After installing a fresh copy of Kubuntu 13.04, I've noticed my favorite browser Firefox freezes after about 2 minutes of not being used. I had to terminate the process and to re-launch the app every time, than it'd freeze again after I switched to another application.
The issue was caused by the following add-ons that came installed and enabled by default:
Global menu bar integration
Ubuntu Firefox Modifications.
After disabling the add-ons Firefox started behaving as expected.