Worst ever. It has made a mess into my bookmarks in Firefox, putting all in a "dupe" file ; when I wrote it off, it drove most of my items into "Other bookmarks" in a complete disorder.
Presumably you selected "mark all" and then "move marked bookmarks to dupe".
Note that firefox makes regular backups of your bookmarks: You can restore to a previous stage by opening "Bookmarks -> Show all bookmarks" and there "Import and Backup -> Restore".
