You may have moved that folder by accident into another folder.

If you haven't closed Firefox yet then you can try to undo that operation in the Bookmarks Manager (Library) via Organize > Undo (Ctrl+Z)

• Bookmarks > Show All Bookmarks

Otherwise try to open the bookmarks in the sidebar (Ctrl+B) and click in the bookmark area to set focus to that area and then quickly press the first letters of the lost folder to see if that brings up that lost folder.
The search bar only searches bookmarks and not folders and doesn't show the location of bookmarks that are found.
You need to click at the left of an icon in that area to prevent opening the bookmark.

Thank you so much. As you suggested, the folder was put into another one. Happy New Year!