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 (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

This will replace all current bookmarks and you lose bookmarks that were added since that backup was created, so you may want to create an HTML backup beforehand and import this backup afterward.

Thanks cor-el,

I tried the procedure that you suggested, and the deleted folder has been restored. Thank you very much.

If you haven't closed Firefox then: menu > bookmarks > at the bottom of the window that opens click Manage Bookmarks > in the Library window that opens click Organize > Undo