If you lose or accidentally delete bookmarks then you can use "Organize -> Undo" in the Bookmarks Manager (Library) that keeps an undo queue for all action done in the current session, but as you have restarted Firefox then this isn't possible and you likely need to restore an automatically created backup in the bookmarkbackups folder. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them The name of an automatically created JSON bookmarks backup in the bookmarkbackups folder includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. You also see this count in the Restore menu drop-down list. *bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.