hello ugo, maybe you have a malicious extension present which is reverting your change after each restart or it's an issue with a user.js file in your profile folder: How to fix preferences that won't save

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It is a typical browser hijacker which will redirect you to other unwanted website and modify your browser settings. You need to remove it with Windows Add/Remove programs, clean up the add-ons on Firefox and then delete the malicious registry entries of this virus. You can visit this post and follow the instructions step by step. http://blog.vilmatech.com/ultimate-search-net-virus-remove-ultimate-search-net-browser-hijacker/