Eric provides some excellent information. Let's not jump on Devon, however. Everybody's ideas and opinions are welcome and appreciated.
I roll up my sidewinder cone & inferno insert, hold the roll together with a rubber band, and put it inside my cookpot. I also put the mesh screen in there, plus a spoon, an emergency firesteel and a ziplock of cotton balls smooshed with vaseline. It all packs very easily. I pop the pot in a cuben fiber stuff bag and slip the two titanium stakes in there, too. Regardless of which stove I would use, I would still have the volume of my pot, so I don't see any volume advantage to the firefly.
I hadn't realized that the firefly wasn't a gassifier by design, so I suppose the efficiency loss is a negative. Also, the couple times that I've used the alcohol burner with the caldera (when day hiking with friends who are big babies and get annoyed by a wee bit of wood smoke), I have been wonderfully impressed with that alcohol burner's efficiency in the caldera. I had formerly been a Vargo Triad titanium alcohol stove user, but got sick of fiddling around with a flimsy aluminum foil windscreen, carrying the alcohol weight, etc. I'm amazed at how well the alcohol burner designed to work with the caldera performs. Wow. Amazing.
I still might buy the firefly, just because I like to try stuff.
But can anybody think of a reason why for me, as the owner of a caldera cone sidewinder with inferno insert, to buy the firefly, other than to satisfy my obsessive GAS (gear aquisition syndrome)? Remember, I'm ONLY interested in woodburning, so all I want to do is compare these two stoves on their woodburning virtues.
Thanks again, Eric, and Devon, too. All comments appreciated.