Firefox sometimes does not save updated preferences due to a problem with the prefs.js file (e.g., it is locked by another process). If you notice other settings not saving, such as your preferred home page, or Tab options, etc., that could be a factor. Your saved settings also can be overridden by other software creating a user.js file. For both of those issues, please see this support article: [[How to fix preferences that won't save]]. Some utility software, such as Advanced SystemCare, will roll back settings changes when you exit Firefox. This is meant to protect you from unwanted changes, but also blocks you from making changes. You should check whether any of your utility, security, or system cleaning software has this feature. A final possibility is malware, but hopefully that won't be the issue here. If you want to supplement your regular security software, you can use some of the free tools listed in our support article: [[Troubleshoot Firefox issues caused by malware]].