This article lists a number of possible reasons that Firefox might be unable to save your preference changes during your session, or that they might be overridden at startup: [[How to fix preferences that won't save]]. Another common issue is a feature of security or utility software that protects you from unwanted browser settings changes by reverting them back to an earlier state. For example, Advanced SystemCare's Surfing Protection feature can do this. You may need to turn off such features, if you have them, in order to keep your changes.