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How to backup and restore these crucial files
You can make mistakes when editing configuration files that would prevent Firefox from working properly or even starting at all. For this reason, you should always back up these files before making changes. Everyone will tell you that, but I'm also going to show you how to do it and, more importantly, how to recover in case you make a serious error.
1. Backup your config files. Your customization files are stored in a folder named Profiles on your hard drive underneath username \ Application Data \ Mozilla \ Firefox. To find the different locations for this folder on Windows NT, 2000, XP, 9x, and Me, check the convenient chart at Mozilla.org. Before making edits, copy the Profiles folder to a backup disk or another safe location.
2. How to recover from errors. If your changes make Firefox unstable, close all instances of Firefox and then copy your preferences files from your backup location to the default location and restart Firefox. In extreme cases, when Firefox won't even start, you can rename the User.js file and/or the Prefs.js file to a temporary name and then start Firefox. The browser will revert to a default configuration, allowing you to transfer your "safe" preferences into a new file.