Is this on a Windows system, or Mac, Linux, etc.? I will assume Windows. What is the startup setting and/or home page changed to when you go into Options to check it? That may help identify the culprit. Also, please check for an optional settings file named user.js file in your profile folder (personal settings folder). If it exists, Firefox uses the settings in that file at startup to override your saved preferences from the previous session. Unless you created that file yourself, it may contains settings from an add-on or external software. You can check and remove the file using the steps in this article: [[How to fix preferences that won't save]]. If you find a file named user (of type JScript) but not user.js, Windows may be hiding file extensions from you. To work with files as accurately as possible, I suggest unhiding them. This Microsoft support article has the steps: [http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions].