Hello, please try restarting Firefox in safe mode [[Troubleshoot Firefox issues using Safe Mode]] see if problem goes out or not, if yes then one of your extension may be causing the problem. Moreover your profile folder has a user.js file, User.js file “does not exist by default. If you haven't created it yourself in the first place or aren't sure what has created it, then the user.js file shouldn't be there at all. you can go ahead and delete the file - it won't affect your stored bookmarks or other personal data... This article will help you [[How to fix preferences that won't save]] please reply back to us.