See this support article: https://support.mozilla.org/en-US/kb/bookmark-folders-organize-your-bookmarks

You can't add/create a folder at the root level (i.e. under All Bookmarks) if that is what you try to do.

There are three bookmarks root folders in the All Bookmarks folder where you can create new bookmarks and new folders. You can't remove these root folders (Delete in the right-click context menu is grayed for the root folders).

• the "Bookmarks Toolbar" folder stores the bookmarks that you see on the Bookmarks Toolbar (Firefox > Options or View > Toolbars)
• the "Bookmarks Menu" folder stores bookmarks that show in the Bookmarks menu and are created via "Bookmark This Page" or via the star (Bookmark This Link) in the right-click context menu
• the "Other Bookmarks" folder (previously named "Unsorted Bookmarks") stores bookmarks that are created by clicking the star on the location bar