I’ve found a problem in FF10 with Ember/Ember Data and doing a save on a model. I’ve traced into things pretty deep and it looks like model.currentState gets deleted during the save, which is definitely not what’s supposed to be happening. Ember.unwatchKey is called on the currentState key and it deletes the meta data for currentState as expected, but for some reason it doesn’t assign the new o_definedProperty to currentState, so model.currentState is then undefined after that.
Using the dailies (March 1st) things are broken with FF10. You can see the self contained version here: http://jsfiddle.net/LNXyp/23/
Using the canary builds, things are actually working: http://jsfiddle.net/LNXyp/26/