Bookmarks hierarchy crashed when I paired with mobile device.
I have lost all my Bookmark's hierarchy in Bookmark Toolbar and Bookmarks Menu most of them moved to Unsorted Bookmarks.
We have seen more reports with bookmarks getting corrupted when Sync is used.
Try to unlink the current sync account and try to restore the bookmarks from the most recent JSON that is still OK.
You can check also for problems with the places.sqlite database file in the Firefox profile folder in case of problems with restoring a JSON backup.