Firefox keeps a full undo stack of changes made to bookmarks for the current session. If you haven't closed Firefox, you can open the Bookmarks Manager (Library; Ctl+Shift+O) and use "Organize -> Undo" (Ctrl+Z) until you see your deleted bookmark(s) reappear. Otherwise you will have to restore an automatically created backup. Note that you lose bookmarks that aren't in the backup, so if necessary create an HTML backup. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them *https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer