Firefox keeps a full undo stack of changes to bookmarks for the current session. So 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 the deleted bookmarks reappear. Otherwise you would have to restore one of the automatically created backup in the bookmarkbackups folder. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them You can look at this tool to inspect a compressed jsonlz4 bookmarks backup and possibly convert this file to HTML to be able to import/merge bookmarks. *https://www.jeffersonscher.com/res/bookbackreader.html