Open your profile folder:
       Click the menu button Fx57menu , click Help and select Troubleshooting Information. The Troubleshooting Information tab will open.
       Under the Application Basics section next to Profile Folder, click Open Folder. Your profile folder will open. 
   Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

   Click the Firefox menu Fx57Menu and select Exit.
   Locate the prefs.js file (and, if present, the prefs.js.moztmp file).
   Delete those files and any prefs-n.js files where n is a number (e.g. prefs-2.js).
   Delete Invalidprefs.js if it exists.
   Restart Firefox. You should now have all your preferences reset. 

   Your problem might also be due to malware. See Troubleshoot Firefox issues caused by malware for details.
   Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software. 

'''will automatically create another one when it needs to. Here's how to delete the prefs.js file. This will remove all your custom settings and settings for many extensions. Open your profile folder: Click the menu button Fx57menu , click Help and select Troubleshooting Information. The Troubleshooting Information tab will open. Under the Application Basics section next to Profile Folder, click Open Folder. Your profile folder will open. Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox. Click the Firefox menu Fx57Menu and select Exit. Locate the prefs.js file (and, if present, the prefs.js.moztmp file). Delete those files and any prefs-n.js files where n is a number (e.g. prefs-2.js). Delete Invalidprefs.js if it exists. Restart Firefox. You should now have all your preferences reset. Other solutions Your problem might also be due to malware. See Troubleshoot Firefox issues caused by malware for details. Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software. Based on information from Preferences not saved (mozillaZine KB) Share this article: http://mzl.la/1BAQrbZ '''