If you haven't closed Firefox yet, then you can use "Organize -> Undo" in the Bookmarks Manager (Library; Ctrl+Shift+O). ---- If you notice you lost bookmarks or accidentally deleted bookmark(s) and you haven't closed Firefox yet then you can use "Organize -> Undo" in the bookmarks manager (Bookmarks -> Show All Bookmarks) to undo this action and restore bookmark(s) you deleted accidentally or otherwise lost/misplaced. You may have to use Undo more than once because Firefox keeps a full undo stack of changes to bookmarks for the current session. Otherwise you can consider to restore the bookmarks from a JSON backup in the bookmarkbackups folder. *Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them This will replace all current bookmarks and you lose bookmarks that were added since that backup was created.