Mozilla's Boot to Gecko, or Firefox OS (and specifically the Gaia UI layer) is not on any hardware you can buy yet or anything, but it's being developed quite transparently, so there is a nice collection of guidelines. Also, I know they used Designing Mobile Interfaces a lot during the UI design phase, so I agree with all their decisions.

Tools & Templates - Firefox OS (last edited 2013-08-27 18:26:57 by shoobe01)