We recently moved from our temporary Mozilla-specific onMozOrientation API for device orientation events - which let you detect what angle you are holding your mobile phone, for example - to the W3C DeviceOrientation spec.
The main difference in the orientation data is that onMozOrientation returned an (x,y,z) vector, whereas the W3C spec returns (alpha,beta,gamma) Euler angles.
The easiest way to understand what those mean is to look at the examples in the W3C spec linked to before (search for "The following code extracts illustrate basic use of the events"). For now on Android we don't give the alpha value, which is the compass heading (azimuth). You can use the beta and gamma values though. Basically, if the user holds the device flat on a surface pointing up, then tilting it towards the user or away changes beta, and tilting to the right or to the left changes gamma.
We hope to implement the rest of the spec soon.