on that device; unless the bookmarks on the Sync server got wiped and that "change" was propagated to the other connected devices ...

BTW, we have had many reports of issues with Sync duplicating bookmarks when Xmarks was being used at the same time as Sync was connected.