Although not the previously mentioned upgrade to Android 4.3, owners of the Xperia Z and Xperia ZL reported receiving updates to their phones' firmware. The update, which keeps the phone at Android 4.2.2, seems to be a mix of fixes and improvements across Sony's flagship smartphones.
Both phones already received a major upgrade last June, when Sony rolled out OTA upgrades bringing the Xperia Z and ZL to the land of Android 4.2. It seems that there have been some performance issues and bugs back then, so this new upgrade, which is already available to some users, might be Sony's attempt to promptly fix the problems.
There is no definite list of what has been fixed, removed, or improved, but users who have received the upgrades reported several changes. Among these are smoother boot animation and home launcher performance, improved touch response, improved image and display quality, and some other fixes here and there. All in all, the upgrade seems to have boosted performance and user experience not by much, but just enough to be noticeable.
There is no new information regarding any upgrade to Android 4.3 for the Xperia Z line of devices. Considering the devices just got Android 4.2 and it was only recently that Android 4.3 was announced, that might not happen any time soon anyway. Sony might also still be busy in rolling out Android 4.2 for the Xperia ZR and the Xperia Tablet Z, which it said should happen sometime around August.