We should note that a container in a Tinderbox document is not a folder – in the sense of a folder in the file system. Similar metaphor, different physical reality.
Tinderbox still syncs with Simplenote. And Simplenote itself can sync with a Dropbox folder.
AAMOF, my nvALT notes are in a Drobbox folder – Simplenote indexes that folder – and I can use Simplenote, nvALT, DEVONthink, or any other markdown-capable editor to edit those files on any platform (Windows, Mac, iOS, etc.).
See Help or aTbRef for how-to help.