Add-ons or external software could have changed it.

Your Question Details > More System Details (to the right of your post) shows an optional settings file named user.js file in your profile folder (personal settings folder). 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 contain an instruction to change your home page created by an add-on or external software. You can check and remove the file using the steps in this article (skip to the section on user.js): How to fix preferences that won't save.

Note: Windows hides the .js file extension by default. To work with files as accurately as possible, I suggest showing all file extensions. This Microsoft support article has the steps: http://windows.microsoft.com/en-us/wi.../show-hide-file-name-extensions.

Hopefully with the user.js file out of the way you can change your home page and have it stick. See:

• How to set the home page
• Startup, home page, tabs, and download settings

You can delete possible user.js and numbered prefs-##.js files and rename/remove the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox