Unfortunately, we don't currently support phone-based Daydream or Gear VR devices. There's both a bunch of extra work to support all of the phone in/out/notification lifecycle scenarios and we have a pretty small engineering team and the long tail of phone/chipset-specific bugs is pretty substantial. We might evaluate support for it or Cardboard in the future, but for right now we're mainly focused on standalone headsets.
That said, we're all in favor of people who want to sideload and play with it! https://github.com/MozillaReality/FirefoxReality/wiki/Sidelo...