Thread can be burned, but does not burn. It just gives you a small fire for 40 minutes or so. The thread is not consumed in the fire. Works for stacks and single items. Works in woodstoves and on open squares. (You can also reach into the fire and remove the burning thread without putting the fire out or hurting yourself, but you can do that with anything, which I suspect is a problem with the way that picking up items is handled, not with the way that fire is handled.)
Reproduction:
Drop or throw any number of threads. Light them on fire. Wait about 40 minutes for the fire to go out.
Have not tested on the latest of experimentals or on the stable.