If you haven't closed the Firefox application yet then you should be able to use Undo (Command + Z) in the Bookmarks Manager (Library). Note that you may have to use Undo more than once, repeat until you see the folders reappear. You can find Undo in the Organize menu, that is the first of the three buttons on the toolbar in the Library. *https://support.mozilla.org/en-US/kb/bookmarks-firefox If you can't use undo then you need to restore a recent compressed .jsonlz4 backup in the bookmarkbackups folder. *Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore "Import & Backup" is the third button on the toolbar in the Library. 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.