Joining up with the mixed reality team is more about how the Mozilla staff working on Servo will be focusing their time and some of the desire we have for things we want to do for mixed reality that are a little invasive and early-stage of the standards process to be experimenting with inside of existing production browser engines.
With regards to a full browser, there's still a pretty big gap before Servo could stand on its own in a browser (https://github.com/servo/servo/wiki/Remaining-work ). Filling that list out is not a higher priority for the Mozilla staff over getting the stuff done we need to experiment with delivering the web on VR & AR devices.