Abstract: Firefly is an untethered multi-user virtual reality (VR) system for commodity mobile devices. It supports more than 10 users to simultaneously enjoy high-quality VR content using a single commodity server, a single WiFi access point, and commercial off-the-shelf (COTS) mobile devices. Firefly employs a series of techniques including offline content preparation, viewport-adaptive streaming with motion prediction, adaptive content quality control among users, to name a few, to ensure good image quality, low motion-to-photon delay, a high frame rate at 60 FPS, scalability with respect to the number of users, and fairness among users. We have implemented Firefly in 17,400 lines of code. We use our prototype to demonstrate, for the first time, the feasibility of supporting 15 mobile VR users at 60 FPS using COTS smartphones and a single AP/server.