The Sony Xperia Z (C660X) may soon be receiving a new firmware upgrade judging by a thread on xda-developers. A poster who has a ‘brown’ developer device says that he has received a new update in the form of build number 10.3.1.A.0.236. This particular Android 4.2.2 version has not been certified on the PTCRB website so far, the latest current firmware is 10.3.A.0.423.
Whether this particular firmware version makes it to consumer devices is unknown right now, as it is likely to be a test version. Even if it doesn’t, it shows that hopefully a new firmware update is on its way soon for Xperia Z devices.