Here's my understanding of how it works. When you long press a bookmark in Firefox and add to home screen, it does not join any existing folder. If you want to move it into a folder, you have to long press it, then drag and drop it on the folder. It's useful to have a blank space on the same home screen as the folder so you don't have to drag it across home screens. But you may be doing something a bit different that I haven't learned about yet.