Usually, Firefox for Android doesn't create any bookmark folders, it just shows a Desktop Bookmarks folder (if you use Sync with a Windows/Mac/Linux installation of Firefox) and then the individual mobile bookmarks. But I've had this device for 3 1/2 years, so maybe a fresher install behaves a bit differently.

Could you test: load a bookmarked page, and on the menu the "Add" item should now be "Edit". In Edit, you can switch the "folder" back to Bookmarks. Hopefully Firefox will take the clue for new bookmarks going forward?