system
8
It seems to be a problem with the FF addon “avast! Online Security” (currently version 8.0.1489).
Disabling the addon solved my problems: Firefox lost all settings but not after restarting the application. It lost it when restarting the system.
Addons like NOSCRIPT & Co. lost their restriction settings. Very annoying. Had to reconfigure all settings after restarting the system. Only Hibernating the system was a workaround but sometimes you have to restart the PC …e.g. when updating Avast 