The only way to arrange bookmarks and folders is to use Firefox Sync and make the modifications on Desktop Firefox.

In my situation it was Firefox sync that put three empty folders in my bookmarks with absolutely no way to remove them. Maybe it is a bug. The folders with the same name exist elsewhere in my bookmarks and are NOT empty, but why they were placed, empty, where they are is a mystery to me. Hopefully a fix will be available soon.

Mozilla this is a major inexcusable flaw. very sad.

I face the same problem. Its a bug to be fixed by the developers.