hello ram085, there's a user.js file present in your profile folder, which is often used by adware to override your custom preferences.

in order to fix that go to firefox > help > troubleshooting information > profile folder - "show folder", then a new window will open up. in this window look out for a file named user.js, in case it is present , delete or rename this file and restart firefox afterwards.
How to fix preferences that won't save

finally you can install the search reset addon which will revert the most common changes by adware programs in firefox: https://addons.mozilla.org/firefox/addon/searchreset/

You can also check the target line in the Firefox desktop shortcut (right-click: Properties) to make sure that nothing is appended after the path to the Firefox program.