Is any kind of XMLHttpRequest methode as CORS supported for FxOS inter app communication today, I know if you set systemXHR permission and set mozSystem:true it works reaching other domains, but is it suppose to work for reaching another app(hosted or packaged) in FxOS or is the same origin policy preventing this as it does for postMessage?
As @kazhik suggest, the best way could be web activity, where you delegate the execution of a task, and another web app (with its own UI) will take care of it.
Firefox OS is introducing new ways of performing communication across applications, still in test you can find new APIs coming in new 1.3 version:
IAC (Inter app communications):
a new way of getting a 'pointer' to a window and perform postMessage: https://wiki.mozilla.org/WebAPI/Inter_App_Communication
DataStore:
share your data across apps : https://wiki.mozilla.org/WebAPI/DataStore
So far I think this APIs require certification but I'm sure will be opened for everyone else in next releases.